What is the significance of the bottom number on a chemical element?

The bottom number on a chemical element, known as the atomic number, is significant because it represents the number of protons in the nucleus of an atom of that element. This fundamental characteristic determines the identity of the element; for example, all atoms with an atomic number of 6 are carbon atoms. The atomic number also directly relates to the element's position on the periodic table and plays a crucial role in its chemical behavior, including how it bonds with other elements.

In contrast to the atomic number, the other options present different concepts: the mass number is typically represented as the top number and refers to the total number of protons and neutrons; ionization energy is a measure of how much energy is required to remove an electron from an atom; and the number of isotopes relates to the variations of an element that have the same number of protons but different numbers of neutrons, which is not indicated by the bottom number. Therefore, the bottom number's primary role is to define the atomic number, solidifying its significance in understanding the properties of elements in chemistry.