Atoms: Definitions & Structure (Edexcel IGCSE Chemistry: Double Science)

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

Term	Definition
Atom	The smallest particle of an element that contains electrons surrounding a nucleus that contains protons and neutrons
Molecule	A group of two or more atoms chemically combined to form an identifiable unit which retains the properties and composition of the substance

• 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 nucleus is positively charged 
• 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

Atomic structure

Protons and neutrons are in the nucleus, and electrons in shells orbiting the nucleus

• Subatomic particles are so small that it is not practical to measure their masses and charges using conventional units (such as grams or coulombs)
• Instead, their masses and charges are compared to each other, and so are called ‘relative atomic masses’ and ‘relative atomic charges’
  • Protons and neutrons have a very similar mass, so each is assigned a relative mass of 1
  • Electrons are 2000 times smaller than a proton and neutron, and so their mass is often described as being negligible
  • These are not actual charges and masses, but rather charges and masses of particles relative to each other

Table of mass and charge of subatomic particles

Particle	Relative Mass	Charge
Proton	1	+1
Neutron	1	0 (neutral)
Electron	1/1840	-1

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

Term	Definition
Atomic number	The number of protons in the nucleus of an atom
Mass number	The number of protons and neutrons in the nucleus of an atom
Isotope	Atoms of the same element which have the same number of protons but a different number of neutrons
Relative atomic mass	The average mass of one atom of an element, taking into account the abundance of all the isotopes for the element. It is relative to 1/12th the mass of an atom of carbon-12.

• The atomic number is equal to the number of protons in an atom
• Since atoms are neutral, then it is also the same as the number of electrons
• The mass number is the number of protons plus neutrons
  • The number of neutrons can thus be calculated by subtracting the atomic number from the mass number
• For example, lithium has an atomic number of , therefore it has 3 protons and 3 electrons
• The mass number of lithium is 7, so it has 7 - 3 = 4 neutrons
• Lithium therefore has:
  • 3 protons
  • 3 electrons
  • 4 neutrons