The Periodic Table

Exercise 5(A)

Question 1

What is the need for classification of elements?

Answer

Reasons for classification of elements are:

1. It helps in studying the elements in an organized manner.
2. It helps in correlating the properties of elements with the fundamental properties of all states of matter.
3. It helps in defining the relationship of one element with another.

Question 2

What was the basis of the earliest attempts made for classification and grouping of elements?

Answer

Earliest attempts or classification and grouping of elements were based on factors such as:

• Density
• Malleability
• Ductility
• Metal or Nonmetal

Question 3

(a) A, B and C are the elements of a Dobereiner's triad. If the atomic mass of A is 7 and that of C is 39, what should be the atomic mass of B?

(b) Why were Dobereiner's triads discarded?

Answer

(a) When the atomic mass of A = 7, and that of C is = 39 then according to Dobereiner's triad, atomic mass of B should be mean value of atomic masses of A and C.

$\dfrac{7 + 39}{2}$ = $\dfrac{46}{2}$ = 23

Hence, atomic mass of B = 23

(b) Reasons for discarding Dobereiner's triads:

1. Dobereiner failed to arrange all the then known elements in the form of triads.
2. The law did not fully apply even within the same family.

Question 4

Explain 'Newland's law of Octaves'. Why was the law discarded?

Answer

According to Newland's law of octaves, when elements are arranged by increasing atomic mass, the properties of every eighth element starting from any element are a repetition of the properties of the starting element.

Reasons for discarding the Newland's law of Octaves:

1. This classification did not work with heavier elements i.e., those lying beyond Calcium. As more and more elements were discovered, they could not be fitted into Newland's Octaves.
2. Newland adjusted two elements Cobalt (Co) and Nickel (Ni) in the same slot, and these were placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements.
3. Iron, which resembles cobalt and nickel in properties has been placed far away from these elements.

Question 5

Did Dobereiner's triads also exist in the columns of Newland's Octaves? Compare and find out.

Answer

Yes, Dobereiner's triads also exist in the columns of Newland's octaves.

Example: Consider the elements lithium (Li), sodium (Na), and potassium (K) present in second column of Newland's classification of elements. If we start with lithium as the 1st element, then the 8th element from it is sodium, and according to Newlands' law of octaves, the properties of the 8th element, sodium should be similar to those of the 1st element, lithium.

Again, if we take sodium as the 1st element, then the 8th element from it is potassium, and according to Newland's law of octaves, the properties of the 8th element, potassium should be similar to those of the 1st element, sodium.

This means that according to Newland's law of octaves, the elements lithium, sodium, and potassium should have similar chemical properties. We also know that lithium, sodium, and potassium form a Dobereiner's triad having similar chemical properties. From this, we conclude that Dobereiner's triads also exist in the columns of Newland's Octaves.

Question 6(a)

Lithium, sodium and potassium elements were put in one group on the basis of their similar properties. What are those similar properties.

Answer

Similar properties of lithium, sodium and potassium were:

1. They have one electron in the outermost shell, so they form unipositive ions.
2. All these are soft metals.
3. They are good reducing agents.
4. These metals impart colour to the flame.

Question 6(b)

The elements calcium, strontium and barium were put in one group or family on the basis of their similar properties. What were those similar properties?

Answer

Similar properties of calcium, strontium and barium were:

1. All are metals.
2. Their oxides are alkaline in nature.
3. Each has valency 2.

Question 7

(a) What was Mendeleev's basis for classification of elements?

(b) Mendeleev's contributions to the concept of a periodic table laid the foundation for the Modern Periodic Table. Give reasons.

Answer

(a) Mendeleev classified elements according to Mendeleev's Periodic Law. Mendeleev's Periodic Law states that physical and chemical properties of elements are a periodic function of their atomic masses.

(b) As many features of the Mendeleev's periodic table were later used when Modern Periodic Table was made so we can say that Mendeleev's contributions to the concept of a periodic table laid the foundation for the Modern Periodic Table. For example:

1. Mendeleev's periodic table contains vertical column called groups and horizontal rows called periods.
2. They were all in eight groups.
3. All elements in a group had similar properties and same valency.
4. In a period elements gradually changed from metallic to non-metallic character.
5. Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
6. He predicted the properties of then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots.
7. He was able to correct the atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.

Question 8

State Mendeleev's periodic law.

Answer

Mendeleev's periodic law states that the physical and chemical properties of elements are a periodic function of their atomic masses.

Question 9

Use Mendeleev's Periodic Table to predict the formula of:

(a) hydrides of carbon and silicon

(b) oxides of potassium, aluminium and barium.

Answer

(a) Carbon is in group 4. Hydride of any element of group 4 is written as RH₄. Hence, the hydride of carbon will have the formula CH₄.

Silicon is also in Group 4. So, the hydride will be SiH₄.

(b) K is in Group 1 and have general oxide formula R₂O. So, the oxide will be K₂O.

Al is in Group 3 and have general oxide formula R₂O₃. So, the oxide will be Al₂O₃.

Ba is in Group 2 and have general oxide formula RO. So, the oxide will be BaO.

Question 10

Which group of elements was missing from Mendeleev's original periodic table ?

Answer

Group zero elements or inert gases were missing from Mendeleev's periodic table as they were not discovered at that time.

Question 11

State the merits of Mendeleev's classification of elements.

Answer

Merits of Mendeleev’s classification of elements:

1. Grouping of elements: He generalized the study of the elements then known to a study of mere eight groups.
2. Gaps of undiscovered elements: Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
3. Prediction of properties of undiscovered elements: He predicted the properties of then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots.
4. Incorrect atomic mass corrected: He was able to correct the atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.

Question 12

Why did Mendeleev leave some gaps in his periodic table of elements? Explain your answer with an example.

Answer

In order to make sure that elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table. These gaps were left for subsequent inclusion of elements not known at that time.

For example,

(i) Eka-aluminium (means one place below aluminium in the group); its atomic mass and chemical properties are quite similar to those of the element gallium discovered later in 1876.

(ii) Properties of eka-silicon are the properties of germanium.

Question 13

The atomic number of an element is more important to the chemist than its relative atomic mass. Why?

Answer

Through an experiment, Henry Mosley found that number of positive charge present in the nucleus of an atom or atomic number is the most fundamental property of an atom.

According to him, if elements are arranged in order of their increasing atomic number, those with similar properties are repeated after regular intervals, i.e., periodicity in the periodic table occurs based on the atomic numbers (number of protons). Hence, the atomic number is more important to a chemist than its relative atomic mass.

Question 14

Consider the following elements: Be, Li, Na, Ca, K. Name the elements of (a) same group (b) same period.

Answer

(a) Same group

1. Li, Na, K belong to the same group IA.
2. Be, Ca belong to the same group IIA.

(b) Same period

1. Li and Be belong to Second period.
2. K, Ca belong to Fourth period.
3. Na belongs to Third period.

Question 15

(a) Name an element whose properties were predicted on the basis of its position in Mendeleev's periodic table.

(b) Name two elements whose atomic weights were corrected on the basis of their positions in Mendeleev's periodic table.

(c) How many elements were known at the time of Mendeleev's classification of elements?

Answer

(a) Gallium (Eka aluminium)

(b) Gold and Platinum

(c) 63

Exercise 5(B)

Question 1

(a) State the modern periodic law.

(b) How many periods and groups are there in the modern periodic table?

Answer

(a) The Modern Periodic Law is stated as, 'the physical and chemical properties of elements are the periodic functions of their atomic number.'

(b) Groups - 18 and Periods - 07

Question 2

What is the main characteristic of the last elements in the periods of a periodic table? What is the general name of such elements?

Answer

Main characteristic of the last element in each period of the Periodic Table are:

1. They have valency equal to zero.
2. They react under very special condition otherwise they don't react.
3. They have stable gas electronic configuration.

General name of such elements is Inert Gases.

Question 3

What is meant in the periodic table by :

(a) a group, and

(b) a period?

Answer

(a) Eighteen vertical columns in the periodic table are called groups.

(b) Seven horizontal rows in the periodic table are called periods.

Question 4

From the standpoint of atomic structure, what determines which element will be the first and which the last in a period of the periodic table ?

Answer

Number of valence electrons determines which element will be the first and which element will be the last in a period.

Question 5

(a) What are the following groups known as?

1. Group 1
2. Group 17
3. Group 18

(b) Name two elements of each group.

Answer

(a) The groups are knows as:

1. Group 1 — Alkali metals
2. Group 17 — Halogens
3. Group 18 — Noble gases

(b) Two elements of each group are:

1. Group 1 — Lithium, Sodium
2. Group 17 — Fluorine, Chlorine
3. Group 18 — Neon, Argon

Question 6

What is the number of elements in the :

(a) 1st period, and

(b) 3rd period, of the modern periodic table?

Answer

(a) 2 Elements

(b) 8 Elements

Question 7

How does the number of (i) valence electrons (ii) valency; vary on moving from left to right:

(a) in the second period of a periodic table?

(b) in the third period of a periodic table?

Answer

(a) In the second period of the periodic table, the number of valence electrons increase from 1 to 8 as we move from left to right. The valency first increases from 1 to 4 from element Lithium (Li) to Carbon (C) and then reduces to zero from element Nitrogen (N) till Neon (Ne).

(b) In the third period of the periodic table, the number of valence electrons increases from 1 to 8 as we move from left to right. The valency first increases from 1 to 4 from element Sodium (Na) to Silicon (Si) and then reduces to zero from element Phosphorus (P) till Argon (Ar).

Question 8

How do atomic structures (electron arrangements) change in a period with an increase in atomic numbers, moving from left to right?

Answer

As we move across a period from left to right, the number of shells remain the same and number of valence electron increases by one. The size of atom decreases because with increase in number of protons, nuclear pull increases, which decreases the size.

Question 9

State the criteria used by Mendeleev in proposing his periodic table?

Answer

The criteria used by Mendeleev in proposing his periodic table was the atomic masses of elements. He classified elements according to Mendeleev's Periodic Law. Mendeleev's Periodic Law states that physical and chemical properties of elements are a periodic function of their atomic masses.

Question 10

How are the positions of Cobalt and Nickel resolved in the Modern periodic table?

Answer

In Mendeleev's periodic table, Cobalt with atomic mass 58.9 preceded Nickel with atomic mass 58.6. This anomaly was resolved in Modern periodic table by arranging the elements based on their atomic number instead of atomic mass.
Cobalt with atomic number 27 is placed in group 9 and Nickel with atomic number 28 is placed in group 10 in the Modern periodic table.

Question 11

Cl³⁵ and Cl³⁷ are two different atoms. Where would they be placed in the periodic table.

Answer

Since, all isotopes of chlorine i.e., Cl³⁵ and Cl³⁷ have the same number of protons so their atomic number is the same. Hence, they are placed at the same position in the periodic table in group 17, period 3.

Question 12

An element X has 3 electrons in its M shell. State its position in the periodic table? Name the element.

Answer

The element having three shells [K, L, M] with three electrons in the valence shell is in group 13 [III A] and period 3. The name of the element is Aluminium.

Question 13

Name an element that has two shells, each filled completely with electrons. State its position in the periodic table.

Answer

Neon (Ne), It is placed in the zero group and 2nd period.

Question 14

Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common.

Answer

Atoms of both Helium and Neon have completely filled valence shells which makes them very stable and unreactive.

Question 15

In which part of a group would you separately expect the elements to have

(a) the greatest metallic character?

(b) the largest atomic size.

Answer

(a) Greatest metallic character is shown in the elements at the bottom of the group.

(b) Largest atomic size is shown in the elements at the bottom of the group.

Question 16

What happens to number of valence electrons in atoms of elements as we go down a group of the periodic table?

Answer

Number of valence electrons remain the same as we go down a group.

Example:

Question 17

Sodium and aluminium have atomic numbers 11 and 13, respectively. They are separated by one element in the periodic table and have valencies 1 and 3 respectively. Chlorine and potassium are also separated by one element in the periodic table (their atomic numbers being 17 and 19, respectively) and yet both have valency 1. Explain.

Answer

Sodium and aluminium both belong to period 3 of the periodic table. They are the first and the third elements of their period belonging to group IA and IIIA, respectively. As they lie towards the left in their period, they exhibit metallic character and being separated by one element exhibit positive valencies of +1 and +3, respectively.
On the other hand, even though chlorine and potassium are also separated by one element, chlorine belongs to period 3 and potassium to period 4 of the periodic table. Chlorine is the rightmost element of period 3 belonging to the Halogens group VIIA. So, it exhibits a negative valency of -1. Potassium is the leftmost element of period 4 belonging to the Alkali metals group IA. So, it exhibits a positive valency of +1.

Question 18

This question refers to elements of the periodic table with atomic numbers from 3 to 18. In the table below, some elements are shown by letters, even though the letters are not the usual symbols of the elements.

(a) Which of these is

(i) a noble gas

(ii) a halogen

(iii) an alkali metal

(iv) an element with valency 4

(b) If A combines with F, what would be the formula of the resulting compound?

(c) What is the electronic configuration of G?

Answer

(a) (i) H and P are noble gases, as they are in group zero and have their octet complete.

(ii) G and O are halogens, as they are in group VIIA and have 7 valence electrons.

(iii) A and I are alkali metals, as they are in group IA and have 1 valence electron.

(iv) D and L have valency 4, as they are in group IVA and have 4 valence electrons.

(b) Valency of A [2, 1] is +1, so it tries to donate an electron and attain a stable state.
Valency of F [2, 6] is -2, so it tries to gain two electrons and attain a stable state. Hence, one F atom takes electron from two A atoms. Therefore, the formula of their compound will be A₂F

(c) Electronic configuration of G is [2, 7] as its atomic number is 9.

Question 19

The position of elements A, B, C, D and E in the periodic table are shown below:

(a) State which are metals, non-metals and noble gas in this table.

(b) State which is the most reactive (i) metal (ii) non-metal.

(c) Which type of ion will be formed by element A, B and C.

(d) Which is larger in size (i) D or E (ii) B or C.

Answer

(a) Metals — A and B, as they belong to alkali metals group and alkaline earth metals group.

Non-metals — C, as it belongs to halogen group 17.

Noble gases — D and E, as they belong to group 18 or zero group.

(b) A is the most reactive metal as Group 1 (Alkali metals) elements are very reactive and the degree of reactivity further increases down the group. C is the most reactive non-metal as it is in Group 17 (Halogens) and the degree of reactivity of halogens decreases down the group.

(c) Type of ion formed by elements A, B and C will be:

1. Element A will lose the one valence electron and become an electropositive ion [i.e., cation A⁺].
2. Element B will lose two valence electrons and become an electropositive ion [i.e., cation B²⁺].
3. Element C will gain one electron, complete its octet and become an electronegative ion [i.e., anion C^-].

(d) Larger in size

1. E is larger in size as the number of shells increases as we move down a group increasing the size of the atom.
2. B is larger in size as atomic size decreases on moving from left to right across a period due to increase in nuclear charge.

Question 20

Write the electronic configuration of the element ³⁵₁₇T.

(a) What is the group number of T?

(b) What is the period number of T?

(c) How many valence electrons are there in an atom of T?

(d) What is the valency of T?

(e) Is it a metal or a non-metal?

(f) State number of protons and neutrons in T.

Answer

The electronic configuration of ³⁵₁₇T is [2, 8, 7]

(a) Group number of T — VIIA [∵ it has 7 valence electrons]

(b) Period number of T — Third period [∵ it has 3 shells]

(c) Valence electrons — Seven [from electronic configuration]

(d) Valency of T is -1. [∵ it needs to gain one electron to complete its octet]

(e) T is a non-metal [∵ it has a tendency to accept electrons]

(f) Protons = 17, Neutrons = 18

Exercise 5(C) — Multiple Choice Type

Question 1

In the periodic table, the elements of a same group have the same:

1. Number of valence electrons
2. Mass number
3. Atomic number
4. Number of electrons

Answer

Number of valence electrons

Reason — The arrangement of elements based on their atomic weights in the Modern Periodic Table puts elements with same number of valence electrons in the same group.

Question 2

The number of elements in the largest period of the periodic table is:

1. 18
2. 8
3. 32
4. 42

Answer

32

Reason — The sixth period contains 32 elements (atomic number 55-86). It is the longest period.

Question 3

Atomic number is a much more fundamental property of an element than its atomic mass. This was emphasised by:

1. Bohr
2. Mosley
3. Mendeleev
4. Newland

Answer

Mosley

Reason — Henry Mosley found that atomic number is a better fundamental property of elements compared to atomic mass which led to the modern periodic law.

Question 4

All the members of group 15 have ................ electrons in their outermost orbit.

1. 3
2. 7
3. 5
4. 8

Answer

5

Reason — Group 15 elements have 5 valence electrons.

Question 5

Which of the following statements is correct with respect to the elements — Na, Mg, Al, Si, P, S and Cl.

1. Each element belongs to the same group.
2. Valency of each element is the same.
3. Each element belongs to the same period.
4. They are all metals.

Answer

Each element belongs to the same period

Reason — Period 3, known as short period, contains 8 elements i.e., sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorous (P), sulphur (S) and chlorine (Cl).

Question 6

Elements with 2 electrons in their outermost shells are known as :

1. Alkali metals
2. Halogens
3. Alkaline earth metals
4. Inert gases

Answer

Alkaline earth metals

Reason — Group 2 elements known as alkaline earth metals, have two electrons in their outermost orbit.

Question 7

Modern periodic table is based on:

1. Atomic weight
2. Atomic number
3. Mass number
4. Molecular mass

Answer

Atomic number

Reason — The Modern Periodic Law is stated as, 'the physical and chemical properties of elements are the periodic functions of their atomic number.'

Question 8

Elements which have their outermost orbits incomplete are:

1. Transition elements
2. Noble gases
3. Inner transition elements
4. Representative elements

Answer

Transition elements, Inner transition elements, Representative elements.

Reason — Noble gases have a stable electronic configuration with their outermost orbits complete.

Question 9

The element with atomic number 16 is placed in the modern periodic table in:

1. 2nd period and VI A group
2. 3rd period and IV A group
3. 3rd period and VI A group
4. 2nd period and IV A group

Answer

3rd period and VI A group

Reason — The element with atomic number 16 has electronic configuration 2,8,6. It has 3 shells so it will be in period 3 and 6 valence electrons so it is placed in VI A group.

Question 10

Typical elements are the elements of:

1. IIIrd group
2. IIIrd period
3. IIrd period
4. IVth period

Answer

IIIrd period

Reason — The third period elements Na, Mg, Al, Si, P, S and Cl summarize the properties of their respective groups and are called typical elements.

Question 11

The strongest reducing agents are the elements of group:

1. Six
2. Seven
3. One
4. Two

Answer

One

Reason — As group 1 elements have 1 valence electrons hence they tend to donate it, attain a stable configuration and get oxidized. Therefore, they are the strongest reducing agents.

Question 12

The most reactive non-metal is:

1. Iodine
2. Oxygen
3. Fluorine
4. Chlorine

Answer

Fluorine

Reason — As reactivity of halogens group decreases as we move downwards, hence fluorine is the most reactive non-metal. Reactivity of fluorine is more than oxygen as it has a stronger tendency to take electrons and complete its octet.

Exercise 5(C) — Very Short Answer Type

Question 1

Match column A with column B

Answer

Question 2

Write down the word that will correctly complete the following sentences:

(a) Relative atomic mass of a light element up to calcium is approximately ............... its atomic number.

(b) The horizontal rows in a periodic table are called ...............

(c) Going across a period left to right, atomic size ...............

(d) Moving left to right in the second period, number of valence electrons ...............

(e) Moving down in the second group, number of valence electrons ...............

Answer

(a) Relative atomic mass of a light element up to calcium is approximately twice its atomic number.

(b) The horizontal rows in a periodic table are called periods.

(c) Going across a period left to right, atomic size decreases.

(d) Moving left to right in the second period, number of valence electrons increases

(e) Moving down in the second group, number of valence electrons remains same

Question 3

Name or state the following with reference to the elements of the first three periods of the periodic table.

(a) Noble gas with duplet arrangement of electrons.

(b) Metalloid in Period 3.

(c) Valency of elements in Group 14 and 15.

(d) Noble gas having electronic configuration 2, 8, 8.

(e) Group whose elements have zero valency.

(f) A covalent compound formed by an element in period 2 and a halogen.

(g) Non-metallic element present in Period 3 of Groups 15 and 16.

(h) An electrovalent compound formed by an alkaline earth metal and a halogen.

(i) Bridge elements of Period 3 of Group 1, 2 and 3.

(j) Alkali metal in Period 3 that dissolves in water giving a strong alkali.

(k) Typical elements of Groups 14 and 15.

(l) Alkaline earth metal in Period 3.

Answer

(a) Helium (He)

(b) Silicon (Si)

(c) 4, 3

(d) Argon (Ar)

(e) Group 18 — Noble gases

(f) Carbon tetrachloride (CCl₄)

(g) Phosphorus and sulphur

(h) Sodium chloride (NaCl)

(i) Lithium (Li) and Magnesium (Mg);
    Beryllium (Be) and Aluminium (Al);
    Boron (B) and Silicon (Si)

(j) Sodium (Na)

(k) Silicon (Si) and phosphorus (P).

(l) Magnesium (Mg)

Question 4

Name the gas used in

(a) filling balloons

(b) light bulbs

(c) bright coloured advertising light works

Answer

(a) Helium

(b) Argon

(c) Neon

Question 5

(a) Name the method by which alkali metals can be extracted.

(b) What is the colour of the flame of sodium and potassium?

Answer

(a) Electrolysis of their molten state.

(b) Flame of sodium is golden yellow, and potassium is lilac.

Exercise 5(C) — Short Answer Type

Question 1

Arrange the following

(a) Elements of group 1, in increasing order of reactivity.

(b) Elements of group 17, in decreasing order of reactivity.

(c) He, Na, Mg (decreasing order of valency).

(d) Chlorine, sodium, magnesium (increasing reducing character).

Answer

(a) Li < Na < K < Rb < Cs

(b) F > Cl > Br > I

(c) Mg > Na > He

(d) Cl < Mg < Na

Question 2

Element P has atomic number 19. To which group and period, does P belong? Is it a metal or a non-metal? Why?

Answer

1A group and 4th period.

P is metal because its electronic configuration is 2,8,1 and and it belongs to the group of alkali metals.

Question 3

An element belongs to the 3rd period and Group IIIA (13) of the periodic table. State

(a) the number of valence electrons,

(b) the valency,

(c) if it is a metal or non-metal?

(d) the name of the element.

Answer

(a) 3, as the electronic configuration of the element is 2,8,3.

(b) +3, as the element will try to donate three valence electrons and attain a stable octet configuration.

(c) Metal, as the element has 3 valence electrons and tries to donate it to attain a stable configuration.

(d) Aluminium (Al), as it is in 3rd period and Group IIIA (13)

Question 4

An element A has 2 electrons in its fourth shell. State:

(a) its atomic number

(b) its electronic configuration

(c) its valency

(d) position in the periodic table

(e) is it a metal or a non-metal?

(f) is it an oxidising or reducing agent?

Answer

(a) Atomic number is 20.

(b) Its electronic configuration is 2, 8, 8, 2.

(c) +2, as the element tries to donate the two valence electrons, gain a stable octet configuration and become an electropositive ion.

(d) Group 2 Period 4

(e) Metal, as the element has 2 valence electrons and tries to donate it to attain a stable configuration.

(f) Reducing agent, as metals are strong reducing agents.

Question 5

(a) Name the first three alkaline earth metals.

(b) Write their reactions with dil. hydrochloric acid.

Answer

(a) Beryllium (Be), Magnesium (Mg) and Calcium (Ca).

(b) Be + 2HCl ⟶ BeCl₂ + H₂

Mg + 2HCl ⟶ MgCl₂ + H₂

Ca + 2HCl ⟶ CaCl₂ + H₂

Question 6

(a) Name the alkali metals. How many electrons(s) do they have in their outermost orbit.

(b) Take any one alkali metal and write its reaction with

(i) Oxygen

(ii) Water

(iii) Acid

Answer

(a) Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr) are alkali metals. They have 1 electron in their outermost orbit.

(b) (i) Alkali metal like sodium reacts rapidly with oxygen forming metal oxide

4Na + O₂ ⟶ 2Na₂O

(ii) Alkali metal like sodium reacts with water violently and produces its hydroxide and hydrogen

2Na + 2H₂O ⟶ 2NaOH + H₂

(iii) Alkali metal react violently with dil. HCl and dil. H₂SO₄ to produce hydrogen

2Na + 2HCl ⟶ 2NaCl + H₂

Question 7

(a) How do alkaline earth metals occur in nature?

(b) Write the electronic configuration of the first two alkaline earth metals.

Answer

(a) Alkaline earth metals are reactive metals, hence they do not occur in free state in nature. Their oxides occur in the earth's crust.

(b) Electronic configuration of beryllium (Be) is [2, 2] and magnesium (Mg) is [2, 8, 2]

Question 8

(a) How many electrons do inert gases have in their valence shells?

(b) Name an element of group 18 which can form compounds.

Answer

(a) They have 8 electrons in their outermost orbits (except He, which has only 2 electrons).

(b) Xenon

Question 9

Give reasons.

(a) Alkali metals are kept in inert solvent.

(b) Alkali metals and halogens do not occur free in nature.

(c) Alkali and alkaline earth metal compounds usually form electrovalent compounds.

(d) Inert gases do not form compounds.

Answer

(a) As alkali metals are very reactive metals, hence they are kept in inert solvents.

(b) As alkali metals and halogens are very reactive, hence, they do not occur free in nature.

(c) As alkali and alkaline earth metal have 1 and 2 valence electrons respectively hence, they tend to donate the valence electrons to attain a stable configuration and form electrovalent compounds.

(d) As the outermost orbits of inert gases are completely filled and they are in a stable state hence they do not form compounds.

Exercise 5(C) — Descriptive Type

Question 1

(a) State the nature of compounds formed when group 17 elements combine with (i) metals (ii) non-metals.

(b) Why group 17 elements are highly reactive?

Answer

(a) (i) Group 17 elements (halogens) can react directly with metals to form metal halides (or salts).

(ii) Group 17 elements (halogens) react with non-metals to form halides like chlorides, bromides and iodides.

(b) Group 17 elements are highly reactive as they need only one electron to complete its octet and attain a stable state.

Question 2

(a) What is the name given to group 17 elements? Why are they called so?

(b) Comment on the (i) reactivity (ii) colour and (iii) physical state of group 17 elements.

Answer

(a) Group 17 elements are called halogens. The name halogen (Greek, halo = sea or salt + gen = producing) means 'salt former'. Group 17 elements form salts, hence, they are called Halogens.

(b) (i) Halogens are the most reactive non-metals. Their reactivity decreases down the group. For example: fluorine is the most reactive and iodine is the least reactive.

(ii) The intensity of the colour of the element increases from pale to dark. Fluorine is a pale yellow gas, chlorine is a greenish-yellow gas, bromine is a reddish-brown liquid and iodine is a violet solid.

(iii) The physical state of the elements changes from gas to solid as we move down the group. Fluorine is a gas, chlorine is a gas, bromine is liquid and iodine is solid.

Question 3

State the uses of Modern Periodic table.

Answer

1. Periodic table has been useful in predicting the existence of new elements.
2. It has been useful in the past in correcting the position of elements in relation to their properties.
3. Study of elements and their compounds has become systematic and easier to remember.
4. Position of an element in the periodic table reveals its:
   1. atomic number
   2. electronic configuration
   3. number of valence electrons
   4. properties
5. Nature of chemical bond, formula of compound formed and properties of that compound can all be predicted from the periodic table.
6. Position of an element in the periodic table reveals:
   1. valency of the element
   2. whether the element is a metal or a non-metal — metals occupy the extreme left positions of the periodic table while non-metals are at the extreme right of the periodic table.

Question 4

Give the merits of Mendeleev's periodic table.

Answer

Merits of Mendeleev’s periodic table :

1. Grouping of elements — He generalized the study of the elements then known to a study of mere eight groups.
2. Gaps of undiscovered elements — Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
3. Prediction of properties of undiscovered elements — He predicted the properties of then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots.
4. Incorrect atomic mass corrected — He was able to correct the atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.

Question 5

State the defects of the table proposed by Mendeleev.

Answer

Defects of table proposed by Mendeleev:

1. Anomalous pairs — The following elements did not follow Mendeleev's principles:
   1. Argon with atomic mass 39.9 precedes potassium with atomic mass 39.1
   2. Cobalt with atomic mass 58.9 precedes nickel with atomic mass 58.6
   3. Tellurium with atomic mass 127.6 precedes iodine with atomic mass 126.9
2. Position of isotopes — According to Mendeleev's periodic table isotopes should be given separate place in the table since they have different atomic mass. But they were not assigned separate places.
3. Grouping of chemically dissimilar elements — Elements such as copper and silver bear no resemblance to alkali metals (lithium, sodium etc.) but they have been placed together in the first group.
4. Separation of chemically similar elements — Elements that are chemically similar, such as gold and platinum have been placed in separate groups.
5. Electron arrangement — It does not explain the electron arrangement of elements.
6. Position of hydrogen — Hydrogen was not given a fixed position. It was considered in group IA as well as in group VIIA because it forms forms both a positive ion, viz. in HCl and a negative ion, viz. in NaH.

Question 6

Give the main characteristics of representative elements.

Answer

1. They include both metals and non-metals. There is a regular gradation from metallic to non-metallic character as one moves from left to right across the period.
2. They form electrovalent as well as covalent compounds with non-metals.
3. Metallic nature increases on moving down any of these seven groups.
4. Metals, which are good conductors of heat and electricity, are present in groups 1 and 2. Non-metals, which are present in groups 16 and 17, are poor conductors of heat and electricity.
5. Some heavier elements, like tin and lead, exhibit variable valencies.

Question 7

Discuss the characteristics of halogens with respect to:

(a) reactivity

(b) intensity of colours

(c) formation of ions.

Answer

(a) Halogens are the most reactive non-metals. Their reactivity decreases down the group. For example: fluorine is the most reactive and iodine is the least reactive.

(b) The intensity of the colour of the element increases from pale to dark. Fluorine is a pale yellow gas, chlorine is a greenish-yellow gas, bromine is a reddish-brown liquid and iodine is a violet solid.

(c) They all form negative ions carrying a single charge [Fluoride ions F^-, Chloride ions Cl^-, Bromide ions Br^-, Iodide ions I^-.]

Question 8

Two elements 'P' and 'Q' belong to the same period of the modern periodic table and are in group 1 and group 2 respectively. Compare the following characteristics in tabular form.

(a) number of electrons in their valence shell.

(b) their tendency to lose electrons.

(c) their metallic characters.

(d) formation of their oxides.

(e) formulae of their chlorides.

Answer

These elements belong to alkali metals and alkaline earth metals, respectively. Below is the comparison of their characteristics:

Exercise 5(C) — Structured/Application/Skill Type

Question 1

(a) Name the scientist who gave this table.

(b) Which group of elements were not known at his time?

(c) This system worked quite well for the .............. (heavier/lighter) elements.

(d) The table shown above relates the properties of the elements to their .............. (atomic number / atomic mass)

(e) Why was this table discarded?

Answer

(a) John Newland

(b) 18th group of inert elements

(c) This system worked quite well for the lighter elements.

(d) The table shown above relates the properties of the elements to their atomic mass.

(e) Reasons for discarding the law of octaves:

1. This classification did not work for heavier elements i.e., those lying beyond calcium. As more and more of elements were discovered, they could not be fitted in this table.
2. Newland adjusted two elements cobalt (Co) and nickel (Ni) in the same slot and these were placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements.
3. Iron, which resembles cobalt and nickle in properties, has been placed far away from these elements.

Question 2

(a) Complete the table.

(b) Classify the elements given in the table as metals and non-metals.

(c) To which group does sodium belong? Why is Hydrogen not considered while describing the elements of this group?

Answer

(a) The completed table is given below:

(b)

(c) Sodium belongs to group 1.
Position of hydrogen is not satisfactory. Hydrogen atom is very small in size and therefore many of its properties are different from group 1 elements (alkali metals). It is a gas at room temperature, while other elements of group 1 are solid metals. Secondly, it does not readily form cations, in most compounds, whereas group 1 metals readily form cations and form only ionic bonds. Due to these reasons Hydrogen is not considered while describing the elements of group 1.