Which property is different for neutral atoms of the two isotopes of the same element?

An element $M$ has an atomic mass $19$ and atomic number $9$. Its ion is represented by

Which of the following is always a whole number?

Which of the following species is isotonic with $_{37}Rb^{86}$?

Consider an imaginary ion ${}_{22}^{48}X^{3-}$. The nucleus contains '$a$'$\%$ more neutrons than the number of electrons in the ion. The value of '$a$' is $.....$ [nearest integer].

Which of the following has the smallest size?