How does the atomic number differ from the mass number?

The atomic number represents the number of protons in an atom, while the mass number is the total of protons and neutrons.

The atomic number, also known as the proton number, is a fundamental property of an atom. It defines the chemical element to which an atom belongs. For example, all atoms with an atomic number of 1 are hydrogen atoms, while all atoms with an atomic number of 2 are helium atoms. The atomic number is always a whole number and it is unique to each element. It is denoted by the symbol 'Z'.

On the other hand, the mass number, denoted by the symbol 'A', is the total number of protons and neutrons in an atom. Neutrons and protons are collectively known as nucleons, and they reside in the nucleus of an atom. The mass number gives an estimate of the atomic mass, which is measured in atomic mass units.

It's important to note that while the atomic number is fixed for a given element, the mass number can vary. This is because an element can have atoms with different numbers of neutrons. These different forms of an element are called isotopes. For example, carbon-12 and carbon-14 are isotopes of carbon. They both have an atomic number of 6 (meaning they have 6 protons), but carbon-12 has a mass number of 12 (6 protons + 6 neutrons), while carbon-14 has a mass number of 14 (6 protons + 8 neutrons).

In summary, the atomic number and mass number are fundamental properties of atoms that help us identify and characterise different elements and their isotopes. The atomic number tells us the number of protons, and thus the identity of the element, while the mass number gives us the total number of protons and neutrons, providing an estimate of the atom's mass.