Atoms: Definitions & Structure (Edexcel IGCSE Chemistry)

• It is important to understand the terms atom and molecule when referring to atomic structure, elements and compounds

Atoms & Molecules Definitions Table

• Elements are made of tiny particles of matter called atoms
• Each atom is made of subatomic particles called protons, neutrons and electrons
• Their size is so tiny that we can't really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass is used
• One relative atomic mass unit is equal to one twelfth the mass of a carbon-12 atom.
• All other elements are measured relative to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units
• Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon
• The relative mass and charge of the subatomic particles are shown below:

The Mass & Charge of Subatomic Particles Table

• You need to know the following terms to describe the properties and characteristics of atoms

Atomic Structure Key Terms Table

Deducing protons, neutrons & electrons

Finding the protons

• The atomic number of an atom and ion determines which element it is
• Therefore, all atoms and ions of the same element have the same number of protons (atomic number) in the nucleus
  • E.g. lithium has an atomic number of 3 (three protons) whereas beryllium has atomic number of 4 (4 protons)

• The number of protons equals the atomic (proton) number
• The number of protons of an unknown element can be calculated by using its mass number and number of neutrons:

Mass number = number of protons + number of neutrons

Number of protons = mass number – number of neutrons

Finding the electrons

• An atom is neutral and therefore has the same number of protons and electrons

Finding the neutrons

• The mass and atomic numbers can be used to find the number of neutrons in ions and atoms:

Number of neutrons = mass number – number of protons