Structure of Atoms

• Atoms are made up of three main subatomic particles – protons, neutrons and electrons.

• Protons and neutrons form a cluster at the centre of the atom, called the nucleus.

• Electrons orbit the nucleus in electron shells, forming an electron cloud.

Atoms are made up of protons, neutrons and electrons.

Protons

• Protons are subatomic particles with a positive charge (+1).

• Protons have a mass of 1 atomic mass unit.

• The number of protons in an atom determines the type of atom.

• Examples

• If an atom has 2 protons, it is a helium atom.

• If an atom has 6 protons, it is a carbon atom.

• If an atom has 26 protons, it is an iron atom.

• The number of protons in an atom is known as the atomic number.

Neutrons

• Neutrons are subatomic particles with no charge.

• Neutrons have a mass of 1 atomic mass unit.

• The number of neutrons in a particular type of atom can vary.

• For example, 99% of carbon atoms have 6 neutrons, while 1% have 7 neutrons.

• The total number of protons and neutrons in an atom is known as the mass number.

• Atoms that have the same number of protons but a different number of neutrons are called isotopes.

• Therefore, isotopes of an element have the same atomic number, but different mass numbers.

Carbon-12 and carbon-13 are isotopes of carbon.

Electrons

• Electrons are subatomic particles with a negative charge (–1).

• Electrons have a mass of 1/1840 of an atomic mass unit.

• Therefore, almost all of the mass of an atom is in the nucleus.

• The number of electrons in an atom equals the number of protons.

• Therefore, atoms have no overall charge.