The atomic numbers of calcium and argon are $20$ and $18$ respectively,but the mass number of both these elements is $40$. What is the name given to such a pair of elements?

$(a)$ An ion $M^{2+}$ contains $10$ electrons and $12$ neutrons. What is the atomic number and mass number of the element $M$?
$(b)$ Is it possible in an atom to have $12$ protons and $13$ electrons? Explain.
$(c)$ Why is helium gas inert?

Write down the electron distribution of chlorine atom. How many electrons are there in the $L$ shell? (Atomic number of chlorine is $17$).

What information do you get from the figure about the atomic number,mass number,and valency of atoms $X$,$Y$,and $Z$? Give your answer in a tabular form.

An ion $X^{2-}$ contains $10$ electrons and $8$ neutrons. What are the atomic number and mass number of element $X$?

$(a)$ Write the names of three isotopes of hydrogen.
$(b)$ State one use of an isotope of uranium.
$(c)$ Isotopes of an element have similar chemical properties. Why?