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 Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. 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 an atom.

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Sub-Atomic Particles

Atoms are made up 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 sometimes called nucleons.
• Electrons are found in shells (energy levels) around the nucleus.
• The nucleus is very small compared to the overall size of the atom — the atom is mostly empty space.
• The nucleus contains almost all the mass of the atom because protons and neutrons have a relative mass of 1, whereas electrons have negligible mass.
• The nucleus has an overall positive charge (due to the protons). Neutrons have no charge.
• Electrons have a negative charge that exactly balances the positive charge of the protons, so atoms are overall electrically neutral.

Exam Tip: The Edexcel specification requires you to know the relative masses and charges of all three sub-atomic particles. Learn the table above thoroughly — it is tested in almost every exam series.

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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 of an atom.
• 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.
• On the periodic table, elements are arranged in order of increasing atomic number.

Mass Number (A)

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

How to Read the Periodic Table Notation

An element is often written as:

A (top left) Z (bottom left) X (element symbol)

For example, for sodium:

• Symbol: Na
• Atomic number (Z): 11 → 11 protons
• Mass number (A): 23 → 23 nucleons (protons + neutrons)

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Calculating Sub-Atomic Particles

Using the atomic number and mass number, you can work out the number of each sub-atomic particle in a neutral atom:

Quantity	How to Calculate
Number of protons	= atomic number (Z)
Number of electrons	= atomic number (Z) — in a neutral atom, protons = electrons
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 to confuse atomic number and mass number. Remember: atomic number is the smaller number (it goes at the bottom). The mass number is the larger number (it goes at the top). "A is Above, Z is below."

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The Size of Atoms

• The radius of an atom is approximately 0.1 nm (1 × 10⁻¹⁰ m).
• The radius of the nucleus is approximately 1 × 10⁻¹⁴ m — about 10,000 times smaller than the atom itself.
• This means the atom is mostly empty space — if an atom were the size of a football stadium, the nucleus would be the size of a marble at the centre.

Comparing the Nucleus and the Atom

Feature	Atom	Nucleus
Radius	~0.1 nm (1 × 10⁻¹⁰ m)	~1 × 10⁻¹⁴ m
Relative size	1	~1/10,000 of the atom
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 Description

The atom can be visualised as a central nucleus containing protons (positive) and neutrons (neutral), surrounded by electrons (negative) in shells at various distances from the nucleus.

graph TD
    A["Atom"] --> B["Nucleus<br/>(centre of the atom)"]
    A --> C["Electron Shells<br/>(around the nucleus)"]
    B --> D["Protons<br/>Relative mass: 1<br/>Relative charge: +1"]
    B --> E["Neutrons<br/>Relative mass: 1<br/>Relative charge: 0"]
    C --> F["Electrons<br/>Relative mass: ~0<br/>Relative 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

Here is a worked model of a lithium atom (Li, atomic number 3, mass number 7) — showing the nucleus, the two electron shells, and the 2,1 electron configuration: