Atomic nucleus and isotopes

CIE A-Level Chemistry· 2.1.1 Atomic structure· 35 min read

1. Nuclide Structure and Notation★☆☆☆☆⏱ 10 min

📘 Definition

Atomic Nucleus

The dense, positively charged central core of an atom, made up of protons and neutrons (collectively called nucleons)

Example:

A neutral carbon-12 atom has a nucleus containing 6 protons and 6 neutrons, with 6 electrons outside the nucleus

Subatomic Particle	Relative Mass	Relative Charge
Proton	1	+1
Neutron	1	0
Electron	~1/1836	-1

Standard nuclide notation $_{Z}^{A} X$ summarizes key information about a nuclide: the top number $A$ is the total nucleon (proton + neutron) count, the bottom number $Z$ is the proton count, and $X$ is the element's chemical symbol. For neutral atoms, the number of electrons equals $Z$ .

📐 Worked Example

Write the full nuclide notation for a neutral aluminium atom with 13 protons and 14 neutrons.

1
Identify the proton number $Z$ (given):
2
$Z = 13$
3
Calculate nucleon number $A$ as sum of protons and neutrons:
4
$A = 13 + 14 = 27$
5
Find the chemical symbol for aluminium: Al
6
Write in standard nuclide notation:
7
$^{27}_{13}Al$

Exam tip:

Always confirm the proton number matches the element on your periodic table — examiners often use mismatched Z/X as a distractor in multiple choice questions.

2. Isotopes: Definition and Properties★★☆☆☆⏱ 12 min

📘 Definition

Isotopes

Atoms of the same element that share the same number of protons (same proton number $Z$ ) but have different numbers of neutrons (different nucleon number $A$ )

Example:

Hydrogen has three isotopes: $_{1}^{1} H$ (protium), $_{1}^{2} H$ (deuterium), and $_{1}^{3} H$ (tritium)

Chemical properties of isotopes are identical: chemical behavior is determined by the number and arrangement of electrons, which is the same for all isotopes of an element. Physical properties (mass, density, boiling point, melting point) differ because of the different mass of the nucleus.

📐 Worked Example

Three particles have the following composition: P (17p, 18n, 17e), Q (16p, 18n, 16e), R (17p, 20n, 17e). Identify which pair are isotopes of the same element.

1
Recall that isotopes have the same proton number $Z$ :
2
Particle P has $Z = 17$ , particle Q has $Z = 16$ , particle R has $Z = 17$
3
Since P and R share the same proton number, they are isotopes of the same element (chlorine)

3. Calculating Relative Atomic Mass★★☆☆☆⏱ 15 min

📘 Definition

Relative Atomic Mass ($A_r$)

The weighted average mass of one atom of an element, accounting for the relative abundance of each of its naturally occurring isotopes, measured on a scale where an atom of carbon-12 has a mass of exactly 12

The general formula for relative atomic mass is:

A_r = \frac{\sum (\text{isotopic mass} \times \text{relative abundance})}{\sum \text{relative abundance}}

📐 Worked Example

Bromine has two naturally occurring isotopes: $^{79} B r$ (abundance 50.7%) and $^{81} B r$ (abundance 49.3%). Calculate the relative atomic mass of bromine.

1
Multiply each isotopic mass by its percent abundance:
2
$(79 \times 50.7) + (81 \times 49.3) = 4005.3 + 3993.3 = 7998.6$
3
Divide by the sum of percent abundances (100 for percent data):
4
$A_r = \frac{7998.6}{100} = 79.99$
5
Final result: $A_{r}$ of bromine ≈ 80.0

✓ Quick check

Test your calculation skill: A sample of magnesium has three isotopes with the following data: $^{24} M g$ (78.6%), $^{25} M g$ (10.1%), $^{26} M g$ (11.3%). What is the correct relative atomic mass?

Calculate $A_{r}$ for magnesium
- 24.00
- 24.33
- 25.00
- 24.10
Reveal answer
24.33 —
Correct: $(24 \times 78.6) + (25 \times 10.1) + (26 \times 11.3) = 2432.7$ , divided by 100 = 24.33

4. Common Pitfalls

Wrong move:

Mixing up nucleon and proton number in nuclide notation

Why:

The order of numbers in notation is easy to reverse when reading quickly

Correct move:

Remember: A (top) = All nucleons, Z (bottom) = number of Protons

Wrong move:

Claiming isotopes have different chemical properties

Why:

Different masses lead to different physical properties, but chemistry depends on electrons

Correct move:

All isotopes of the same element have identical chemical behavior

Wrong move:

Calculating a simple average of isotopic masses, ignoring abundance weighting

Why:

Simple averages only work if all isotopes are equally abundant, which is rare

Correct move:

Always multiply each mass by its abundance before summing and dividing

Wrong move:

Claiming neutral isotopes have different numbers of electrons

Why:

Isotopes only differ in neutron count for neutral atoms

Correct move:

Neutral isotopes have equal numbers of protons and electrons, so electron counts are identical

5. Quick Reference Cheatsheet

Term	Key Information
Proton	Relative mass 1, charge +1
Neutron	Relative mass 1, charge 0
Proton number (Z)	Number of protons, defines the element
Nucleon number (A)	Total protons + neutrons
Neutron count	$A - Z$
Isotopes	Same Z, different A
Relative Atomic Mass	$\frac{\sum ( mass \times abundance )}{\sum abundance}$

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
Identify isotopes from particle data
2023 · 2
Calculate relative atomic mass
2024 · 1
Interpret nuclide notation

Going deeper

SyllabusCIE 9701 2025-2026 SyllabusReference page 12

What's Next

Mastery of the atomic nucleus and isotopes is the foundation for all subsequent topics in chemistry. This knowledge underpins the study of electron configuration, which explains how elements bond and react, and is essential for understanding mass spectrometry, a key technique used to identify isotopes and organic compounds. Calculating relative atomic mass is also a core skill required for all mole and stoichiometry calculations, which appear throughout the CIE A-Level Chemistry syllabus. This topic also introduces the concepts you will need for the further study of radioactivity in nuclear chemistry later in the course.