The Nuclear Model of the Atom

The nuclear model places a tiny, dense, positively charged nucleus at the atom’s centre with electrons orbiting far out, and it is the foundation for isotopes, decay equations and half-life. Examiners test it directly through the alpha-particle scattering experiment, the single most famous evidence question in 0625.

What is the nuclear model of the atom?

An atom has a tiny, dense, positively charged nucleus at its centre. Negatively charged electrons orbit the nucleus at relatively large distances. Almost all the atom’s mass sits in the nucleus, and almost all of its volume is empty space. The nucleus contains two types of particle, together called nucleons: protons and neutrons.

Particle	Relative charge	Relative mass	Location
Proton	$+1$	1	nucleus
Neutron	0	1	nucleus
Electron	$-1$	about $\dfrac{1}{2000}$	orbiting the nucleus

Two numbers define a nucleus. The proton number ($Z$) is the number of protons; it fixes the element and equals the number of electrons in a neutral atom. The nucleon number ($A$) is the total of protons plus neutrons. Neutrons $= A - Z$. An atom that loses electrons becomes a positive ion; gaining electrons makes it negative. The nucleus never changes during ionisation.

What did the alpha-scattering experiment prove?

Researchers fired alpha particles at very thin gold foil and detected where they went. Three observations, three conclusions: learn them as pairs.

Observation	Conclusion
Most alpha particles passed straight through	Most of the atom is empty space
A few deflected through small angles	A concentrated positive charge repels the (positive) alpha particles
A very small number bounced back (deflected $> 90^\circ$)	The nucleus is tiny, dense and contains most of the mass

This replaced the earlier “plum pudding” picture, in which charge spread evenly through the atom. Spread-out charge could never bounce an alpha particle backwards. Extended candidates must give both the observations and the matched conclusions; Core candidates need the resulting model.

Worked Exam Question

In the alpha-scattering experiment, about 1 in 8000 alpha particles deflected through more than $90^\circ$. (a) Explain what this observation shows about the atom. (3 marks) (b) State why alpha particles, rather than neutrons, were suitable for this experiment. (1 mark)

Solution. (a) Back-scattering needs a very large repulsive force. So the atom contains a concentrated region of positive charge. Because so few particles bounce back, that region (the nucleus) must be very small, and it must hold most of the atom’s mass to turn a fast alpha particle around. (b) Alpha particles are positively charged, so the positive nucleus repels them; neutrons are uncharged and would not deflect electrostatically.

Mark scheme:

• B1: charge in the atom is concentrated, not spread out.
• B1: that concentration is positive (repels alpha particles).
• B1: the nucleus is very small / most of the atom is empty space, and dense.
• B1: alpha particles are charged, so they experience the electrostatic force.

Common Mistakes

• Saying electrons deflect the alpha particles. Fix: electrons are about $8000\times$ lighter than alphas; only the nucleus can repel them. Name the nucleus.
• Observations without conclusions. Fix: write each as a pair, “most pass through, SO most of the atom is empty space”.
• Confusing $Z$ and $A$. Fix: $Z$ = protons (bottom number), $A$ = nucleons (top number). Neutrons $= A - Z$.
• “The nucleus gains electrons” in ionisation. Fix: ionisation only adds or removes orbital electrons; the nucleus is untouched.
• Calling the nucleus “big because it is heavy”. Fix: it is tiny, roughly 1/10,000 of the atom’s diameter, but contains nearly all the mass.

Exam Technique Tip

Scattering questions are marked observation-plus-inference. Use a two-column habit even in prose: one sentence of evidence, then “this shows that…” and the conclusion. Three such pairs is a complete answer to almost any version of this question, including the 6-mark extended-response form on Paper 4.

How This Is Examined

A CS subtopic. Papers 1 and 2 ask quick MCQs on particle charges, masses and the meaning of Z and A. Paper 3 (Core) tests the model itself and ion formation. Paper 4 (Extended) brings the full scattering experiment, usually 3-5 marks, occasionally as part of a 6-mark response on evidence for the nuclear atom. There is no practical-paper presence. Most Malaysian students meet a version of this in KSSM science first; the IGCSE difference is the demand for matched observation-conclusion pairs rather than the story alone.