Atomic Structure

Atomic Structure

Question 1

State the meaning of the term –

(a) atom

(b) molecule

(c) radical – with a suitable example.

Answer

(a) Atom — An atom is the smallest particle of an element that exhibits all the properties of that element. It may or may not exist independently but takes part in every chemical reactions.

Example : O — Oxygen atom, H — Hydrogen atom

(b) Molecule — A molecule is the smallest particle of an element or compound, made up of two or more atoms of an element or compound which has independent existence exhibiting all of their properties.

Example : O₂ — Oxygen molecule, H₂O — Water molecule

(c) Radical — A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.

Example : NH₄⁺ — Ammonium radical

Question 2

Draw the basic structure of an atom representing its –

(a) Subatomic particles

(b) Nucleus

(c) Orbits or shells.

Answer

Below labelled diagram shows the basic structure of an atom:

Question 3

In the Modern Periodic Table – atoms of all elements are arranged according to the their increasing order of – atomic numbers. An atom of nitrogen has seven protons and seven electrons. State what would be its atomic number.

Answer

Atomic number [z] = no. of protons [p] = no. of electrons [e] = 7

Therefore, Atomic number of nitrogen is 7.

Question 4

State the main points of contradiction of Dalton’s atomic theory by the Modern Atomic Theory.

Answer

Contradiction of Dalton’s atomic theory by the Modern Atomic Theory:

Question 5

Atoms of the same element or different elements combine to form a molecule. Give two examples each of –

(a) atoms of the same element

(b) atoms of different elements combining to form a molecule.

Answer

(a) O₂ — Oxygen molecule and N₂ — Nitrogen molecule

(b) H₂O — Water molecule and NaCl — Sodium chloride molecule

Question 6

Explain the term – 'atomicity'. Give a reason why –

(a) Helium is considered monoatomic, but hydrogen a diatomic molecule.

(b) Ozone is considered triatomic, but phosphorus a tetratomic molecule.

Answer

Atomicity — It is the number of atoms present in a molecule of an element.

(a) Helium (He) is considered monoatomic because it contains only one atom of helium while hydrogen contains two atom of hydrogen (H₂) therefore it is considered a diatomic molecule.

(b) Ozone is considered triatomic because it is composed of three atoms of Oxygen (O₃) while phosphorous is composed of four atoms (P₄), therefore, it is considered a tetratomic molecule.

Question 7

State the atomicity of the following:

(a) Neon

(b) Oxygen

(c) Chlorine

(d) Argon

(e) Sulphur

Answer

(a) Monoatomic

(b) Diatomic

(c) Diatomic

(d) Monoatomic

(e) Octatomic or Polyatomic

Question 8

'A radical takes part — unsplit in a chemical reaction and retains its identity in reactions'. With reference to the radical:

(a) ammonium

(b) bisulphate

(c) carbonate — state the group of atoms present in each radical.

Answer

(a) Ammonium [NH₄]: 1 nitrogen atom and 4 hydrogen atoms

(b) Bisulphate [HSO₄]: 1 hydrogen atom, 1 sulphur atom and four oxygen atoms

(c) Carbonate [CO₃] : 1 carbon atom and 3 oxygen atoms

Question 9

Differentiate between the following with suitable examples — A basic radical and a acid radical.

Answer

Question 10

Name the cation and the anion in the salt – calcium sulphate.

Answer

In calcium sulphate [CaSO₄], the cation is [Ca²⁺] and anion is [SO₄^2-]

Question 11

Explain the term – 'valency'. State giving reasons the valency of –

(a) chlorine in hydrogen chloride

(b) sulphate in sulphuric acid

(c) nitrogen in ammonia

(d) magnesium in magnesium oxide

Answer

Valency of an element or radical is the number of hydrogen atoms which can combine with or displace one atom of the element or radical forming a compound.

(a) One atom of hydrogen combines with one atom of chlorine to form HCl, hence the valency of chlorine in hydrogen chloride is 1^-.

(b) Two atoms of hydrogen combine with one radical of sulphate to form sulphuric acid (H₂SO₄), hence the valency of sulphate in sulphuric acid is 2^-.

(c) Three atoms of hydrogen combine with one atom of nitrogen to form ammonia (NH₃) hence the valency of nitrogen in ammonia is 3^-.

(d) We know that valency of oxygen is 2^-. Hence, in magnesium oxide [MgO], valency of magnesium is 2⁺.

Question 12

Give the symbol and valency of the following elements and radicals.

(a) Potassium

(b) Sodium

(c) Hydrogen

(d) Calcium

(e) Aluminum

(f) Magnesium

(g) Zinc

(h) Chlorine [chloride]

(i) Sulphur [sulphide]

(j) Nitrogen [nitride]

(k) Oxygen [oxide]

(l) Ammonium

(m) Bicarbonate

(n) Nitrate

(o) Bisulphate

(p) Bisulphite

(q) Hydroxide

(r) Carbonate

(s) Sulphate

(t) Sulphite

Answer

Question 13

Give an example of three elements exhibiting variable valency.

Answer

Three elements exhibiting variable valency are:

1. Iron (Fe) can show a valency of 2 and 3.
2. Copper (Cu) can show a valency of 1 and 2.
3. Lead (Pb) can show a valency of 2 and 4.

Question 14

State what is meant by the term – chemical formula. Give the chemical formula of –

(a) sodium hydroxide

(b) sodium nitrate

(c) sodium bicarbonate

(d) sodium sulphite

(e) magnesium nitrate

(f) ammonium sulphate

(g) carbonic acid

(h) calcium phosphate

Answer

Chemical formula is the representation of a substance by means of symbols. It denotes in a compound, the number of atoms of each element present.

(a) Sodium hydroxide

$\text{Na}^{1+} \phantom{\nearrow} \text{OH}^{1-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{OH}} \Rightarrow \underset{\phantom{}1{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{OH}} \\[0.5em]$

So, we get the formula as $\bold{Na}\bold{OH}$

(b) Sodium nitrate

$\text{Na}^{1+} \phantom{\nearrow} \text{NO}_3^{1-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{NO}_3} \Rightarrow \underset{\phantom{}1{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{NO}_3} \\[0.5em]$

So, we get the formula as $\bold{Na}\bold{NO}_3$

(c) Sodium bicarbonate

$\text{Na}^{1+} \phantom{\nearrow} \text{HCO}_3^{1-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{HCO}_3} \Rightarrow \underset{\phantom{}1{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{HCO}_3} \\[0.5em]$

So, we get the formula as $\bold{Na}\bold{HCO}_3$

(d) Sodium sulphite

$\text{Na}^{1+} \phantom{\nearrow} \text{SO}_3^{2-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{SO}_3} \Rightarrow \underset{\phantom{}2{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{SO}_3} \\[0.5em]$

So, we get the formula as $\bold{Na}_2\bold{SO}_3$

(e) Magnesium nitrate

$\text{Mg}^{2+} \phantom{\nearrow} \text{NO}_3^{1-} \\[0.5em] \overset{\phantom{1}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{NO}_3} \Rightarrow \underset{\phantom{2}{1}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{NO}_3} \\[0.5em]$

So, we get the formula as $\bold{Mg}\bold{(NO}_3)_2$

(f) Ammonium sulphate

$\text{NH}_4^{1+} \phantom{\nearrow} \text{SO}_4^{2-} \\[0.5em] \overset{\phantom{1}{1}}{\text{NH}_4} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{\phantom{2}{2}}{\text{NH}_4} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{SO}_4} \\[0.5em]$

So, we get the formula as $\bold{(NH}_4)_2\bold{SO}_4$

(g) Carbonic acid

$\text{H}^{1+} \phantom{\nearrow} \text{CO}_3^{2-} \\[0.5em] \overset{\phantom{1}{1}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{CO}_3} \Rightarrow \underset{\phantom{2}{2}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{CO}_3} \\[0.5em]$ So, we get the formula as $\bold{H}_2\bold{CO}_3$

(h) Calcium phosphate

$\text{Ca}^{2+} \phantom{\nearrow} \text{PO}_4^{3-} \\[0.5em] \overset{\phantom{1}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \overset{3}{\text{PO}_4} \Rightarrow \underset{\phantom{2}{3}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{PO}_4} \\[0.5em]$

So, we get the formula as $\bold{Ca}_\bold{3}\bold{(PO_4)_2}$

Question 15

'Dilute hydrochloric acid is added to sodium hydroxide'. Give a word equation and a molecular equation of the same. Is the equation written, a balanced equation. Give reasons.

Answer

Word equation:

Sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water

Molecular equation:

NaOH + HCl ⟶ NaCl + H₂O

In the above equation,

As, the number of atoms of each element of reactants = the number of atoms of each element of products. Hence, the equation is balanced.

Question 16

State what the group number in the Modern Periodic Table signifies. Give a reason why elements in the same group, have similar valency and similar chemical properties.

Answer

Group number signifies the number of electrons in the outer shell of an atom of the element.

As the number of electrons in the outer shell of all elements in a group is the same. Hence all elements in the same group of the periodic table have same valency and similar chemical properties.

Objective Type Questions

Question 1

Fill in the blanks with the correct words from the brackets.

1. A molecule of an element is ............... [water/chlorine/ammonia].

2. A radical with one atom of sulphur and three atoms of oxygen is ............... [bisulphite/sulphate/sulphite].

3. An element with variable valency is ............... [sodium/copper/calcium]

4. The formula of aluminum sulphate is ............... [AlSO₄ / Al₂(SO₄)₃ / Al₃(SO₄)₂]

5. The element lithium in the Modern Periodic Table with valency 1⁺ belongs to the group ............... [2/1]

Answer

1. A molecule of an element is chlorine.

2. A radical with one atom of sulphur and three atoms of oxygen is sulphite.

3. An element with variable valency is copper.

4. The formula of aluminum sulphate is Al₂(SO₄)₃.

5. The element lithium in the Modern Periodic Table with valency 1⁺ belongs to the group 1.

Question 2

Select the correct answer from A, B, C, D and E for each statement given below-

A: Molecule B: Orbits C: Atom D: Valency E: Radical

1. The basic unit of matter not capable of independent existence.

2. The smallest particle of a substance – capable of independent existence and which retains the characteristics of the substance.

3. An elementary particle of matter – of a group of atoms of different elements behaving as a single unit with charge on the group.

4. Imaginary path around the nucleus of an atom in which electrons revolve.

5. The term for the combining capacity of an element or the number of hydrogen atoms with which it combines or replaces.

Answer

1. C: Atom

2. A: Molecule

3. E: Radical

4. B: Orbits

5. D: Valency

Question 3

Match the radicals in List I with their correct names from List II.

Answer

Question 4

State the correct formula of the following from the list given:

1. Calcium Hydroxide - Ca₂OH/ Ca(OH)₂ /CaOH

2. Sodium Bisulphite - NaHSO₄/Na₂HSO₃/ NaHSO₃

3. Ammonium Sulphate - (NH₄)₂SO₄/NH₄SO₃/ (NH₄)₂SO₃

4. Aluminium Phosphate - Al₂(PO₄)₃ / AlPO₄/ Al₂(PO₄)₂

5. Copper [II] nitrate - Cu(NO₃)₂/Cu₂NO₃/CuNO₃

Answer

1. Ca(OH)₂

2. NaHSO₃

3. (NH₄)₂SO₄

4. AlPO₄

5. Cu(NO₃)₂

Question 5

Name the following

1. A molecule of a compound containing two oxygen atoms and one nitrogen atom.

2. The radical containing one nitrogen and four hydrogen atoms.

3. The compound formed when iron [Fe²⁺] combines with oxygen [O^2-].

4. The group of the Modern Periodic Table to which calcium [Ca²⁺] belongs.

5. A non-metal which forms a polyatomic molecule with more than three atoms.

Answer

1. Nitrogen dioxide [NO₂]

2. Ammonium [NH₄]

3. Iron (II) oxide [FeO]

4. Group 2

5. Phosphorous (P₄)