The nucleus of a hydrogen atom is known as:

Which of the following statements are true for an element?
$(i)$ Atomic number $=$ number of protons $+$ number of electrons
$(ii)$ Mass number $=$ number of protons $+$ number of neutrons
$(iii)$ Atomic mass $=$ number of protons $=$ number of neutrons
$(iv)$ Atomic number $=$ number of protons $=$ number of electrons

Name the particles which determine the mass of an atom.

$(a)$ Define the following terms with one example each:
$(i)$ Isotope
$(ii)$ Isobar
$(b)$ Name the elements whose isotopes are used in:
$(i)$ Nuclear Reactor
$(ii)$ Treatment of cancer

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 $\%$)?

Write five applications of isotopes in various fields.