Isotopes of an element have

The average atomic mass of a sample of an element $X$ is $16.2 \, u$. What are the percentages of isotopes $_8^{16}X$ and $_8^{18}X$ in the sample (in $\%$)?

Define valency by taking examples of silicon and oxygen.

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.

Write the electronic configuration of any one pair of isotopes and isobars.

Draw a sketch of Bohr's model of an atom with three shells.