The relative atomic mass of an element, such as boron, is not always a whole number due to the presence of different isotopes of that element. Isotopes are variants of an element that have the same number of protons but different numbers of neutrons in their nuclei. Since the atomic mass of an element is a weighted average of the masses of its isotopes, taking into account their relative abundances, the presence of multiple isotopes can lead to a non-integer atomic mass.
In the case of boron, it has two stable isotopes: boron-10 and boron-11. Boron-10 has 5 protons and 5 neutrons, while boron-11 has 5 protons and 6 neutrons. The relative abundance of these isotopes in nature is approximately 19.9% for boron-10 and 80.1% for boron-11.
To calculate the relative atomic mass of boron, we take into account the mass of each isotope and its relative abundance. The formula for calculating the relative atomic mass is:
Relative atomic mass = (mass of isotope 1 × relative abundance of isotope 1) + (mass of isotope 2 × relative abundance of isotope 2) + ...
Using the masses and abundances of boron-10 and boron-11, we get:
Relative atomic mass of boron = (10.012 u × 0.199) + (11.009 u × 0.801) ≈ 10.81 u
The resulting relative atomic mass of boron is not a whole number because it is an average of the masses of its isotopes weighted by their abundances.