Elements, Compounds and Symbols: Unit Review

Introduction

This unit has taken you from the simplest substances — elements — to the language chemists use to describe everything built from them. You have learned element symbols, how elements combine into compounds, how formulae record those combinations, how valency predicts formulae, and how to calculate formula masses. This review draws the threads together so that you can see how each idea supports the next, and it gives you a chance to test yourself across the whole unit.

Core explanation

Elements and symbols. An element contains only one type of atom and cannot be broken down chemically. About 118 elements are known, of which roughly 90 occur naturally. Each has a symbol of one or two letters: the first is always a capital, any second letter is lower case (C, Ca, Cl). Some symbols come from Latin names, such as Na (natrium) for sodium, Fe (ferrum) for iron and Pb (plumbum) for lead. The periodic table arranges elements in groups (columns) and periods (rows), with metals on the left, non-metals on the right and metalloids between them.

Compounds. A compound contains two or more elements chemically combined in a fixed ratio. It has new properties, unlike its elements, and it can be separated only by chemical reactions. Unlike a mixture, its composition is constant — the law of constant composition. Compounds are broadly ionic (a metal with a non-metal, forming a lattice of ions) or molecular (non-metals sharing electrons in molecules).

Formulae. A formula shows which elements are present and in what ratio. Subscripts count the atoms before them; brackets group a polyatomic ion so a subscript multiplies the whole group; coefficients in front count whole units. Molecular formulae show actual atom counts, while ionic formulae show the simplest ratio.

Valency and writing formulae. Valency is an element's combining power. Hydrogen has valency 1; oxygen 2; nitrogen 3 in ammonia; carbon 4. For ions, valency equals the size of the charge. To write a formula, balance valencies or charges, often by crossing them, then bracket polyatomic ions and simplify.

Name Ions Formula --- --- --- Sodium oxide Na⁺, O²⁻ Na₂O Aluminium chloride Al³⁺, Cl⁻ AlCl₃ Calcium nitrate Ca²⁺, NO₃⁻ Ca(NO₃)₂ Iron(III) sulfate Fe³⁺, SO₄²⁻ Fe₂(SO₄)₃

Naming. Ionic names give the positive ion first and the negative ion second, with -ide for single-element ions and -ate or -ite for oxygen-containing polyatomic ions. Roman numerals show the charge of metals with variable valency. Covalent names use prefixes such as di-, tri- and tetra-.

Masses. Relative atomic mass (Ar) compares an atom's average mass with one-twelfth of a carbon-12 atom. Adding the Ar values of all atoms in a formula gives Mr, and dividing an element's share by Mr gives its percentage by mass.

Step-by-step reasoning

A complete example linking the unit's skills, for "magnesium hydroxide":

1. Ions: Mg²⁺ and OH⁻. 2. Balance: one Mg²⁺ needs two OH⁻. 3. Formula: Mg(OH)₂, with brackets. 4. Mr = 24 + (2 × 16) + (2 × 1) = 58. 5. % Mg = 24 ÷ 58 × 100 = 41.4%.

Visual explanation

Imagine a staircase with five steps: symbols at the bottom, then compounds, then formulae, then valency and naming, and finally masses at the top. Each step rests on the one below; you cannot calculate Mr without a correct formula, and you cannot write the formula without knowing symbols and valencies.

Real-world analogy

Learning chemistry's language is like learning to read music. Symbols are the notes, formulae are the chords, valency is the set of rules for which notes fit together, and calculations are like counting the beats. Once these basics are secure, you can read and write much more complex "pieces".

Real-world example

A food scientist checking a label that says "calcium carbonate added" uses every skill in this unit: recognising the elements, writing CaCO₃, calculating its Mr of 100 and working out that 40% of its mass is calcium, which tells her how much calcium the product supplies.

Why?

Why does chemistry need such a precise system? Millions of compounds are known, and chemists across the world must describe them without confusion. Symbols, formulae and systematic names form a universal language in which each substance has one clear description, whatever the reader's spoken language.

Common misconception

"Formulae are just abbreviations of names." A formula carries much more information than a shortened name: it shows exactly which elements are present, in what ratio, and allows masses and compositions to be calculated. A name like "iron oxide" is ambiguous; FeO and Fe₂O₃ are not.

Worked example

Question: Write the formula of ammonium sulfate and calculate its Mr.

Reasoning: Ions NH₄⁺ and SO₄²⁻; two ammonium ions balance one sulfate, so the formula is (NH₄)₂SO₄. Atoms: 2 N, 8 H, 1 S, 4 O. Mr = 28 + 8 + 32 + 64.

Answer: (NH₄)₂SO₄, Mr = 132.

Quick check

1. What is the difference between an element and a compound? Answer: An element contains only one type of atom; a compound contains two or more elements chemically combined in a fixed ratio.

Exam focus

Unit questions often combine skills: name to formula, then Mr, then percentage by mass. Accuracy at each step matters because errors carry forward. Revise symbol capitalisation, the common polyatomic ions, Roman numerals and the definitions of element, compound, mixture and relative atomic mass.

Advanced insight

The ideas of this unit lead directly to the mole, the chemist's counting unit. The Mr of a substance in grams contains the same number of formula units — about 6.02 × 10²³ — for every substance. This link between relative masses and real masses allows chemists to measure out atoms by weighing.

Summary

Elements are single types of atom, written with one- or two-letter symbols and arranged in the periodic table. Compounds combine elements in fixed ratios and have new properties. Formulae, built using valency, subscripts and brackets, record these ratios, and systematic names describe them in words. Relative atomic and formula masses allow compositions to be calculated.

Practice questions

1. Write the symbols for potassium, iron and chlorine. Answer: K, Fe and Cl. 2. Write the formula of aluminium oxide. Answer: Al₂O₃, since two Al³⁺ ions (+6) balance three O²⁻ ions (−6). 3. Name the compound Cu(NO₃)₂. Answer: Copper(II) nitrate. 4. Calculate the Mr of potassium carbonate, K₂CO₃. Answer: (2 × 39) + 12 + (3 × 16) = 78 + 12 + 48 = 138. 5. Give two differences between a compound and a mixture. Answer: A compound has a fixed composition and can be separated only by chemical reactions; a mixture has variable composition and can be separated by physical methods.