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

The key idea is Atomic Notation: the mass number (A) is written as a superscript, and the atomic number (Z) as a subscript, both to the left of the element symbol. The format is $^{A}_{Z}\text{X}$.

1. In $^{79}_{35}\text{Br}$, the superscript 79 is the mass number (protons + neutrons) and the subscript 35 is the atomic number (protons). This is the standard, acceptable notation.
2. The symbol $^{79}\text{Br}$ is acceptable because the atomic number is implied by the element symbol (Br always has Z = 35), so the subscript can be omitted.
3. In $^{35}_{79}\text{Br}$, the superscript 35 would incorrectly represent the mass number (too small for Br, which has ~45 neutrons), and the subscript 79 would wrongly suggest Z = 79 (gold, not bromine). This violates the fixed meaning of superscript and subscript positions. …

1. The core convention: what each symbol means Every atom is identified by its atomic number $Z$ (number of protons), which determines the element. The mass number $A$ (protons + neutrons) distinguishes isotopes of that element. The standard notation is:

Here, $X$ is the chemical symbol, $A$ is the superscript on the left, and $Z$ is the subscript on the left. For bromine-79, $Z = 35$ and $A = 79$, so the correct form is $^{79}_{35}\text{Br}$.

2. Why $^{79}\text{Br}$ is acceptable

   In many contexts, the atomic number $Z$ is omitted because the element symbol itself already tells you $Z$ (bromine always has $Z = 35$). So $^{79}\text{Br}$ is a shorthand that still clearly means “bromine-79”. It is widely used in textbooks and exam papers.

3. Why $^{35}_{79}\text{Br}$ is not acceptable

   This swaps the positions: $A$ (79) is written as a subscript and $Z$ (35) as a superscript. The convention is fixed — superscript always means mass number, subscript always means atomic number. Writing $^{35}_{79}\text{Br}$ would be read as “element with $Z = 79$ and $A = 35$”, which is nonsense (gold-35 does not exist). It is a direct violation of the standard.

4. Why $^{35}\text{Br}$ is not acceptable …