Atomic Structure

• All substances are made of tiny particles of matter called atoms which are the building blocks of all matter

• Each atom is made of subatomic particles called protons, neutrons, and electrons

• The protons and neutrons are located at the centre of the atom, which is called the nucleus

• The electrons move very fast around the nucleus in orbital paths called shells

• The mass of the electron is negligible, hence the mass of an atom is contained within the nucleus where the protons and neutrons are located

Proton Number

• The atomic number (or proton number) is the number of protons in the nucleus of an atom

• The symbol for atomic number is Z

• It is also the number of electrons present in a neutral atom and determines the position of the element on the Periodic Table

Mass Number

• The Nucleon number (or mass number) is the total number of protons and neutrons in the nucleus of an atom

• The symbol for nucleon number is A

• The nucleon number minus the proton number gives you the number of neutrons of an atom

• Note that protons and neutrons can collectively be called nucleons.

• The atomic number and mass number of an element can be shown using atomic notation

• The Periodic Table shows the elements together with their atomic (proton) number at the top and relative atomic mass at the bottom - there is a difference between relative atomic mass and mass number, but for your exam, you can use the relative atomic mass as the mass number (with the exception of chlorine)