What is defined as the number of protons in an atom, helping to identify the element?

The atomic number is defined as the number of protons in the nucleus of an atom, and it plays a crucial role in identifying the element itself. Each element on the periodic table is uniquely represented by its atomic number, which determines its position on the table and its chemical properties. For example, hydrogen has an atomic number of 1 because it has one proton, while oxygen has an atomic number of 8 due to its eight protons.

Understanding the atomic number is fundamental in chemistry because it not only distinguishes one element from another but also influences how elements behave in chemical reactions. The atomic number is essential for identifying isotopes and understanding electron configuration as well, since the number of electrons typically matches the number of protons in a neutral atom.

In contrast, mass number refers to the total number of protons and neutrons in an atom’s nucleus, atomic weight is a weighted average of the masses of an element's isotopes, and isotopic notation provides specific information about the number of protons and neutrons in a particular isotope, but does not define the element itself.