Question

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

Answer 1

To answer this question you must recall the rules for the IUPAC nomenclature of the symbol of an element given by the International Union of Pure and Applied Chemistry. The rules are given in such a way so as to make sure that each element has a unique symbol. This type of expression of the symbol of elements is very useful when writing the radioactive reactions which involve the disintegration of the element and emission of neutrons.

Complete answer:
In the denotation of the type $_Z^AX$ of an element, A represents the atomic mass of the element and Z represents the atomic number of the element which is represented by the symbol X.
The mass numbers or atomic masses, however, can be the same for two elements or different for the same element depending upon the number of neutrons present in the nucleus of the atom. Thus, it is important to specify the atomic mass in order to identify the element.
As a result, the symbols $_{35}^{79}Br$ and $^{79}Br$ are accepted as we can figure out the atomic number of bromine even if it is not specified. The symbol $_{79}^{35}Br$ is not acceptable as the atomic number of an element cannot be less than the atomic mass and the symbol $^{35}Br$ cannot be used as it is important to specify the atomic mass.

Note:
We know that the atomic number of each element is unique to itself. Two different elements cannot have the same number of protons and thus the same atomic number. Hence, the atomic number of an element can be determined using the symbol of the element.