ZIMSEC O-Level Combined Science (4003)

Matter, Atoms and Bonding: notes

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States of Matter and the Particle Model

Particle model: all matter is made of tiny particles that are always moving.
Solid: particles are close together, in fixed positions, and only vibrate. It has a fixed shape and volume.
Liquid: particles are close but can slide past each other. It has a fixed volume and takes the shape of its container.
Gas: particles are far apart and move fast in all directions. It fills its container.
Diffusion: the spreading of particles from high to low concentration, caused by their random movement. It is faster in gases than in liquids.

Mixture: two or more substances mixed together but not chemically joined. Each keeps its own properties.
Filtration: separates an insoluble solid from a liquid, such as sand from water.
Distillation: separates a solvent from a solution by boiling and condensing the vapour, such as pure water from salty water.
Chromatography: separates substances by how far they travel up paper, which shows the different colours in an ink.
Crystallisation: separates a dissolved solid by evaporating some solvent and cooling until crystals form.
Purity: a pure substance has only one kind of particle, and it has sharp, fixed melting and boiling points.

Element: a substance made of only one type of atom.
Compound: two or more elements chemically joined in fixed ratios.
Protons and neutrons: both are in the nucleus. Protons are positive and neutrons have no charge.
Electrons: negative particles arranged in shells around the nucleus.
Atomic number and mass number: atomic number is the number of protons. Mass number is protons plus neutrons.
Isotopes: atoms of the same element with the same number of protons but different numbers of neutrons.

Ionic bonding: metal atoms give electrons to non-metal atoms, forming ions held by strong attraction. Ionic compounds have high melting points and conduct electricity when molten or dissolved.
Covalent bonding: non-metal atoms share pairs of electrons. Simple covalent substances usually have low melting points and do not conduct electricity.
Metallic bonding: positive metal ions sit in a sea of delocalised electrons. This explains why metals conduct electricity and can be bent.

Relative atomic mass (Ar): the average mass of an atom compared with carbon-12.
Relative formula mass (Mr): the sum of the Ar values of all atoms in a formula.
Mole: the amount of substance containing 6.02 × 10²³ particles. Number of moles = mass ÷ Mr.
Worked example: find the moles in 36 g of water (H₂O). Ar: H = 1, O = 16. Mr = (2 × 1) + 16 = 18. Moles = 36 ÷ 18 = 2 mol.

Confusing atomic number with mass number.
Forgetting to multiply by the number of atoms in a formula, such as the 2 hydrogens in H₂O.
Saying that particles expand when a solid melts. It is the spacing between particles that changes.
Thinking ions are neutral. They carry a charge.
Calling ionic compounds "molecules". They form giant lattices.