$A$ sodium cation $(Na^{+})$ has a different number of electrons from:

An atom having atomic mass number $13$ has $7$ neutrons. What is the atomic number of the atom?

Number of neutrons in ${}^{12}C$ is

$_{20}X^{40}$ and $_{21}X^{40}$ are

In the two elements,${ }_{Z_1} A^{M_1}$ and ${ }_{Z_2} B^{M_2}$,the following relations are true. $M_1 \neq M_2$ and $Z_1 \neq Z_2$ but $M_1-Z_1=M_2-Z_2$. These elements are ($M$ is atomic weight,$Z$ is atomic number)

Give the definition and examples of isobars and isotopes.