Specification Point 1.14:
Know what is meant by the terms atom and molecule


Term Definitition
Atom The smallest particle of an element that consists of electrons surrounding a nucleus that contains protons and neutrons.
Molecule The smallest particle of an element or compound that can exist independently.  Molecules are formed when atoms join together by chemical bonds.
Specification Point 1.15:
Know the structure of an atom in terms of the positions, relative masses and relative charges of sub-atomic particles

The Structure of an Atom

Atoms are made up of subatomic particles called protons, neutrons and electrons.

The nucleus is composed of protons and neutrons and the electrons orbit around the nucleus in different shells.

A model diagram for an atom is shown below:

Atom structure, Edexcel IGCSE Chemistry
Particle Relative charge Relative mass
Proton +1 1
Neutron 0 1
Electron -1 1/1836
Specification Point 1.16:
Know what is meant by the terms Atomic number, mass number, isotopes and relative atomic mass (Ar)


Term Definition
Atomic number The number of protons in the nucleus of an atom.
Mass number The sum of the number of protons and neutrons in the nucleus of an atom.
Isotopes Atoms of the same element that contains the same number of protons and electrons but a different number of neutrons. Therefore, they have the same atomic number but a different mass number.
Relative atomic mass (Ar) Weighted average mass of one atom of an element, taking into account the abundances of all the isotopes of that element. It is measured as a ratio to 1/12 of the mass of an atom of carbon-12.

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.