What Matter Is Made Of
The changes we see around us — burning, rusting, the taste of different foods — can be explained once we understand what matter is made of. As far back as the fourth century BC, the Greek philosophers Democritus and Leucippus suggested that matter is made up of tiny, indivisible particles, but they had no way to test the idea. It wasn’t until the early 19th century that the English scientist John Dalton put this on a scientific footing. Dalton’s suggestions, though two centuries old, remain the basis of modern chemical thought and are known today as Dalton’s atomic theory.
Key Definitions
- Atom: the smallest particle of an element that can take part in a chemical reaction. (An atom can only be split by a nuclear reaction, never by an ordinary chemical one.)
- Element: a pure substance made entirely of one type of atom.
- Compound: a pure substance formed when two or more atoms of different elements combine chemically.
- Molecule: the smallest particle of an element or compound that can exist freely and still keep the chemical properties of that substance. Note that all compounds are molecules, but not all molecules are compounds — O2 and O3 are molecules but not compounds, while CO2 and NH3 are both.
Chemical Symbols
There are around 118 known elements, arranged in the Periodic Table, and each has its own unique name and symbol. A few rules govern how these symbols are written:
- An element’s symbol always starts with a capital letter.
- Many symbols simply use the first letter of the element’s name (e.g. O for Oxygen, N for Nitrogen).
- Where that first letter is already taken by another element, the first two letters are used, with the second letter written in lower case (e.g. Ca for Calcium, so it isn’t confused with C for Carbon).
- Some symbols come from the element’s Latin name rather than its English one — for example Fe (ferrum) for Iron, Cu (cuprum) for Copper, Ag (argentum) for Silver.
Other common symbols worth knowing at this level: Zn for Zinc, Br for Bromine, and I for Iodine.
Valency
When atoms combine to form compounds, they do so in fixed, whole-number ratios — for example, a water molecule always needs exactly 2 hydrogen atoms for every 1 oxygen atom. This combining power of an element is called its valency, and more precisely, it is the number of electrons an atom loses, gains, or shares during a chemical reaction.
An ion is an atom, or group of atoms, carrying a net electric charge, formed when atoms gain or lose electrons. Losing electrons gives a positive ion; gaining electrons gives a negative ion. A monoatomic ion is made of just one atom (e.g. Na⁺, Cl⁻). A polyatomic ion, or radical, is a group of atoms that behaves as a single charged unit (e.g. OH⁻, CO₃²⁻, NH₄⁺).
| Common ions | Symbol | Valency |
|---|---|---|
| Hydrogen, Sodium, Potassium, Silver, Ammonium | H⁺, Na⁺, K⁺, Ag⁺, NH₄⁺ | 1 |
| Magnesium, Calcium, Zinc, Copper(II), Iron(II) | Mg²⁺, Ca²⁺, Zn²⁺, Cu²⁺, Fe²⁺ | 2 |
| Aluminium, Iron(III) | Al³⁺, Fe³⁺ | 3 |
| Chloride, Bromide, Iodide, Hydroxide, Nitrate | Cl⁻, Br⁻, I⁻, OH⁻, NO₃⁻ | 1 |
| Oxide, Sulphide, Carbonate, Sulphate | O²⁻, S²⁻, CO₃²⁻, SO₄²⁻ | 2 |
| Phosphate | PO₄³⁻ | 3 |
Writing Chemical Formulae
A chemical formula uses element symbols to represent a compound, with a subscript number showing how many atoms of each element are present. To work out a formula from the name of a compound:
- Identify the ions that make up the compound.
- Write the symbol of each ion, with its valency underneath.
- Swap the valencies over, writing each as a subscript on the other ion.
- Simplify the ratio where possible, and drop the charges.
Examples: sodium oxide is Na₂O (Na⁺ swaps with O²⁻); magnesium chloride is MgCl₂ (Mg²⁺ swaps with Cl⁻); calcium carbonate is CaCO₃ (Ca²⁺ and CO₃²⁻ have equal valency, so the ratio simplifies to 1:1); ammonium sulphate is (NH₄)₂SO₄.
Naming Compounds
To name a compound, first work out whether it’s formed from a metal and a non-metal, or from a metal and a radical.
- If the compound is a metal combined with a non-metal, the metal’s name stays the same and the non-metal’s ending changes to “-ide”. For example: NaCl is sodium chloride, MgCl₂ is magnesium chloride, H₂O is hydrogen oxide (water).
- If the compound contains a radical, the metal’s name stays the same and the radical’s ending changes to “-ate” — except for hydroxide, which keeps its name. For example: Na₂SO₄ is sodium sulphate, FeSO₄ is iron(II) sulphate, NaOH is sodium hydroxide.
