The Atom and the Nucleus

Physics· 5.1.1, 5.1.2 (2026-2028 syllabus)· 15 min read

1. Core: Atomic Structure and Subatomic Particles★★☆☆☆⏱ 4 min

📘 Definition

Atom

The smallest particle of an element that can take part in a chemical reaction, consisting of a central positively charged nucleus surrounded by negatively charged orbiting electrons.

All atoms are made of three fundamental subatomic particles: protons, neutrons, and electrons. Protons and neutrons are tightly packed in the central nucleus, which makes up nearly all of the atom's mass but occupies less than 1/10,000 of its total volume. Electrons orbit the nucleus in discrete energy levels (shells) at large distances relative to the size of the nucleus.

Subatomic Particle	Relative Charge	Relative Mass
Proton	+1	1
Neutron	0	1
Electron	-1	~1/1840 (negligible)

📐 Worked Example

State the relative charge and mass of a neutron, and identify where it is located in an atom.

1
1. Recall subatomic particle properties from the reference table.
2
1. Relative charge: Neutrons have no charge, so relative charge = 0.
3
1. Relative mass: Neutrons have the same mass as protons, so relative mass = 1.
4
1. Location: Neutrons are found in the central nucleus of the atom.

Exam tip:

You will be expected to memorise the relative charge and mass of all three subatomic particles for both core and extended exams. Do not mix up the negligible mass of electrons with the mass of protons/neutrons.

2. Core: Atomic Notation and Isotopes★★☆☆☆⏱ 4 min

📘 Definition

Proton Number (Atomic Number, Z)

The number of protons in the nucleus of an atom of an element, unique to each element and determining its position in the periodic table.

📘 Definition

Nucleon Number (Mass Number, A)

The total number of protons and neutrons (collectively called nucleons) in the nucleus of an atom.

Atoms are electrically neutral because the number of positively charged protons is equal to the number of negatively charged electrons. The number of neutrons in an atom of the same element can vary.

📘 Definition

Isotope

Atoms of the same element (same proton number Z) that have different numbers of neutrons (different nucleon number A).

Isotopes have identical chemical properties because chemical reactions depend only on the arrangement of electrons, which is the same for all isotopes of the same element. They may have different physical properties, such as density or radioactivity.

📐 Worked Example

Carbon has two common isotopes: carbon-12 and carbon-14. Carbon has a proton number of 6. State the number of protons, neutrons, and electrons in each isotope.

1
1. For carbon-12: Proton number Z = 6, so number of protons = 6.
2
1. Neutral atom, so number of electrons = number of protons = 6.
3
1. Nucleon number A = 12, so number of neutrons = A - Z = 12 - 6 = 6.
4
1. For carbon-14: Proton number is still 6, so protons = 6, electrons = 6.
5
1. Nucleon number = 14, so neutrons = 14 - 6 = 8.

Exam tip:

When asked to explain why isotopes have the same chemical properties, always reference that they have the same number of electrons (and same electron arrangement) – this is the mandatory marking point.

3. Extended Only: Evidence for the Nuclear Model★★★☆☆Extended only⏱ 4 min

Before the nuclear model was proposed, scientists used the 'plum pudding' model of the atom, which described the atom as a sphere of positive charge with negatively charged electrons spread evenly through it, like plums in a pudding. The Geiger-Marsden experiment (also called the alpha scattering experiment) disproved this model and provided evidence for the nuclear model.

📐 Worked Example

Describe the three key observations from the Geiger-Marsden alpha scattering experiment, and explain what each observation tells us about the structure of the atom.

1
1. Observation 1: Most alpha particles passed straight through the gold foil without deflection. Conclusion: Most of the volume of the atom is empty space.
2
1. Observation 2: A small number of alpha particles were deflected through large angles (more than 90 degrees). Conclusion: The atom has a very small, central, positively charged nucleus that contains most of the atom's mass.
3
1. Observation 3: Very few alpha particles bounced straight back towards the source. Conclusion: The nucleus is extremely small compared to the total size of the atom, and has a very high mass and positive charge.

Exam tip:

When answering questions about the alpha scattering experiment, always link each observation explicitly to its conclusion – do not list observations without explaining what they prove, as you will lose marks.

4. Extended Only: The Nucleus in More Detail — Fission and Fusion★★★★☆Extended only⏱ 6 min

This section covers Supplement-only content for Extended (Paper 2/4) candidates. Core (Paper 1/3) candidates may skip it.

Because each proton carries a relative charge of $+ 1$ and each neutron is neutral, the relative charge on a nucleus is equal to its proton number $Z$ . Because each proton and each neutron has a relative mass of $1$ (and electrons are negligible), the relative mass of a nucleus is approximately equal to its nucleon number $A$ .

📘 Definition

Nuclear Fission

The splitting of a large (heavy) unstable nucleus into two smaller nuclei, usually releasing two or three neutrons and a large amount of energy.

Example:

A representative fission reaction: a uranium-235 nucleus absorbs a neutron, becomes unstable, and splits.

📐 Worked Example

Write a representative nuclide equation for the nuclear fission of uranium-235 after it absorbs a slow neutron, and describe the mass and energy changes qualitatively.

1
1. A slow neutron $_{0}^{1} n$ is absorbed by the uranium-235 nucleus, which becomes unstable and splits into two smaller (daughter) nuclei plus more neutrons. The exact daughter nuclei vary; one common outcome is barium and krypton.
2
${}^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n}$
3
1. Check the balance: nucleon numbers $235 + 1 = 141 + 92 + 3$ ; proton numbers $92 + 0 = 56 + 36 + 0$ . Both balance.
4
1. Mass and energy (qualitative, no values): the total mass after fission is slightly less than before, and this small loss of mass is released as energy. The released neutrons can go on to cause further fission (a chain reaction).

📘 Definition

Nuclear Fusion

The joining of two light (small) nuclei to form a heavier nucleus, releasing a large amount of energy. Fusion requires very high temperatures and pressures.

Example:

Fusion of hydrogen nuclei is the process that powers the Sun and other stars.

📐 Worked Example

Write a representative nuclide equation for the nuclear fusion of a deuterium nucleus and a tritium nucleus (two isotopes of hydrogen), and describe the mass and energy changes qualitatively.

1
1. Two light hydrogen nuclei join to form a helium nucleus plus a neutron.
2
${}^{2}_{1}\text{H} + {}^{3}_{1}\text{H} \rightarrow {}^{4}_{2}\text{He} + {}^{1}_{0}\text{n}$
3
1. Check the balance: nucleon numbers $2 + 3 = 4 + 1$ ; proton numbers $1 + 1 = 2 + 0$ . Both balance.
4
1. Mass and energy (qualitative, no values): the mass of the helium nucleus plus the neutron is slightly less than the total mass of the original two nuclei, and this small loss of mass is released as energy.

Exam tip:

Remember the direction: fission = a big nucleus splitting; fusion = small nuclei joining. Both release energy from a small loss of mass, and in both the nucleon numbers and the proton numbers must balance in the nuclide equation.

5. Summary and Key Takeaways★☆☆☆☆⏱ 2 min

Section summary

▸
All atoms contain a central positively charged nucleus (protons + neutrons) surrounded by orbiting negatively charged electrons.
▸
Proton number (Z) = number of protons; nucleon number (A) = number of protons + neutrons.
▸
Isotopes are atoms of the same element with different numbers of neutrons, identical chemical properties, and different physical properties.
▸
Extended only: The Geiger-Marsden alpha scattering experiment provides evidence for the nuclear model of the atom.
▸
Extended only: A nucleus has a relative charge equal to its proton number Z and a relative mass approximately equal to its nucleon number A.
▸
Extended only: Nuclear fission splits a heavy nucleus into smaller nuclei, and nuclear fusion joins light nuclei; both release energy from a small loss of mass (described qualitatively, without values).

6. Common Pitfalls

Wrong move:

Stating that electrons have a mass of 1, or that neutrons have a negative charge.

Why:

Mixing up subatomic particle properties is a common error that loses easy marks.

Correct move:

Memorise the reference table of relative charge and mass for all three particles, and test yourself regularly.

Wrong move:

Calculating number of neutrons as Z - A instead of A - Z.

Why:

Confusing nucleon number and proton number in the calculation leads to incorrect results.

Correct move:

Remember that nucleon number (A) is the sum of protons and neutrons, so subtract proton number (Z) from A to get the neutron count.

Wrong move:

Explaining identical chemical properties of isotopes by referencing same number of protons, not electrons.

Why:

Chemical reactions depend on electron arrangement, not proton count, so this answer will not get full marks.

Correct move:

Always explicitly state that isotopes have the same number of electrons and same electron arrangement when answering this question.

Wrong move:

Stating that most alpha particles were deflected in the Geiger-Marsden experiment.

Why:

Overstating the number of deflected particles leads to incorrect conclusions about atomic structure.

Correct move:

Remember that most alpha particles passed straight through, only a small number were deflected, and very few bounced back.

Wrong move:

Claiming the nucleus contains electrons.

Why:

Electrons orbit the nucleus outside; they are not found inside the nucleus.

Correct move:

Only protons and neutrons are located in the nucleus; electrons are in energy shells surrounding the nucleus.

7. Quick Reference Cheatsheet

Concept	Core/Extended	Key Fact
Subatomic particles	Core	Proton: +1 charge, 1 mass; Neutron: 0 charge, 1 mass; Electron: -1 charge, negligible mass
Proton number (Z)	Core	Number of protons, unique to each element, equals number of electrons in neutral atom
Nucleon number (A)	Core	Total number of protons + neutrons in nucleus
Isotopes	Core	Same Z, different A; same chemical properties, different physical properties
Geiger-Marsden experiment	Extended	Most alpha pass through = atom mostly empty; few deflected = small positive massive nucleus
Nucleus charge and mass	Extended	Relative charge of nucleus = proton number Z; relative mass of nucleus ≈ nucleon number A
Nuclear fission	Extended	Heavy nucleus splits into two smaller nuclei + neutrons; small mass loss released as energy (nuclide equation, no values)
Nuclear fusion	Extended	Light nuclei join into a heavier nucleus; small mass loss released as energy; powers stars (nuclide equation, no values)

8. Frequently Asked

What is the difference between nucleon number and proton number?

Proton number (Z) is the total number of protons in the nucleus, unique to each element. Nucleon number (A) is the total number of protons and neutrons in the nucleus.

Why do isotopes have identical chemical properties?

Chemical reactions depend only on the arrangement of electrons, which is identical for all isotopes of the same element, as they have the same number of protons and electrons.

Going deeper

Official SyllabusCIE IGCSE Physics 0625 2026-2028 Syllabus

What's Next

Now that you have mastered the structure of the atom and nucleus, you are ready to move on to radioactivity, the next core topic in the CIE IGCSE Physics 0625 nuclear physics unit. You will use your knowledge of isotopes to understand why some atoms are radioactive, and how radioactive decay works. For extended learners, you will also build on the Geiger-Marsden experiment to understand how nuclear radiation interacts with matter. Make sure you practice calculating subatomic particle counts for different isotopes before moving on, as this skill is essential for all subsequent nuclear physics topics.