Question

Symbols $_{79}^{35}\textrm{Br}$ and $_{79}^{}\textrm{Br}$ can be written, whereas symbols $_{35}^{79}\textrm{Br}$ and $_{35}^{}\textrm{Br}$ are not acceptable. Answer briefly.

Answer 1

The general convention of representing an element along with its atomic mass (A) and atomic number (Z) is $_{Z}^{A}\textrm{X}$. Hence, $_{35}^{79}\textrm{Br}$ is acceptable $_{35}^{79}\textrm{Br}$ is not acceptable.
$_{79}^{}\textrm{Br}$ can be written but $_{35}^{}\textrm{Br}$cannot be written because the atomic number of an element is constant, but the atomic mass of an element depends upon the relative abundance of its isotopes. Hence, it is necessary to mention the atomic mass of an element.