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Atom & Isotopes

Key idea: Learn atom structure, proton number Z, nucleon number A, and isotopes, including how to find protons, neutrons and electrons in exam questions (O Level).

  • SEC G3 Physics 2027
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Learning objectives

  • Describe atomic composition
  • Use proton number, nucleon number and isotope
  • Use and interpret nuclide notation
  • Explain random and spontaneous nuclear decay
  • Describe alpha, beta-minus and gamma radiation
  • Compare ionising effect and penetrating power
  • Use nuclide equations for radioactive decay
  • Explain background radiation
  • Use half-life in tables and decay curves
  • State radioactivity applications and hazards
  • Evaluate uses and hazards using half-life and radiation properties
  • Relate fission and fusion to nuclear-energy release

1. Definition

A. Atom

An atom has a small, positively charged nucleus containing protons and neutrons, with negatively charged electrons outside the nucleus.

B. Isotopes

Isotopes are atoms of the same element with the same proton number Z but different numbers of neutrons. They therefore have different nucleon numbers A.

What you need for this course

You should be able to describe the structure of an atom and the meaning of isotopes.

2. Key Ideas

  • Proton number Z = number of protons (and equals number of electrons in a neutral atom).
  • Nucleon number A = number of protons + neutrons.
  • Neutrons: N = A - Z.
  • The nucleus contains almost all the mass of the atom.
ParticleLocationRelative chargeRole in A or Z
protonnucleus+ 1counted in both A and Z
neutronnucleus0counted in A only
electronoutside nucleus-1counted in neither A nor Z
Atom composition and isotopesA schematic atom shows protons and neutrons in a central nucleus and electrons outside. Beside it, carbon-12 and carbon-14 are compared: both have six protons, while carbon-14 has two more neutrons.Composition of an atomp+nnp+e−e−positive nucleus: protons + neutronsnegative electrons outside the nucleusExample: carbon isotopes126C6 protons6 neutrons146C6 protons8 neutronssame Z → same elementdifferent neutron number → isotopes
An atom has a small positive nucleus containing protons and neutrons, with negatively charged electrons outside it. Isotopes have the same proton number but different neutron numbers. The drawing is schematic and not to scale.

3. Detailed Explanations

A. Proton number Z and element identity

Z identifies the element. If two atoms have the same Z, they are the same element (even if their neutrons differ).

B. Nucleon number A and neutrons

Nucleon number counts the particles in the nucleus:

A = Z + N ⇒ N = A - Z

C. What isotopes change (and what they don’t)

  • Isotopes have the same Z → same element.
  • Neutral atoms of the isotopes have the same number and arrangement of electrons, so they have the same chemical properties.
  • Their different neutron numbers can give them different nuclear properties; some isotopes are stable and some are radioactive.
Exam-safe phrasing

Isotopes have the same proton number, but different numbers of neutrons.

4. Common Mistakes

  • Using A as the proton number (it is the nucleon number).
  • Forgetting N = A - Z.
  • Saying isotopes have different proton numbers (they don’t).
  • Assuming electrons are counted in A. Nucleon number counts only protons and neutrons.

5. Exam Tips

  1. Write the key relationships: A = Z + N and N = A - Z.
  2. For “isotopes”, lead with the defining comparison: same proton number, different neutron number.
  3. Do not assume electrons = Z unless the atom is stated or shown to be neutral.

6. Worked Examples

Modelled example 1

Neutrons in an atom

Core

Problem

How many neutrons are in ¹⁶₈O?
Study the worked solution

  1. Read the notation

    Method

    Identify A = 16 and Z = 8.

    Reason

    The upper number counts nucleons and the lower number counts protons.

    Working

    A = 16, Z = 8

  2. Subtract protons from nucleons

    Method

    Calculate N = A-Z.

    Reason

    Nucleons are protons plus neutrons.

    Working

    N = 16-8 = 8

Guided practice 2

Protons, neutrons, electrons

About 5 min

Problem

For neutral ²³₁₁Na, state the numbers of protons, neutrons and electrons.

Use neutrality only for the electron count

Hints

Hint 1: nucleus
Z gives protons; A-Z gives neutrons.
Hint 2: neutral atom
Total positive and negative charges balance.
View solution step by step

  1. Count nuclear particles

    Method

    Give 11 protons and 12 neutrons.

    Reason

    Z = 11 and N = 23-11.

    Working

    p = 11, n = 12

  2. Count electrons

    Method

    Give 11 electrons.

    Reason

    A neutral atom has equal proton and electron numbers.

    Working

    e = 11

Common misconception 3

Identifying isotopes

Find and correct the mistake

Learner response

A learner says ¹²₆C and ¹⁴₆C are different elements because their upper numbers differ. Diagnose the claim.

Use proton number for identity

View solution step by step

  1. Identify the element

    Method

    State that both are carbon.

    Reason

    Both have proton number Z = 6, which fixes element identity.

    Working

    Z = 6 for both

  2. Identify the isotope difference

    Method

    State that they are isotopes with 6 and 8 neutrons.

    Reason

    The nucleon numbers differ while proton number is the same.

    Working

    12-6 = 6; 14-6 = 8

Examiner practice 4

Finding A and Z

3 marks

Examination question

A nucleus contains 12 protons and 13 neutrons. State Z, calculate A and write its nuclide notation. [3 marks]

Use proton count to identify the element

View solution step by step

  1. Find identifiers

    2 marks

    Method

    Set Z = 12 and add nucleons to get A = 25.

    Reason

    Proton number counts protons; nucleon number counts protons plus neutrons.

    Working

    Z = 12, A = 12 + 13 = 25

  2. Write the nuclide

    1 mark

    Method

    Write magnesium with A upper left and Z lower left.

    Reason

    Z = 12 identifies magnesium.

    Working

    ²⁵₁₂Mg

Challenge 5

Neutrons and electrons from A and Z

Minimal support

Two-rule transfer

For neutral ⁴⁰₂₀Ca, determine neutron and electron counts and justify each using a different rule.

Separate nucleon subtraction from charge neutrality

Hints

Hint 1: neutrons
Use N = A-Z.
Hint 2: electrons
A neutral atom has electron count equal to proton count.
View solution step by step

  1. Find neutrons

    Method

    Subtract Z from A.

    Reason

    Nucleon number includes protons and neutrons.

    Working

    N = 40-20 = 20

  2. Find electrons

    Method

    Use the neutral-atom condition.

    Reason

    20 proton charges require 20 electron charges for zero net charge.

    Working

    e = 20

7. Mind Stretchers

Mind stretcher 1: Element vs isotopeExtension

Why does changing the number of neutrons change nuclear stability but not element identity?

Show Answer

Element identity depends on the number of protons (Z). Neutrons affect forces and stability inside the nucleus, so changing neutrons can change stability without changing the element.

Mind stretcher 2: Tiny nucleus, huge massExtension

Why does the nucleus contain most of the atom’s mass even though it is so small?

Show Answer

Almost all the mass is in the protons and neutrons, and they are packed into the nucleus. Electrons have a much smaller mass, so they contribute very little to the total mass even though they occupy most of the atom’s volume.

8. Practice and next step

Check particle numbers in the Radioactivity Quiz, then continue to Nuclide Notation.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027