For the symbols $H$,$D$,and $T$,tabulate the number of three sub-atomic particles found in each of them.

What are isobars? Give one example.

$(a)$ 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?

In response to a question,a student stated that in an atom,the number of protons is greater than the number of neutrons,which in turn is greater than the number of electrons. Do you agree with the statement? Justify your answer.

Two elements are denoted as follows:
$_{20}^{40}A$ and $_{18}^{40}B$
$(a)$ Find the number of electrons present in $A$ and $B$.
$(b)$ Find the number of nucleons present in $A$ and $B$.
$(c)$ Explain the special 'term' used to represent $A$ and $B$ and define the same.

An ion $X^{2+}$ contains $18$ electrons and $20$ neutrons. Calculate the atomic number and mass number of element $X$. Name the element $X$.