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AQA ChemistryAtomic structure and the periodic table

Atomic structure

Describe the structure of the atom and use atomic number and mass number correctly.

Start here

The key idea

Atoms are made of a nucleus containing protons and neutrons, surrounded by electrons in shells.The atomic number gives the number of protons; the mass number gives the total of protons and neutrons.

Equation to know

neutrons = mass number − atomic number

Atomic Structure
nucleusp + nelectrons in shellsprotons set element

Use the labels to explain the scientific relationship shown.

Revision notes

The bit that matters

Keep the idea tight, then use the worked example to practise the exact exam wording.

1

Sub-atomic particles

An atom consists of a nucleus containing protons and neutrons, with electrons arranged in shells around the nucleus.Protons carry a relative charge of +1, electrons carry −1, and neutrons are neutral.The mass of an atom is concentrated in the nucleus because electrons have negligible mass compared to protons and neutrons.

2

Atomic number and mass number

The atomic number is the number of protons in an atom and uniquely identifies the element.The mass number is the total number of protons and neutrons.In a neutral atom the number of electrons equals the number of protons.The number of neutrons is found by subtracting atomic number from mass number.

3

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.Because the number of electrons is the same, isotopes of an element have the same chemical properties.The relative atomic mass of an element is the weighted mean mass of all its naturally occurring isotopes relative to carbon-12.

4

Development of the atomic model

Early models of the atom, including Dalton's solid sphere and Thomson's plum pudding model, were revised as new evidence emerged.The Geiger-Marsden alpha-particle scattering experiment showed that most of the atom is empty space and the positive charge is concentrated in a tiny, dense nucleus.Bohr later proposed that electrons orbit the nucleus in fixed energy levels (shells), and later models introduced sub-shells and orbitals.

Key terms

Definitions to learn

Atomic number

The number of protons in the nucleus of an atom; it identifies the element.

Mass number

The total number of protons and neutrons in the nucleus of an atom.

Isotope

Atoms of the same element with the same atomic number but different numbers of neutrons.

Relative atomic mass

The weighted mean mass of an atom of an element compared to one-twelfth of the mass of carbon-12.

Ion

An atom or group of atoms that has gained or lost electrons to have an overall charge.

Worked example

An atom has an atomic number of 17 and a mass number of 35. State the number of protons, neutrons and electrons.

1

Protons = atomic number = 17.

2

Electrons = protons (neutral atom) = 17.

3

Neutrons = mass number − atomic number = 35 − 17 = 18.

Final answer

Protons = 17, Electrons = 17, Neutrons = 18.

Exam habit

Always calculate neutrons as mass number minus atomic number.Never read off neutron number directly from the periodic table.

Watch out

Do not confuse mass number with atomic number. The mass number is the larger value.

Examiner tips

How to score full marks

  • 1Always calculate neutrons = mass number − atomic number; never assume neutrons equal protons.
  • 2When describing isotopes, state 'same number of protons, different number of neutrons' — both parts are needed for the mark.
  • 3In atomic structure questions, link any change in electron number to the formation of a positive (loss) or negative (gain) ion.
Practice questions

Try these yourself

Open each answer only after you have explained the full chemical process.

1Define the term 'atomic number'.[1 mark]
Mark scheme
  1. 1.Recall what atomic number tells you.
The number of protons in the nucleus of an atom (1).
2State the relative charge of a proton and a neutron.[2 marks]
Mark scheme
  1. 1.Compare each particle's charge.
Proton: +1 (1); Neutron: 0 (1).
3Explain why atoms have no overall charge.[2 marks]
Mark scheme
  1. 1.Compare proton and electron numbers and charges.
Atoms have equal numbers of protons (positive) and electrons (negative) (1), so the charges cancel out (1).
4An atom of calcium has atomic number 20 and mass number 40. State the number of neutrons.[1 mark]
Mark scheme
  1. 1.Use mass number − atomic number.
Neutrons = 40 − 20 = 20 (1).
5Define isotopes and give one example.[2 marks]
Mark scheme
  1. 1.Link to same element, different neutron number.
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons (1). Example: carbon-12 and carbon-14 (1).
6Describe the plum pudding model of the atom.[2 marks]
Mark scheme
  1. 1.State where positive charge and electrons are according to this model.
The plum pudding model proposed that the atom was a ball of positive charge (1) with electrons embedded throughout it (1).
7Explain how the Geiger-Marsden experiment led to the nuclear model replacing the plum pudding model.[4 marks]
Mark scheme
  1. 1.Describe the results and what they indicated.
Most alpha particles passed straight through the gold foil (1), but some were deflected at large angles and a few bounced back (1). This showed that most of the atom is empty space (1) and the positive charge is concentrated in a small, dense nucleus (1).
8Explain why an atom becomes a positive ion when it loses an electron.[3 marks]
Mark scheme
  1. 1.Compare proton and electron numbers after the change.
Losing an electron means the atom has more protons than electrons (1), so there are more positive charges than negative charges (1), giving an overall positive charge (1).
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