Atomic Structure and Isotopes

This lesson covers the structure of the atom, the sub-atomic particles, atomic number, mass number, nuclear notation and isotopes. Understanding atomic structure is essential for the Radioactivity section of the Edexcel GCSE Combined Science specification (1SC0).

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The Structure of the Atom

An atom consists of a small, dense, positively charged nucleus surrounded by negatively charged electrons that orbit in shells (energy levels).

Part of the atom	Location	Detail
Nucleus	Centre	Contains protons and neutrons; very small compared with the atom; contains almost all the mass
Electron shells	Around the nucleus	Electrons orbit in shells at different energy levels

graph TD
    subgraph "Atom"
    N["Nucleus<br/>(protons + neutrons)"] --> E1["1st electron shell"]
    E1 --> E2["2nd electron shell"]
    E2 --> E3["3rd electron shell"]
    end

Size of the Atom

• The radius of an atom is about $1 \times 10^{-10}$1×10−10 m (0.1 nm).
• The radius of the nucleus is about $1 \times 10^{-14}$1×10−14 m — roughly 10 000 times smaller than the atom.
• Most of the atom is empty space.

Exam Tip: A very common exam question asks you to compare the size of the nucleus to the atom. State that the nucleus is about 10 000 times smaller than the atom, or that the atom is mostly empty space.

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Sub-Atomic Particles

Particle	Relative mass	Relative charge	Location
Proton	1	+1	Nucleus
Neutron	1	0 (neutral)	Nucleus
Electron	Very small (≈ 1/1836)	−1	Shells around the nucleus

Key Points

• An atom has equal numbers of protons and electrons, so the overall charge is zero.
• The mass of an electron is so small it is considered negligible compared with protons and neutrons.
• The number of protons defines which element an atom is.

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Atomic Number and Mass Number

Term	Symbol	Definition
Atomic number (proton number)	$Z$Z	The number of protons in the nucleus
Mass number (nucleon number)	$A$A	The total number of protons + neutrons in the nucleus

Calculating the Number of Neutrons

$\text{Number of neutrons} = A - Z = \text{Mass number} - \text{Atomic number}$Number of neutrons=A−Z=Mass number−Atomic number

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Nuclear Notation

Nuclear (isotope) notation shows the mass number and atomic number of an atom:

$^{A}_{Z}X$ZA​X

Where $X$X is the chemical symbol.

Examples

Element	Symbol	Atomic number (Z)	Mass number (A)	Protons	Neutrons	Electrons
Hydrogen	$^{1}_{1}\text{H}$11​H	1	1	1	0	1
Carbon-12	$^{12}_{6}\text{C}$612​C	6	12	6	6	6
Carbon-14	$^{14}_{6}\text{C}$614​C	6	14	6	8	6
Uranium-235	$^{235}_{92}\text{U}$92235​U	92	235	92	143	92

Exam Tip: In nuclear notation, the top number is always the mass number (the bigger number) and the bottom number is the atomic number (the smaller number). Remember: A is at the top (A for Above).

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Isotopes

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

Feature	Same in all isotopes	Different in isotopes
	Number of protons	Number of neutrons
	Number of electrons (in a neutral atom)	Mass number
	Chemical properties	Physical properties (e.g. density, stability)

Example: Isotopes of Carbon

Isotope	Protons	Neutrons	Mass number	Stable?
Carbon-12 ($^{12}_{6}\text{C}$612​C)	6	6	12	Yes
Carbon-13 ($^{13}_{6}\text{C}$613​C)	6	7	13	Yes
Carbon-14 ($^{14}_{6}\text{C}$614​C)	6	8	14	No — radioactive

Why Are Some Isotopes Radioactive?

If a nucleus has too many or too few neutrons relative to protons, it becomes unstable. An unstable nucleus will decay by emitting radiation to become more stable. These isotopes are called radioactive isotopes (or radioisotopes).

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The Development of the Atomic Model

The understanding of atomic structure has evolved over time:

Model	Scientist(s)	Key idea
Solid sphere	Dalton (early 1800s)	Atoms are tiny, indivisible solid spheres
Plum pudding	Thomson (1897)	Atom is a sphere of positive charge with electrons embedded in it (after discovering the electron)
Nuclear model	Rutherford (1911)	Most of the atom is empty space; the mass and positive charge are concentrated in a tiny nucleus
Bohr model	Bohr (1913)	Electrons orbit the nucleus in fixed shells (energy levels) at specific distances

graph LR
    A["Dalton<br/>Solid sphere"] --> B["Thomson<br/>Plum pudding"]
    B --> C["Rutherford<br/>Nuclear model"]
    C --> D["Bohr<br/>Electron shells"]

Rutherford's Alpha Scattering Experiment

Rutherford fired alpha particles at a thin gold foil and observed:

Observation	Conclusion
Most alpha particles passed straight through	Most of the atom is empty space
Some were deflected at small angles	The nucleus has a positive charge (repelling the positive alpha particles)
Very few bounced back (> 90°)	The nucleus is very small and very dense, containing most of the mass

Exam Tip: You need to know the three key observations from the alpha scattering experiment and what each one tells us about the atom. This is a very popular exam question (often 6 marks).

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Ions

An ion is a charged atom. It forms when an atom gains or loses electrons.

Change	Result
Atom loses electrons	Becomes a positive ion (cation) — more protons than electrons
Atom gains electrons	Becomes a negative ion (anion) — more electrons than protons

The number of protons does not change when an ion forms — only the number of electrons changes.

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Common Misconceptions