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What is inside an atom, and how do isotopes give an element its relative atomic mass?

Sub-atomic particles and their relative masses and charges, working out particle numbers from atomic and mass number, isotopes, and calculating relative atomic mass.

A focused answer to the WJEC GCSE Science Double Award Unit 2 topic on atoms, covering protons, neutrons and electrons and their relative masses and charges, atomic and mass number, particles in atoms and ions, isotopes, and calculating relative atomic mass.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. The sub-atomic particles
  3. Atomic number and mass number
  4. Working out the particles in an ion
  5. Isotopes
  6. Relative atomic mass
  7. Try this

What this dot point is asking

WJEC Double Award Unit 2 wants you to describe the structure of the atom and the relative masses and charges of its particles, work out the number of each particle in an atom or ion, define isotopes, and calculate relative atomic mass.

The sub-atomic particles

The relative masses and charges are:

  • Proton: relative mass 11, relative charge +1+1, in the nucleus.
  • Neutron: relative mass 11, relative charge 00, in the nucleus.
  • Electron: relative mass negligible, relative charge −1-1, in shells around the nucleus.

Almost all the mass of the atom is in the nucleus, because electrons have negligible mass. An atom is neutral overall because it has equal numbers of protons and electrons.

Atomic number and mass number

For sodium, atomic number 1111 and mass number 2323: there are 11 protons, 11 electrons and 23−11=1223 - 11 = 12 neutrons.

Working out the particles in an ion

An ion forms when an atom loses or gains electrons, so it has a charge. The number of protons and neutrons does not change - only the electrons do.

  • A positive ion has lost electrons, so it has fewer electrons than protons. For example, Na+\text{Na}^+ has 11 protons but only 10 electrons.
  • A negative ion has gained electrons, so it has more electrons than protons. For example, Cl−\text{Cl}^- has 17 protons and 18 electrons.

Isotopes

Because isotopes have the same number of electrons, they have identical chemical properties (chemistry depends on the electrons). For example, chlorine-35 and chlorine-37 both have 17 protons, but chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons.

Relative atomic mass

The relative atomic mass (ArA_r) of an element is the weighted mean mass of its atoms, taking into account the masses of the isotopes and how common each one is (its abundance):

Ar=∑(isotope mass×abundance)total abundanceA_r = \frac{\sum (\text{isotope mass} \times \text{abundance})}{\text{total abundance}}

Because it is an average, the relative atomic mass is usually not a whole number (chlorine is 35.535.5).

Try this

Q1. State the relative charge and relative mass of an electron. [2 marks]

  • Cue. Charge −1-1; mass negligible (almost zero).

Q2. How many neutrons are in an atom with mass number 23 and atomic number 11? [1 mark]

  • Cue. 23−11=1223 - 11 = 12 neutrons.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC style4 marksAn atom has a mass number of 39 and an atomic number of 19. State the number of protons, neutrons and electrons, and explain how you found the number of neutrons.
Show worked answer →

A Unit 2 structured question. Reward: the atomic number 19 equals the number of protons, so there are 19 protons (1). A neutral atom has equal protons and electrons, so there are 19 electrons (1). The number of neutrons is the mass number minus the atomic number: 39−19=2039 - 19 = 20 neutrons (1), with the explanation that neutrons = mass number minus atomic number (1). A common slip is to subtract the wrong way round or to forget protons and electrons are equal in a neutral atom.

WJEC style3 marksChlorine exists as chlorine-35 (75 percent) and chlorine-37 (25 percent). Calculate the relative atomic mass of chlorine.
Show worked answer →

A Unit 2 calculation. Reward: the relative atomic mass is the weighted mean of the isotope masses: Ar=(75×35)+(25×37)100=2625+925100=3550100=35.5A_r = \dfrac{(75 \times 35) + (25 \times 37)}{100} = \dfrac{2625 + 925}{100} = \dfrac{3550}{100} = 35.5 (1 mark for method, 1 for working, 1 for answer). Markers credit the weighted-mean method (multiplying each mass by its abundance) and the answer 35.5. A common error is to take a simple average of 35 and 37 (giving 36) instead of weighting by abundance.

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