2.41 Symbols and 79Br can be written, whereas symbols and 35Br are not acceptable. Answer briefly.

The general convention of representing an element along with its atomic mass (A) and atomic number (Z) is XZA

Hence, Br3579 is acceptable but is Br7935 not acceptable.
Br79 can be written but Br35 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