Question

Why is the atomic weight of bromine listed on the periodic table not a whole number?

Answer 1

Hint: Bromine is an atomic number 35 chemical element with the symbol Br. It's the third-lightest halogen, and at room temperature, it's a seething red-brown liquid that quickly evaporates to create a similar-colored vapour. It has characteristics that are halfway between chlorine and iodine. Bromine is a highly reactive element that only occurs in nature as colourless soluble crystalline mineral halide salts, similar to table salt.

Complete answer:
The mass of an atom is its atomic mass. Although the kilogramme (symbol: kg) is the SI unit of mass, atomic mass is frequently represented in the non-SI unit dalton, which is defined as ${\displaystyle \frac{1}{12}}$ times the mass of a single carbon-12 atom at rest. The nucleus' protons and neutrons account for virtually all of an atom's total mass, with electrons and nuclear binding energy playing a modest role. As a result, when stated in daltons, the numeric value of the atomic mass is approximately equal to the mass number.
The stable isotopes of bromine are ${}^{79}Br$ and ${}^{81}Br$ . The only two natural isotopes are ${}^{79}Br$ and ${}^{81}Br$ , with ${}^{79}Br$ accounting for 51 percent of natural bromine and 81Br for the remaining 49 percent. Both have a nuclear spin of $-{\displaystyle \frac{3}{2}}$ and can be used for nuclear magnetic resonance, however ${}^{81}Br$ is better. The almost equal distribution of the two isotopes in nature aids mass spectroscopy identification of bromine-containing molecules. The other bromine isotopes are all radioactive and have half-lives that are too short to exist naturally.
The atomic mass stated on the Periodic Table is the WEIGHTED AVERAGE of the individual isotopes, and all elements have an envelope of isotopes.
Bromine, to a first approximation, has TWO isotopes, therefore the weighted average of the isotopes yields the stated atomic mass.
Atomic weight of Br is 79.9 amu.

Note:
The relative isotopic mass and the atomic mass of an isotope relate to a specific isotope of an element. Because most substances are not isotopically pure, the elemental atomic mass, which is the average (mean) atomic mass of an element, weighted by the quantity of the isotopes, is a handy measurement. The weighted mean relative isotopic mass of a (typical naturally-occurring) combination of isotopes is the dimensionless (standard) atomic weight.