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1.1 Atoms

GCSE Chemistry · AQA · Chapter 1: Atomic Structure

A free GCSE Chemistry lesson for AQA (8462) from OBA School: atoms, elements and their symbols, the periodic table, compounds and a first look inside the atom. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe an atom as the smallest part of an element that can exist
  • Use chemical symbols and the periodic table to identify elements, and say whether an element is a metal or a non-metal
  • Explain the difference between an element and a compound, and count the atoms in a formula
  • Name simple compounds from the elements they contain
  • Describe the parts of an atom and how small atoms are

Key words

  • Atom: The smallest part of an element that can exist.
  • Element: A substance made of only one type of atom. There are about 100 elements.
  • Group: A column of the periodic table. Elements in a group have similar chemical properties.
  • Chemical symbol: One or two letters that stand for an element, e.g. O for oxygen, Na for sodium.
  • Compound: Two or more elements chemically combined in fixed proportions.
  • Molecule: Two or more atoms joined together by chemical bonds, e.g. O₂, H₂O.
  • Formula: Symbols and numbers showing which atoms, and how many of each, are in a substance.
  • Nucleus: The tiny centre of an atom. It contains protons and neutrons.
  • Electron: A very small negative particle that moves around the nucleus in shells.

Atoms and elements

All substances are made of atoms. An atom is the smallest part of an element that can exist. An element contains only one type of atom, so every atom of carbon is a carbon atom. There are about 100 different elements.

Key fact: A molecule can be an element (O₂, two oxygen atoms) or a compound (H₂O). A mixture contains two or more substances that are not chemically combined (you meet mixtures in 1.3).

Each element has a chemical symbol. The first letter is always a capital and any second letter is lower case. This matters: Co is cobalt, but CO is carbon monoxide (a compound of carbon and oxygen). Some symbols come from old Latin names.

  • hydrogen; Symbol H; Element chlorine; Symbol Cl; Element sodium; Symbol Na
  • carbon; Symbol C; Element calcium; Symbol Ca; Element potassium; Symbol K
  • nitrogen; Symbol N; Element magnesium; Symbol Mg; Element iron; Symbol Fe
  • oxygen; Symbol O; Element aluminium; Symbol Al; Element copper; Symbol Cu
  • sulfur; Symbol S; Element zinc; Symbol Zn; Element lead; Symbol Pb

The periodic table

The elements are listed in the periodic table in order of atomic number. The columns are called groups and the rows are called periods. Elements in the same group have similar chemical properties. There are eight main groups (1 to 7, and Group 0), with the block of transition metals in between Groups 2 and 3.

  • Elements to the left of the staircase line are metals. More than three quarters of all elements are metals.
  • Elements to the right of the line are non-metals. Hydrogen is a non-metal, even though it sits above Group 1.
  • A few elements that touch the line, such as silicon (Si) and germanium (Ge), have some metal and some non-metal properties. They are called metalloids (semi-metals).
  • Some groups have names: Group 1 = the alkali metals, Group 7 = the halogens, Group 0 = the noble gases.

Exam tip: If an exam asks whether an element is a metal, find it in the periodic table and check which side of the staircase it is on. Bismuth (Bi) is a metal; selenium (Se) is a non-metal.

Compounds

When elements react, their atoms join with atoms of other elements to make compounds. In a compound the elements are always combined in the same fixed proportions, so water is always H₂O. A compound has different properties from the elements it is made from. Sodium is a soft, very reactive metal and chlorine is a toxic green gas, but sodium chloride is the salt you put on chips.

A compound can only be separated into its elements by a chemical reaction, not by physical methods such as filtering or distilling.

A formula tells you which elements are in a compound and the ratio of their atoms. In ammonia, NH₃, there are 3 hydrogen atoms for every 1 nitrogen atom. No small number after a symbol means “1”.

  • Metal first, then non-metal. Two elements only: ending changes to -ide.; Example magnesium + oxygen → magnesium oxide (MgO) potassium + bromine → potassium bromide (KBr)
  • Ending -ate: the compound also contains oxygen.; Example calcium carbonate (CaCO₃): Ca, C and O copper sulfate (CuSO₄): Cu, S and O

Worked example: counting atoms in a formula

How many atoms are in one formula unit of Al₂(SO₄)₃?
The small number after a bracket multiplies everything inside it.
Al: 2 · S: 1 × 3 = 3 · O: 4 × 3 = 12
Total = 2 + 3 + 12 = 17 atoms of 3 different elements.

Answer: 17 atoms

Exam tip: A big number in front multiplies the whole formula: 3H₂O means 3 molecules, so 6 H atoms and 3 O atoms.

Inside the atom

Atoms are made of even smaller subatomic particles. The nucleus at the centre contains protons (positive) and neutrons (no charge). Electrons (negative) move around the nucleus in shells. An atom has the same number of electrons as protons, so it has no overall charge.

Atoms are tiny. The radius of an atom is about 0.1 nm (1 × 10⁻¹⁰ m). The radius of the nucleus is less than 1/10 000 of the radius of the atom, but almost all of the mass of the atom is in the nucleus.

Worked example: working with the size of an atom

1 nm = 1 × 10⁻⁹ m, so 0.1 nm = 0.1 × 10⁻⁹ m = 1 × 10⁻¹⁰ m.
How many atoms of radius 0.1 nm would fit side by side across 1 mm?
Diameter of one atom = 2 × 1 × 10⁻¹⁰ = 2 × 10⁻¹⁰ m. 1 mm = 1 × 10⁻³ m.
(1 × 10⁻³) ÷ (2 × 10⁻¹⁰) = 5 × 10⁶

Answer: about 5 million atoms

Common mistakes

  • Writing symbols with the wrong capitals, e.g. “CL” or “cl” for chlorine. Only the first letter is a capital: Cl.
  • Forgetting that a bracket multiplies everything inside it: Ca(OH)₂ has 2 O and 2 H, so 5 atoms in total.
  • Thinking a compound keeps the properties of its elements. Sodium chloride is not explosive in water like sodium.
  • Confusing -ide and -ate: sodium sulfide is Na₂S (no oxygen); sodium sulfate is Na₂SO₄.
  • Saying the electrons are in the nucleus. Only protons and neutrons are in the nucleus.

Revision questions

Try these, then check your answers below each one.

Question 1. What is an atom? (Choose from: The smallest part of an element that can exist, A substance made of two or more elements chemically combined, A particle with an electric charge, The positive centre of an element.)

Answer: The smallest part of an element that can exist

Working: An atom is the smallest part of an element that can exist. A substance made of two or more elements chemically combined is a compound; a charged particle is an ion.

Question 2. What is the chemical symbol for potassium?

Answer: K

Working: Potassium is K, from its Latin name kalium. (P is phosphorus and Po is polonium.)

Question 3. True or false? A compound can be separated into its elements by filtration.

Answer: False

Working: False. The elements in a compound are chemically combined, so a chemical reaction is needed to separate them. Filtration is a physical method for mixtures.

Related lessons

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