Atomic Structure

This lesson covers the nuclear model of the atom — the arrangement of protons, neutrons and electrons, and the key numbers used to describe atoms — as required by the Edexcel GCSE Physics specification (1PH0), Topic 6: Radioactivity. You need to understand the structure of the atom, the meaning of atomic number and mass number, what isotopes are, and the relative sizes of atoms and nuclei.

The Nuclear Model of the Atom

All matter is made of atoms. Each atom consists of a central nucleus surrounded by electrons that orbit in shells (energy levels).

The Nucleus

The nucleus is at the centre of the atom.
It contains two types of particle: protons and neutrons (collectively called nucleons).
The nucleus is positively charged because protons are positive and neutrons are neutral.
The nucleus contains almost all the mass of the atom.
The nucleus is extremely small compared to the overall size of the atom.

Electrons

Electrons orbit the nucleus in shells (energy levels) at different distances from the nucleus.
Electrons have a negative charge.
Electrons have negligible mass compared to protons and neutrons.

Sub-Atomic Particles

Sub-Atomic Particle	Relative Mass	Relative Charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	Very small (~1/1836)	−1	Shells around the nucleus

Key Facts

Protons and neutrons have approximately the same mass (relative mass = 1).
Electrons have a much smaller mass — so small that it is often considered negligible at GCSE level.
The charge on a proton (+1) is equal in size but opposite in sign to the charge on an electron (−1).
Neutrons carry no charge — they are electrically neutral.

Exam Tip: You must memorise the relative mass and charge of all three sub-atomic particles. This is one of the most commonly tested facts across all GCSE physics and chemistry papers.

Why Atoms Are Electrically Neutral

In a neutral atom, the number of protons equals the number of electrons. The positive charges from the protons exactly cancel the negative charges from the electrons:

Total positive charge = number of protons × (+1)
Total negative charge = number of electrons × (−1)
If protons = electrons, then overall charge = zero

This is why atoms have no overall electrical charge.

Exam Tip: If a question asks why atoms are neutral, you must state that the number of protons equals the number of electrons, so the positive and negative charges cancel out. Simply saying "it has no charge" without explanation will not earn full marks.

Atomic Number and Mass Number

Every atom is described by two key numbers:

Atomic Number (Z) — also called the Proton Number

The atomic number is the number of protons in the nucleus.
It defines which element the atom is — all atoms of the same element have the same atomic number.
In a neutral atom, the atomic number also equals the number of electrons.

Mass Number (A) — also called the Nucleon Number

The mass number is the total number of protons and neutrons in the nucleus.
Mass number = protons + neutrons

Calculating the Number of Neutrons

Number of neutrons = mass number − atomic number = A − Z

Worked Examples

Example 1: Carbon-12

Atomic number (Z) = 6, Mass number (A) = 12
Protons = 6
Electrons = 6 (neutral atom)
Neutrons = 12 − 6 = 6

Example 2: Uranium-238

Atomic number (Z) = 92, Mass number (A) = 238
Protons = 92
Electrons = 92 (neutral atom)
Neutrons = 238 − 92 = 146

Example 3: Helium-4

Atomic number (Z) = 2, Mass number (A) = 4
Protons = 2
Electrons = 2 (neutral atom)
Neutrons = 4 − 2 = 2

Exam Tip: Remember the formula: neutrons = mass number − atomic number. The mass number is always the larger number. A useful mnemonic: "A is Above, Z is below" — referring to how they are written in nuclear notation.

Isotopes

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.

Because isotopes have the same number of protons (and therefore the same number of electrons in a neutral atom), they have the same chemical properties. However, they have different masses because they have different numbers of neutrons.

Examples of Isotopes

Isotope	Protons	Neutrons	Mass Number
Carbon-12	6	6	12
Carbon-13	6	7	13
Carbon-14	6	8	14
Hydrogen-1 (protium)	1	0	1
Hydrogen-2 (deuterium)	1	1	2
Hydrogen-3 (tritium)	1	2	3
Uranium-235	92	143	235
Uranium-238	92	146	238

Key Points About Isotopes

Same proton number (atomic number) — so they are the same element.
Different mass number — because they have a different number of neutrons.
Same chemical properties — because chemical behaviour depends on electron configuration, which is the same.
Different physical properties — for example, different mass, different density.
Some isotopes are stable, while others are unstable (radioactive).

Exam Tip: A very common exam mistake is to say isotopes have a different number of protons. They do NOT. Isotopes have the SAME number of protons but a DIFFERENT number of neutrons. This changes the mass number but not the atomic number.

Size of the Atom and the Nucleus

Understanding the relative sizes of atoms and nuclei is important:

The radius of an atom is approximately 1 × 10⁻¹⁰ m (0.1 nm).
The radius of a nucleus is approximately 1 × 10⁻¹⁴ m.
The nucleus is therefore about 10,000 times smaller than the atom.
The atom is mostly empty space.

Putting This in Perspective

If an atom were scaled up to the size of a football stadium (~100 m across), the nucleus would be the size of a marble (~1 cm across) sitting at the centre. The electrons would be tiny specks orbiting somewhere in the stands.

Feature	Approximate Radius	Comparison
Atom	~1 × 10⁻¹⁰ m	Football stadium
Nucleus	~1 × 10⁻¹⁴ m	Marble at the centre
Ratio	Atom is ~10,000× larger	—

Structure of the Atom — Diagram

graph TD
    A["Atom"] --> B["Nucleus<br/>(centre, very small, dense)"]
    A --> C["Electron Shells<br/>(surround the nucleus)"]
    B --> D["Protons<br/>Mass: 1, Charge: +1"]
    B --> E["Neutrons<br/>Mass: 1, Charge: 0"]
    C --> F["Electrons<br/>Mass: ~0, Charge: −1"]

    style A fill:#2c3e50,color:#fff
    style B fill:#c0392b,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#95a5a6,color:#fff
    style F fill:#3498db,color:#fff

Summary

Atoms have a nucleus (protons + neutrons) surrounded by electrons in shells.
Protons: charge +1, relative mass 1, found in the nucleus.
Neutrons: charge 0, relative mass 1, found in the nucleus.
Electrons: charge −1, negligible mass, found in shells around the nucleus.
Atomic number (Z) = number of protons; defines the element.
Mass number (A) = protons + neutrons.
Neutrons = A − Z.
In a neutral atom, protons = electrons → the atom is electrically neutral.
Isotopes are atoms of the same element with the same proton number but different numbers of neutrons (different mass number).
The atom radius is ~10⁻¹⁰ m; the nucleus radius is ~10⁻¹⁴ m — the atom is mostly empty space.