Atomic Number and Mass Number

How do we identify different elements, and what makes them unique? It depends on their atoms and their properties.

Each element has a symbol for easy identification, like H for Hydrogen and He for Helium.

But what makes an atom different? The key lies in the number of protons in its nucleus, called the atomic number. For instance, helium, with two protons, has an atomic number of 2. Since atoms are neutral, the atomic number also reflects the number of electrons.

Atoms also contain neutrons in the nucleus. Adding protons and neutrons gives the mass number, which represents the atom’s total mass. Electrons have negligible mass and contribute little.

The mass of protons and neutrons is measured in atomic mass units, amu, where 1 amu is approximately 1.673 × 10^-27 kilograms. For instance, helium has two protons and two neutrons, giving it a mass number of 4 amu.

All elements are represented using their symbol, with the mass number written on top and the atomic number below.