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Elements & Isotopes

Isotopes and nuclide notation: use proton and nucleon numbers to count protons, neutrons and electrons in atoms and ions.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • define proton (atomic) number and nucleon (mass) number
  • interpret and use nuclide notations such as 126 C
  • define the term isotopes
  • deduce the numbers of protons, neutrons and electrons in atoms and ions given proton and nucleon numbers.

Isotope questions become straightforward when you read Z and A in a fixed order, then use “neutrons = A-Z”.

1. Definition

An element is a pure substance made of one type of atom only. Isotopes are atoms of the same element with the same proton number (Z) but different nucleon numbers (A) (different numbers of neutrons).

2. Key Ideas

  • Proton number (Z) identifies the element.
  • Nucleon number (A) counts protons + neutrons.
  • Neutrons = A - Z.
  • For an ion, adjust the electron count from the charge; the nucleus is unchanged.

3. Detailed Explanations

Quick Recall (isotope logic)
  • Same element = same Z (same number of protons).
  • Isotopes: same Z, different A → different neutrons.
  • Ion charge changes the number of electrons, not protons or neutrons.

A. What Z and A Mean (Nuclide Notation)

Nuclide notation is written as A_(Z)X where:

  • Z = proton number = number of protons
  • A = nucleon number = protons + neutrons

So the number of neutrons is: A - Z.

Recall: atomic structure

Revise proton number and electron counts in Atoms if needed.

B. Counting Electrons in Ions

Nuclide notation tells you the nucleus. The ion charge tells you how the electron count differs from the proton count.

  • A 1 + ion has lost one electron: electrons = Z-1.
  • A 2 + ion has lost two electrons: electrons = Z-2.
  • A 1- ion has gained one electron: electrons = Z + 1.

For example, ²⁴₁₂Mg²⁺ has 12 protons, 12 neutrons and 10 electrons.

4. Common Mistakes

  • Saying isotopes have different proton numbers (wrong). Same Z = same element.
  • Writing neutrons = A + Z (wrong). It is A - Z.
  • Using isotope notation without labels (write A_(Z)X clearly, not random superscripts).
  • Adding a positive charge to the proton count. Ion formation changes electrons, not the nucleus.

5. Exam Tips

Fast isotope definition

“Same Z, different A (different neutrons).”

6. Worked Examples

Modelled example 1

Identifying Isotopes

Core

Problem

Two atoms have these compositions:

  • Atom X: 6 protons, 6 neutrons, 6 electrons
  • Atom Y: 6 protons, 8 neutrons, 6 electrons

Determine whether X and Y are isotopes and explain.

Study the worked solution

  1. Test whether they are the same element

    Method

    Compare proton numbers.

    Reason

    Proton number defines the element.

    Working

    Both have Z = 6, so both are carbon atoms.

  2. Test whether their nucleon numbers differ

    Method

    Compare neutron counts and hence nucleon numbers.

    Reason

    Isotopes of one element differ in neutron number.

    Working

    For X, A = 6 + 6 = 12; for Y, A = 6 + 8 = 14.

  3. State the relationship

    Working

    X and Y are isotopes of carbon: same Z, different A.

Guided practice 2

Counting Neutrons From Notation

About 4 min

Problem

An atom is ³¹₁₅P. Calculate its number of neutrons.

Read A and Z before subtracting

Hints

Hint 1: identify the two numbers
The upper number is A = 31 and the lower number is Z = 15.
Hint 2: remove the proton contribution
Use neutrons = A-Z.
View solution step by step

  1. Read the notation

    Method

    Identify nucleon and proton numbers.

    Reason

    The neutron count is not written directly in nuclide notation.

    Working

    A = 31, Z = 15.

  2. Calculate neutron number

    Working

    Neutrons = A-Z = 31-15 = 16.

Common misconception 3

Error Analysis (Fix the Claim)

Find and correct the mistake

Learner response

A student says: “Isotopes have different chemical properties because they have different masses.” Locate the first error and correct the claim using electron arrangement and mass.

Separate chemical from physical properties

Chemical properties at O-Level

View solution step by step

  1. Locate the first error

    Method

    Reject the claim that different neutron numbers change chemical behaviour.

    Reason

    Chemical reactions depend mainly on electron arrangement, especially valence electrons.

    Working

    Isotopes have the same proton number and, as neutral atoms, the same electron configuration.

  2. Correct the property comparison

    Method

    State that chemical properties are the same but some physical properties can differ.

    Reason

    The electron arrangements are the same, while isotope masses differ because neutron counts differ.

    Working

    Same chemical properties; potentially different mass-dependent physical properties.

Examiner practice 4

Count particles in an ion

3 marks

Examination question

State the numbers of protons, neutrons and electrons in ²⁷₁₃Al³⁺. [3 marks]

Read the nucleus before applying the charge

View solution step by step

  1. Read the proton number

    1 mark

    Method

    Use the lower number, Z = 13.

    Reason

    Proton number is the number of protons.

    Working

    Protons = 13.

  2. Find the neutron count

    1 mark

    Method

    Subtract proton number from nucleon number.

    Reason

    A counts protons and neutrons together.

    Working

    Neutrons = 27-13 = 14.

  3. Apply the ion charge

    1 mark

    Method

    Remove three electrons from the neutral-atom count.

    Reason

    A 3 + charge means the atom has lost three electrons.

    Working

    Electrons = 13-3 = 10.

Challenge 5

Predict Properties of Isotopic Molecules

Minimal support

Properties transfer

One chlorine-gas sample contains molecules made only from chlorine-35 atoms. Another contains molecules made only from chlorine-37 atoms. Compare their chemical behaviour and their diffusion rates at the same temperature.

Use electrons for chemistry and mass for diffusion

Chemical behaviour
Faster-diffusing sample

Hints

Hint 1: separate the property causes
Chemical behaviour depends on electron configuration; gas diffusion rate at fixed temperature depends on particle mass.
Hint 2: compare the isotope masses
A molecule containing chlorine-35 atoms is lighter than the corresponding molecule containing chlorine-37 atoms.
View solution step by step

  1. Compare chemical behaviour

    Method

    Predict the same chemical behaviour.

    Reason

    Chlorine-35 and chlorine-37 atoms have the same proton number and electron configuration.

    Working

    Both samples have the same valence-electron arrangement and O-Level chemical properties.

  2. Compare diffusion rates

    Method

    Predict faster diffusion for the chlorine-35 sample.

    Reason

    Its molecules are lighter, so at the same temperature they have a greater average speed.

    Working

    Chlorine-35 sample: lower molecular mass → faster diffusion.

7. Mind Stretchers

Mind stretcher 1: Compare isotope ionsExtension

Question: Compare ³⁵₁₇Cl⁻ and ³⁷₁₇Cl⁻. State what is the same and what is different in their proton, neutron and electron counts.

Show Answer

Both ions have 17 protons because they are chlorine. Both have 18 electrons because each has gained one electron to form a 1- ion.

³⁵Cl⁻ has 35-17 = 18 neutrons, while ³⁷Cl⁻ has 37-17 = 20 neutrons.

So the proton and electron counts are the same; the neutron counts differ.

Mind stretcher 2: Hydrogen Isotopes (Read the Notation)Extension

Question: Hydrogen has three isotopes: ¹₁H, ²₁H, and ³₁H. State the number of neutrons in each.

Show Answer
  • ¹₁H: neutrons = 1 - 1 = 0
  • ²₁H: neutrons = 2 - 1 = 1
  • ³₁H: neutrons = 3 - 1 = 2

8. Quiz

Quiz time

Ready to check your understanding? Try the interactive quiz, then review any questions you missed.

Go to quiz