Ordinary chlorine gas has two isotopes : $^{35}${17}\text{Cl} and $^{37}{17}\text{Cl}$ in the ratio of 3 : 1. Calculate the relative atomic mass [atomic weight] of chlorine.

The atom of aluminium is represented by ²⁷Al₁₃. Write down the number of :

(a) electrons

(b) protons

(c) neutrons

(d) the arrangement of electrons

in the different orbits or shells in one atom of aluminium.

Elements A, B and C have atomic numbers 9, 20 and 10 respectively.

(i) State which one is (1) a non-metal; (2) a metal; (3) chemically inert.

(ii) Write down the formula of the compound formed by two of the above elements.

Define : Proton, Electron, Neutron.