Relative Masses and Charges of Protons, Neutrons and Electrons

Atoms are composed of smaller particles known as protons, neutrons, and electrons.
Protons are positively charged particles that make up part of the nucleus of an atom. They have a relative mass of 1, and a relative charge of +1.
Neutrons, similar to protons, form part of the nucleus. They hold no charge and are hence neutral. Like protons, they have a relative mass of 1.
Electrons are tiny particles that orbit the nucleus in what are called energy levels or shells. They hold a negative charge (relative charge of -1). Despite carrying charge, their mass is considered negligible when considering the atomic mass and hence have a relative mass of about 1/2000.
It’s the number of protons in an atom that determines its atomic number, and thus its place on the Periodic Table.
A neutral atom has an equal number of protons and electrons, balancing out the overall charge to be zero.
The atomic mass number of an atom (often displayed as a superscript before the element symbol) is the sum of the number of protons and neutrons in its nucleus.
A specific variation of an atom with a different number of neutrons is called an isotope.
The relative atomic mass (Ar) often found on the Periodic Table is an average mass taking into account the mass numbers of all the different isotopes of that element.