An atom is the smallest particle that can be described as a chemical.

  • At the centre of an atom is a nucleus containing protons which are positively charged sub-atomic particles. The number of protons determines what chemical the atom is. A hydrogen atom has a single proton.
  • The nucleus also contains a number of neutrons, sub-atomic particles with no charge.
  • Surrounding the nucleus are very small negatively charged particles called electrons, held in place by their attraction to the positively charged nucleus.
  • Quantum mechanics is used to describe the orbital motion of electrons around a nucleus.
  • In an atom, the number of electrons always matches the number of protons.
  • If an electron is removed from an atom the result is a charged particle called an ion.
  • For a single atom, the number of protons and neutrons combined determines its atomic mass.
  • Atomic mass is a measure of the total mass of protons and neutrons in an atom and is used to order elements in the periodic table.