Find out the mass number of oxygen and sulphur atoms.

Describe Bohr's model of the atom.

Find the valency of chlorine,sulphur,and magnesium.

Helium atom has an atomic mass of $4 \, u$ and two protons in its nucleus. How many neutrons does it have?

For the following statements,write $T$ for True and $F$ for False.
$(a)$ $J$.$J$. Thomson proposed that the nucleus of an atom contains only nucleons.
$(b)$ $A$ neutron is formed by an electron and a proton combining together. Therefore,it is neutral.
$(c)$ The mass of an electron is about $\frac{1}{2000}$ times that of a proton.
$(d)$ An isotope of iodine is used for making tincture iodine,which is used as a medicine.

What are the limitations of Rutherford's model of the atom?