The Language of Chemistry

The One Thing

Chemistry represents substances and reactions through symbols, formulae, and balanced equations. Valency determines how atoms and ions combine, while conservation of mass requires equal numbers of each type of atom on both sides of a chemical equation.

Reactions, Processes and Experiments

Key Terms

• Element: A pure substance made up of only one kind of atom.
• Atom: The smallest particle of an element that takes part in a chemical reaction.
• Molecule: The smallest independent particle of a substance that can exist freely and shows the properties of that substance.
• Chemical symbol: A short representation of an element, usually made from one or two letters, such as H for hydrogen and Na for sodium.
• Atomicity: The number of atoms present in one molecule of an element; oxygen is diatomic because O₂ contains two atoms.
• Ion: An electrically charged atom or group of atoms formed by loss or gain of electrons.
• Cation: A positively charged ion formed when an atom loses one or more electrons.
• Anion: A negatively charged ion formed when an atom gains one or more electrons.
• Radical: A charged atom or group of atoms that behaves as a single unit in a compound, such as OH⁻ or SO₄²⁻.
• Valency: The combining capacity of an atom, ion, or radical, generally determined by the number of electrons lost, gained, or shared.
• Variable valency: The ability of some elements to show more than one valency, such as iron with valencies II and III.
• Chemical formula: A symbolic representation showing the elements and the number or ratio of their atoms or ions in a substance.
• Molecular formula: A formula showing the actual number of atoms of each element in one molecule, such as H₂O.
• Empirical formula: A formula showing the simplest whole-number ratio of atoms in a compound.
• Formula unit: The simplest ratio of ions in an ionic compound, such as NaCl.
• Chemical equation: A shorthand representation of a chemical reaction using symbols and formulae.
• Reactants: The substances that take part in a chemical reaction and are written on the left side of an equation.
• Products: The new substances formed in a chemical reaction and written on the right side of an equation.
• Balanced chemical equation: An equation with an equal number of atoms of each element on both sides.
• Coefficient: A number placed before a formula to show how many molecules or formula units are involved.
• Subscript: A small number written within a formula to show the number of atoms of an element in one molecule or formula unit.

Easily Confused

• Coefficient vs subscript: A coefficient shows the number of molecules or formula units involved, whereas a subscript shows the composition of one molecule or formula unit.
• Molecular formula vs empirical formula: A molecular formula gives the actual number of atoms in one molecule, whereas an empirical formula gives the simplest whole-number ratio.
• Atom vs molecule: An atom is the smallest particle of an element taking part in a reaction, whereas a molecule is the smallest independent particle of a substance that can exist freely.
• Cation vs anion: A cation is positively charged because electrons have been lost; an anion is negatively charged because electrons have been gained.
• Chemical symbol vs chemical formula: A symbol represents an element or one atom of it, whereas a formula represents the composition of a substance.
• Radical vs ion: An ion may be a single charged atom or a charged group; a radical is a charged atom or group that behaves as one unit in a compound.
• Changing coefficients vs changing subscripts: Coefficients may be changed to balance an equation; subscripts must not be changed because they alter the substance’s formula.
• Valency vs variable valency: Valency is an atom’s combining capacity; variable valency means that an element can show more than one valency, as iron does with II and III.

What Gets Asked

• Identifying or writing symbols: Questions may require correct symbols such as C, Cl, Mg, Na, K, Fe, Cu, Ag, and Au. Marks are lost by failing to use a capital first letter and a lowercase second letter.
• Using valency to construct formulae: Questions may involve valencies of hydrogen, sodium, potassium, chlorine, hydroxide, magnesium, calcium, oxygen, sulphide, aluminium, or phosphate. Marks are lost by failing to balance total positive and negative charge.
• Writing formulae containing radicals: Common radicals include ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, carbonate CO₃²⁻, sulphate SO₄²⁻, and phosphate PO₄³⁻. Marks are lost by omitting parentheses when more than one radical is required, as in Ca(OH)₂ and Al₂(SO₄)₃.
• Interpreting chemical formulae: Questions may use H₂O, CO₂, NaCl, MgCl₂, Al₂O₃, or CaCO₃ to test the elements and ratios present. Marks are lost by confusing subscripts with coefficients.
• Balancing equations: Questions may require the example 2H₂ + O₂ → 2H₂O. Marks are lost by changing subscripts rather than coefficients or by leaving unequal numbers of atoms on the two sides.
• Interpreting equations and conditions: Questions may ask students to identify reactants, products, state symbols, or conditions such as heat, light, pressure, or a catalyst. Marks are lost by placing reactants and products on the wrong sides or misinterpreting (s), (l), (g), (aq), or Δ.