Everything from the 6 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
An element is a substance made of only one type of atom. A compound is a substance made of two or more elements chemically combined in a fixed ratio, and it has new properties of its own. A mixture contains two or more substances that are not chemically combined: its composition can vary and its parts can be separated by physical methods such as a magnet, filtering or distillation.
A molecule is not the same thing as a compound. Oxygen, O2, and nitrogen, N2, are made of molecules but contain only one type of atom, so they are elements. A compound needs at least two different elements, as in carbon dioxide, CO2.
An atom has a small central nucleus of protons and neutrons, surrounded by electrons in shells. The proton number (atomic number) is the number of protons; in a neutral atom it also equals the number of electrons. The mass number (nucleon number) is protons plus neutrons. Electrons fill shells in order: 2 in the first, then 8, then 8 for the first 20 elements. The number of outer-shell electrons gives the group (I to VII), and the number of occupied shells gives the period.
The mass number is not the number of neutrons. It counts protons and neutrons together, so always subtract: neutrons = mass number − proton number. Also check which number is which: the mass number is always the larger one.
Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons, so they have different mass numbers. (Extended) Isotopes of an element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration. The relative atomic mass, Ar, is the average mass of the isotopes weighted by their percentage abundances.
Do not just average the mass numbers. For chlorine, (35 + 37) ÷ 2 = 36 is wrong because there are three times as many 35Cl atoms as 37Cl atoms. Each mass must be weighted by its abundance, and the total divided by 100, not by 2.
An ion is a charged particle formed when an atom loses or gains electrons. Metal atoms lose their outer electrons to form positive ions; non-metal atoms gain electrons to form negative ions, and both usually end with a noble-gas configuration. An ionic bond is the strong electrostatic attraction between oppositely charged ions. (Extended) Ionic compounds form a giant lattice of alternating positive and negative ions.
In calcium fluoride there are two separate F− ions, each with a single negative charge. Do not draw an F2 unit or write F22−: the small 2 in CaF2 counts ions, it does not mean the fluorine atoms are bonded together.
Molten or dissolved ionic compounds conduct because ions move and carry the charge. Writing that 'electrons move' through molten sodium chloride scores no marks: free electrons are the explanation for metals and graphite, not for ionic compounds.
A covalent bond is formed when a pair of electrons is shared between two atoms, usually non-metals. By sharing, each atom reaches a noble-gas configuration. Simple molecular substances have low melting and boiling points and are poor conductors of electricity. (Extended) Their melting points are low because only the weak intermolecular forces between molecules are overcome; the strong covalent bonds inside the molecules do not break.
Each bond line stands for one shared pair, two electrons. A double bond is two pairs, four electrons; a triple bond three pairs, six electrons. Drawing CO2 with single bonds leaves each oxygen with only seven outer electrons, so always check that every atom other than hydrogen ends with eight.
When water boils, the O-H covalent bonds do not break; the molecules simply move apart and the steam is still H2O. An answer saying 'covalent bonds are weak' or 'covalent bonds break on boiling' loses the mark: it is the intermolecular forces that are weak.
In a giant covalent structure a huge number of atoms are joined by strong covalent bonds in a continuous network. In diamond each carbon atom is bonded to four others, so it is very hard, has a very high melting point and does not conduct. In graphite each carbon atom is bonded to three others in flat layers of hexagons; there are weak forces between the layers and one delocalised electron per atom, so graphite is soft, slippery and conducts. (Extended) Metallic bonding is the electrostatic attraction between a lattice of positive ions and a sea of delocalised electrons.
Graphite is soft because the forces between its layers are weak, not because its covalent bonds are weak. The bonds within each layer are very strong, which is why graphite still has a very high melting point. 'Graphite has weak covalent bonds' is a classic lost mark.
Metals conduct because delocalised electrons move, not ions. In a solid metal the positive ions stay in the lattice. Mixing this up with ionic compounds, which conduct only when their ions are free to move, is a common error.