Atomic Structure

Atoms are made up of three different particles - protons, neutrons and electrons. They are called sub-atomic particles.
Protons and neutrons are tightly packed together in the centre of an atom, forming the nucleus of the atom. Protons and neutrons are known as nucleons.
An atom contains an equal number of positively charged protons and negatively charged electrons.
Proton number
- Also known as atomic number
- Represented by the symbol Z
- Different atoms have different numbers of protons

Nucleon number
- Total number of protons and neutrons in an atom
- Represented by the letter A
- Also known as mass number
Nucleon number (A) = number of protons + number of neutrons

Isotopes
- Atoms of the same element with the same number of protons but different number of neutrons
- Same chemical properties but slightly different physical properties
- Emit high energy radiation called radioisotopes

Electrons
- Move around the nucleus in regions known as electron shells
- The way the electrons are arranged in an atom is also known as the atoms' electron structure or electronic configuration

Valence Electrons
- The shell that is furthest from the nucleus
- Also known as outer shell
- Chemical properties of an element depend on the number of valence electrons