What is the atomic number of an element?

The atomic number of an element is defined as the number of protons found in the nucleus of its atoms. This fundamental property uniquely identifies each element on the periodic table. For instance, hydrogen has an atomic number of 1 because it contains one proton, while carbon has an atomic number of 6 because it contains six protons.

The atomic number plays a crucial role in determining the chemical behavior and identity of the element, as it directly influences the number of electrons surrounding the nucleus in a neutral atom. The arrangement of these electrons determines how the element interacts with other elements, which is essential for understanding chemical reactions and bonding.

In contrast, the other options describe different concepts related to atomic structure but do not accurately define atomic number. For example, the sum of neutrons and protons refers to the atomic mass, while the total number of electrons pertains to the charge of the atom, and the number of isotopes relates to variations of atoms with the same number of protons but different numbers of neutrons. Each of these distinctions is important in their own right but does not capture what constitutes the atomic number.