7.6Chapter 7: Inside Atoms

Chapter 7 Review: Inside Atoms — Alhussan Chemistry - Stage 8 Revision Notes & Topical Questions

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Lesson summary

7.6 Chapter 7 Review: Inside Atoms — Alhussan International Academy — Chemistry (Stages 7–9) notes: Revise atomic structure, atomic and mass numbers, isotopes and the development of atomic models.Nucleus — it contains protons (charge +1, mass 1) and neutrons (charge 0, mass 1).El…

Neutral atom
Atomic number
Relative atomic mass
Improving models
Most particles pass through
Some are deflected

Part 1 of 5Structure of the atom

Structure of the atom

Nucleus

Nucleus it contains protons and neutrons (charge 0, mass 1).
- (charge +1, mass 1)

Electrons

Electrons they have charge -1 and negligible mass, and are arranged in shells around the nucleus.

Neutral atom

Neutral atom it has equal numbers of protons and electrons.

Shells

Shells the first shell holds 2 electrons and the next two hold up to 8.
Outer electrons decide chemical properties.

Models

Models analogies such as eggs, fruit and nuts can be used to model atoms, for example beryllium with 4 protons, 5 neutrons and 4 electrons has 9 nucleons.

Part 2 of 5Numbers, isotopes and ions

Atomic number

Atomic number the number of protons.

Mass number

Mass number protons + neutrons.

Neutrons = mass number - atomic number.

Isotopes

Isotopes they have the same atomic number but different mass numbers.

Ions

Ions they form when atoms gain or lose electrons.
Na⁺ has 11 protons and 10 electrons.
O^2- has 8 protons and 10 electrons.

Relative atomic mass

Relative atomic mass the weighted average mass of the isotopes.

Chlorine's value is 35.5 because it is a mixture of chlorine-35 and chlorine-37 in the ratio of their abundance.

Part 3 of 5History of the atom

Date	Scientist	Contribution
1803	Dalton	Solid indivisible atoms
1897	Thomson	Discovered the electron; plum pudding model
1909 to 1911	Rutherford, Geiger, Marsden	Gold foil experiment; nuclear model
1913	Bohr	Electrons in shells
1932	Chadwick	Discovered the neutron

Improving models
- each step came from new experimental evidence, so models change when evidence does not fit.

Part 4 of 5Gold foil observations

Gold foil observations

Most particles pass through

Most particles pass through the atom is mostly empty space.

Some are deflected

Some are deflected the nucleus is positive and repels the positive alpha particles.

A few rebound

A few rebound the nucleus is dense and holds most of the mass.

Why Rutherford's model is better

Why Rutherford's model is better it explains the gold foil results, which Thomson's model cannot.

Part 5 of 5Forces in the atom

Attraction and repulsion

Attraction and repulsion opposite charges attract and like charges repel.

Electrons

Electrons they are held by attraction to the nucleus, and outer electrons are held more weakly so they are lost or shared easily.

Nucleus

Nucleus it is held together by the strong nuclear force.

Periodic table

Periodic table the number of protons orders the elements, and the number of outer electrons decides group properties and reactivity.

Key terms

Term	Meaning
Atomic number	the number of protons in an atom.
Isotope	an atom with the same protons but different neutrons.
Nucleus	the centre of the atom containing protons and neutrons.
Electron shell	an energy level for electrons.
Ion	a charged particle formed by gaining or losing electrons.

Avoid these mistakes

Mixing up atomic and mass numbers — atomic number counts protons only; mass number counts protons and neutrons.

Saying electrons are in the nucleus — the nucleus contains only protons and neutrons.

Not recognising the charge of an ion — a positive ion has lost electrons and a negative ion has gained them.

Mixing up the scientists — Thomson found the electron, Rutherford the nucleus and Chadwick the neutron.

Last updated: 13 October 2026