Summary

• A chemical subunit is the smallest building block of a pure substance that is unique to that substance.

• Atoms are the simplest type of chemical subunit.

• Atoms can exist individually or connected to other atoms by chemical bonds.

• Molecules are discrete arrangements of atoms joined by chemical bonds.

• Lattices are networks of atoms joined by chemical bonds.

• Pure substances are made up of one type of chemical subunit.

• They have defined physical and chemical properties.

• Elements are pure substances made up of one type of atom.

• These atoms may exist individually, or as molecules or lattices.

• Elements cannot be broken down into simpler substances.

• Compounds are pure substances made up of two or more different types of atoms.

• These structures may be molecules or lattices.

• Compounds can be broken down (chemically) into simpler substances.

• 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.

• Protons have a mass of 1 atomic mass unit and a positive charge (+1).

• Neutrons have a mass of 1 atomic mass unit and no charge.

• Electrons have a mass of 1/1840 of an atomic mass unit and a negative charge (–1).

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

• The atomic number of an atom is equal to the number of protons in the atom.

• It determines the type of atom.

• The mass number of an atom is equal to the total number of protons and neutrons in the atom.

• This is the number of particles in the nucleus, which largely determines the mass of an atom.

• Electron configuration describes the arrangement of electrons in shells around the nucleus of an atom.

• A maximum of 2 electrons can occupy the first shell.

• A maximum of 8 electrons can occupy the second shell.

• A maximum of 18 electrons can occupy the third shell, but the fourth shell will begin to fill once the third shell contains 8 electrons.

• A maximum of 8 electrons can occupy the valence shell (outermost shell).

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