Question

Question 2.41 Symbols Br(79-35) and Br(79) can be written, whereas symbols Br(35-79) and Br(35) are not acceptable.

Class XI Structure of Atom Page 67

Answer 1

79 35Br and 79Br are acceptable way of representation whereas 35 79Br and 35Br are not acceptable because there is a rule in IUPAC nomenclature to write the higher number(mass number) on top and the lower number (atomic number ) at the bottom. This type of expressing the elements is very useful in writing radioactive disintegration reactions where neutrons are emitted.

Answer 2

see above attachment , if X is any elements then subscript shows atomic number ( Z) of element X and superscript shows the atomic mass ( A ) of that element.

hence,
³⁵₇₉Br and ³⁵Br are not acceptable.
we have to write it in such a way that atomic mass number on superscript position and atomic number on subscript position .
hence, ⁷⁹₃₅Br and ³⁵Br are acceptable.