Atomic Structure

This lesson covers the internal structure of the atom — the three sub-atomic particles and how they are arranged — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to know the relative mass and charge of protons, neutrons and electrons, and be able to use atomic number and mass number to determine the number of each particle in any atom.

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Sub-Atomic Particles

Atoms are made of three types of sub-atomic particle: protons, neutrons and electrons.

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

Key Points

• Protons and neutrons are found in the nucleus at the centre of the atom. Together they are called nucleons.
• Electrons orbit the nucleus in shells (energy levels).
• The nucleus is very small compared to the atom — the atom is mostly empty space.
• The nucleus contains almost all of the atom's mass because protons and neutrons each have a relative mass of 1, while electrons have negligible mass.
• The nucleus has an overall positive charge due to its protons. Neutrons have no charge.
• In a neutral atom, the number of electrons equals the number of protons, so the positive and negative charges balance and the atom has no overall charge.

Exam Tip: Learn the table of sub-atomic particles thoroughly — it is tested in almost every exam series. Remember: protons and neutrons are in the nucleus; electrons are in shells around it.

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Atomic Number and Mass Number

Every element is defined by two key numbers:

Atomic Number (Z)

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

Mass Number (A)

• The mass number is the total number of protons and neutrons in the nucleus.
• Mass number = number of protons + number of neutrons.
• It is also called the nucleon number.

Standard Notation

An element can be written as:

ᴬ_Z X where A is the mass number (top), Z is the atomic number (bottom), and X is the element symbol.

For example, sodium: ²³₁₁Na

• Atomic number = 11 → 11 protons, 11 electrons
• Mass number = 23 → 23 nucleons
• Neutrons = 23 − 11 = 12

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Calculating the Number of Sub-Atomic Particles

Quantity	How to Calculate
Number of protons	= atomic number (Z)
Number of electrons	= atomic number (Z) in a neutral atom
Number of neutrons	= mass number (A) − atomic number (Z)

Worked Examples

Example 1: Carbon (C) — Atomic number = 6, Mass number = 12

• Protons = 6
• Electrons = 6
• Neutrons = 12 − 6 = 6

Example 2: Sodium (Na) — Atomic number = 11, Mass number = 23

• Protons = 11
• Electrons = 11
• Neutrons = 23 − 11 = 12

Example 3: Chlorine (Cl) — Atomic number = 17, Mass number = 35

• Protons = 17
• Electrons = 17
• Neutrons = 35 − 17 = 18

Example 4: Iron (Fe) — Atomic number = 26, Mass number = 56

• Protons = 26
• Electrons = 26
• Neutrons = 56 − 26 = 30

Exam Tip: A common mistake is confusing atomic number and mass number. Remember: atomic number is the smaller number, and it goes at the bottom. Mass number is the larger number, and it goes at the top. Think: "A is Above, Z is below."

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The Size of Atoms

Feature	Value
Radius of an atom	~0.1 nm (1 × 10⁻¹⁰ m)
Radius of the nucleus	~1 × 10⁻¹⁴ m
Ratio	The atom is about 10,000 times larger than the nucleus

If an atom were the size of a football stadium, the nucleus would be the size of a marble at the centre. The atom is mostly empty space.

Comparing the Atom and the Nucleus

Feature	Atom	Nucleus
Radius	~0.1 nm	~1 × 10⁻¹⁴ m
Mass	Very small overall	Contains almost all the mass
Charge	Neutral (overall)	Positive (due to protons)

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Structure of the Atom — Diagram

graph TD
    A["Atom"] --> B["Nucleus<br/>(centre)"]
    A --> C["Electron Shells<br/>(around 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

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Why Atoms Are Electrically Neutral

In a neutral atom, the number of protons (each with charge +1) equals the number of electrons (each with charge −1):

• Total positive charge = number of protons × (+1)
• Total negative charge = number of electrons × (−1)
• Since protons = electrons → overall charge = zero

Exam Tip: If asked why an atom is neutral, always explain that the number of positive protons equals the number of negative electrons, so the charges cancel out. Simply stating "it has no charge" is not a sufficient answer.

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Summary

• Atoms contain three sub-atomic particles: protons (+1 charge, mass 1), neutrons (0 charge, mass 1) and electrons (−1 charge, negligible mass).
• Protons and neutrons are in the nucleus; electrons orbit in shells.
• Atomic number (Z) = number of protons; mass number (A) = protons + neutrons.
• Number of neutrons = mass number − atomic number.
• In a neutral atom, the number of protons equals the number of electrons.
• The nucleus is about 10,000 times smaller than the atom — the atom is mostly empty space.
• The atomic number uniquely identifies an element.

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