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C1.1 Atoms

GCSE Combined Science · AQA · Chapter C1: Atomic Structure

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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 use standard form for the size of atoms

Key words

  • Atom: The smallest part of an element that can exist.
  • Element: A substance made of only one type of atom. It cannot be broken down into simpler substances by chemical methods.
  • Chemical symbol: One or two letters that stand for an element, e.g. O for oxygen, Na for sodium.
  • Group / period: A column / a row of the periodic table. Elements in the same group have similar chemical properties.
  • 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, containing protons and neutrons. Electrons move around it 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 in a lump of copper is a copper atom. There are about 100 different elements.

Key fact: A molecule can be an element (N₂, two nitrogen atoms) or a compound (CO₂). A mixture contains two or more substances that are not chemically combined (see C1.3).

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, e.g. Na (sodium), K (potassium), Fe (iron), Pb (lead).

  • 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 groups and the rows are 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 between Groups 2 and 3.

  • Elements to the left of the staircase line (and towards the bottom) are metals. More than three quarters of all elements are metals: shiny, good conductors, high melting points, malleable.
  • Elements to the right of the line (and towards the top) are non-metals: usually dull, brittle, poor conductors with low melting points. Hydrogen is a non-metal.
  • A few elements that touch the line, such as silicon (Si) and germanium (Ge), have some metal and some non-metal properties. They are hard to classify.
  • Some groups have names: Group 1 = the alkali metals, Group 7 = the halogens, Group 0 = the noble gases.

Exam tip: If you are asked whether an element is a metal, find it in the periodic table and check which side of the staircase it is on. Barium (Ba) is a metal; bromine (Br) 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 carbon dioxide is always CO₂. A compound has different properties from the elements it is made from. Iron is magnetic and sulfur is a yellow powder, but iron sulfide is a black solid that is not attracted to a magnet.

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 methane, CH₄, there are 4 hydrogen atoms for every 1 carbon atom. No small number after a symbol means “1”.

  • Metal first, then non-metal. Two elements only: ending changes to -ide.; Example calcium + chlorine → calcium chloride (CaCl₂) lithium + oxygen → lithium oxide (Li₂O)
  • Ending -ate: the compound also contains oxygen.; Example sodium carbonate (Na₂CO₃): Na, C and O zinc sulfate (ZnSO₄): Zn, S and O

Worked example: counting atoms in a formula

How many atoms are in one formula unit of Fe₂(SO₄)₃?
The small number after a bracket multiplies everything inside it.
Fe: 2 · S: 1 × 3 = 3 · O: 4 × 3 = 12
Total = 2 + 3 + 12 = 17 atoms of 3 different elements. (A big number in front multiplies the whole formula: 4NH₃ has 4 N and 12 H atoms.)

Answer: 17 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 (energy levels). 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, about 1 × 10⁻¹⁴ m, yet almost all of the mass of the atom is in the nucleus. It is like a pea in the middle of a sports stadium.

Worked example: working with the size of an atom

1 nm = 1 × 10⁻⁹ m. A magnesium atom has a radius of 0.16 nm. Write this in metres in standard form.
0.16 × 10⁻⁹ m = 1.6 × 10⁻¹⁰ m
How many magnesium atoms would fit side by side across 0.8 mm?
Diameter = 2 × 1.6 × 10⁻¹⁰ = 3.2 × 10⁻¹⁰ m. 0.8 mm = 8 × 10⁻⁴ m.
(8 × 10⁻⁴) ÷ (3.2 × 10⁻¹⁰) = 2.5 × 10⁶

Answer: 1.6 × 10⁻¹⁰ m; about 2.5 million atoms

Common mistakes

  • Writing symbols with the wrong capitals, e.g. “MG” or “mg” for magnesium. Only the first letter is a capital: Mg.
  • Forgetting that a bracket multiplies everything inside it: Mg(NO₃)₂ has 2 N and 6 O, so 9 atoms in total.
  • Thinking a compound keeps the properties of its elements. Iron sulfide is not magnetic like iron.
  • Confusing -ide and -ate: potassium sulfide is K₂S (no oxygen); potassium sulfate is K₂SO₄.

Revision questions

Try these, then check your answers below each one.

Question 1. Which statement describes an atom? (Choose from: The smallest part of an element that can exist, Two or more elements chemically combined, A charged particle, A mixture of protons and molecules.)

Answer: The smallest part of an element that can exist

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

Question 2. The boxes show the particles in four substances. Each colour is a different type of atom. Which box shows a compound? (Choose from: A, B, C, D.)

Answer: D

Working: In D, every particle contains two different types of atom joined together, so it is a compound. A and C are elements, and B is a mixture.

Question 3. What is the chemical symbol for iron?

Answer: Fe

Working: Iron is Fe. Only the first letter is a capital.

Related lessons

Continue in the full course

This lesson is section C1.1 (AGCSC1801) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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