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G2 Radioactivity: Atomic Foundations and Nuclide Notation

Key idea: Build a G2 model of the atom, distinguish proton and nucleon number, identify isotopes, and interpret nuclide notation accurately.

  • SEC G2 Science Physics component 2027
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Learning objectives

  • Describe atomic composition
  • Use proton number, nucleon number and isotope
  • Use and interpret nuclide notation

1. Definition

An atom has a small, positively charged nucleus containing protons and usually neutrons. Ordinary hydrogen has one proton and no neutrons. Negatively charged electrons surround the nucleus.

A nuclide is a particular type of nucleus with a specified number of protons and neutrons. We write it as

^A_ZX

where Z is the proton number, A is the nucleon number, and X is the element symbol.

By the end of this lesson, you can read this notation, find each particle count and recognise isotopes.

2. Key ideas

ParticleChargeLocationContribution to nucleon number
Proton+ 1Nucleus1
Neutron0Nucleus1
Electron-1Outside the nucleus0
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
Protons and neutrons are in the nucleus; electrons are outside it. Carbon-12 and carbon-14 are isotopes because they have the same six protons but different neutron numbers. The drawing is schematic and not to scale.
  • The proton number identifies the element.
  • The nucleon number counts protons and neutrons: A = Z + N.
  • Therefore the neutron number is N = A-Z.
  • A neutral atom has equal numbers of protons and electrons.
  • Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

3. Detailed explanation

Why the nucleus is positive

Every proton has charge + 1, while every neutron has charge 0. The nucleus is therefore positive. An atom is extremely small, but almost all its mass is concentrated in this nucleus because electrons have much less mass than protons and neutrons.

Why proton number fixes the element

Changing the number of neutrons can produce another isotope of the same element. Changing the number of protons produces a different element. For example, every carbon atom has six protons; a nucleus with seven protons is nitrogen, whatever its neutron count.

Reading a nuclide symbol

For ²³₁₁Na:

  1. The lower number gives 11 protons.
  2. The upper number gives 23 nucleons.
  3. The number of neutrons is 23-11 = 12.
  4. A neutral sodium atom has 11 electrons.

The upper number is not the neutron number. It includes both protons and neutrons.

Recognising isotopes

³⁵₁₇Cl and ³⁷₁₇Cl are isotopes because both have proton number 17, so both are chlorine. Their neutron numbers are 18 and 20, so their nuclei are not identical.

4. Common mistakes

  • Treating A as the neutron number. This is tempting because A is written beside the nucleus, but it counts all nucleons. Use N = A-Z.
  • Calling two different elements isotopes. Isotopes must have the same proton number. Compare Z before comparing A.
  • Putting electrons inside the nucleus. Protons and neutrons form the nucleus; electrons occupy the region outside it.
  • Assuming every atom is radioactive. Nuclide notation identifies a nucleus, but the notation alone does not prove that it is unstable.

5. Exam tips

  • For “state the number” questions, give the number directly and label the particle.
  • For “show that these are isotopes”, state both parts: same proton number, different neutron number.
  • Write the element symbol with A at upper left and Z at lower left. Swapping them changes the meaning.
  • Check that proton number plus neutron number returns the stated nucleon number.

6. Worked examples

Modelled example 1

Counting particles in chlorine-37

Core

Problem

A neutral atom is written ³⁷₁₇Cl. Find its numbers of protons, neutrons and electrons.

Study the worked solution

  1. Read the proton number

    Method

    Use the lower number, Z = 17.

    Reason

    The proton number defines the element and equals its number of protons.

    Working

    Protons = 17.

  2. Find the neutron number

    Method

    Subtract proton number from nucleon number.

    Reason

    The nucleon number counts protons and neutrons together.

    Working

    Neutrons = 37-17 = 20.

  3. Use electrical neutrality

    Method

    Match the electron count to the proton count.

    Reason

    A neutral atom has zero net charge.

    Working

    Electrons = 17; answer: 17 protons, 20 neutrons and 17 electrons.

Common misconception 2

Testing an isotope claim

Find and correct the mistake

Learner claim

“⁴⁰₁₈Ar and ⁴⁰₂₀Ca are isotopes because they have the same nucleon number.” Diagnose the error.

Try this before viewing the solution

Number that must match

View solution step by step

  1. Apply the definition

    Method

    Compare proton numbers first.

    Reason

    Isotopes must be atoms of the same element.

    Working

    18 ≠ 20, so these are argon and calcium, not one element.

  2. Repair the claim

    Method

    Reject equal nucleon number as sufficient evidence.

    Reason

    Different proton and neutron combinations can have the same total.

    Working

    Equal A does not make nuclides isotopes; equal Z and different A does.

Challenge 3

Transfer the notation to an ion

Minimal support

Problem

A particle is written ²⁴₁₂Mg²⁺. Determine its proton, neutron and electron counts, explaining how the charge changes only one count.

Try this before viewing the solution

Hints

Hint 1: nucleus
Use Z for protons and A-Z for neutrons.
Hint 2: charge
A positive ion has fewer electrons than protons.
View solution step by step

  1. Count nuclear particles

    Method

    Use Z = 12 and A = 24.

    Reason

    Ion formation does not change the nucleus.

    Working

    Protons = 12; neutrons = 24-12 = 12.

  2. Apply the charge

    Method

    Remove two electrons from the neutral count.

    Reason

    Losing two negative charges leaves net charge 2 +.

    Working

    Electrons = 12-2 = 10.

7. Mind stretchers

Mind stretcher 1: Identify the element from particle countsExtension

A neutral atom contains 15 protons and 16 neutrons. Write its nuclide symbol and explain which number identifies the element.

Show answer

The proton number is 15, which identifies phosphorus, P. The nucleon number is 15 + 16 = 31, so the symbol is ³¹₁₅P. Proton number identifies the element because changing it changes the nuclear charge and element identity.

Mind stretcher 2: Compare three nuclidesExtension

Decide which pair are isotopes: ²⁴₁₂Mg, ²⁵₁₂Mg and ²⁵₁₃Al. Justify your choice without relying only on the symbols.

Show answer

²⁴₁₂Mg and ²⁵₁₂Mg are isotopes. They both contain 12 protons, but they contain 12 and 13 neutrons respectively. The aluminium nuclide has 13 protons, so it is a different element.

Check particle accounting independently

A neutral atom has nuclide notation ²⁷₁₃Al. Find all three particle counts before checking.

Repair the first wrong step

A learner reads ³¹₁₅P as “31 neutrons and 15 protons, so 46 nucleons”. Correct the neutron count, then compare it with ³²₁₅P.

Relationship between the two neutral atoms

Practice and next step

Use the Radioactivity topic check for focused practice and feedback. Then continue to Decay and radiation.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G2 Science Physics component
Edition
SEC G2 Science Physics component 2027