Atomic Structure and Isotopes

This lesson covers the structure of the atom, the properties of subatomic particles, atomic number, mass number and isotopes. This is part of the AQA GCSE Combined Science Trilogy specification (8464, section 6.4.1 — Atoms and Isotopes).

---

The Structure of the Atom

Every atom consists of a small, dense nucleus at the centre, surrounded by electrons orbiting in shells (energy levels).

Part of the atom	Contains	Charge	Relative mass
Nucleus	Protons and neutrons	Positive (due to protons)	Nearly all the mass of the atom
Electron shells	Electrons	Negative	Very little mass

Subatomic Particles

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

---

Size of the Atom

• The radius of an atom is approximately $1 \times 10^{-10}$1×10−10 m (about 0.1 nm or 1 ångström).
• The radius of the nucleus is approximately $1 \times 10^{-14}$1×10−14 m — about 10 000 times smaller than the atom.
• Most of the atom is empty space — the electrons orbit at relatively large distances from the tiny nucleus.

graph TD
    A["Atom<br/>radius ≈ 10⁻¹⁰ m"] --> B["Nucleus<br/>radius ≈ 10⁻¹⁴ m<br/>(protons + neutrons)"]
    A --> C["Electron shells<br/>(electrons orbit at<br/>large distances)"]
    A --> D["Mostly empty space"]

---

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

The number of neutrons = mass number − atomic number = $A - Z$A−Z.

How to Read Nuclear Notation

$^{A}_{Z}X$ZA​X

For example, carbon-12: $^{12}_{6}\text{C}$612​C has 6 protons, 6 neutrons and (in a neutral atom) 6 electrons.

---

Atoms and Ions

In a neutral atom, the number of electrons equals the number of protons — the charges balance.

If an atom gains or loses electrons, it becomes an ion:

• Losing electrons → positive ion (more protons than electrons)
• Gaining electrons → negative ion (more electrons than protons)

Exam Tip: The number of protons never changes in chemical reactions — only electrons are gained or lost. If the number of protons changed, it would become a different element.

---

What Are Isotopes?

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons (and therefore different mass numbers).

Isotope	Protons	Neutrons	Mass number
Carbon-12 ($^{12}_{6}\text{C}$612​C)	6	6	12
Carbon-13 ($^{13}_{6}\text{C}$613​C)	6	7	13
Carbon-14 ($^{14}_{6}\text{C}$614​C)	6	8	14

Key Points About Isotopes

• Isotopes have the same chemical properties (same number of electrons, same reactions).
• Isotopes have different physical properties (different mass, different density).
• Some isotopes are stable, others are unstable (radioactive) — unstable isotopes undergo radioactive decay.

---

The Development of the Atomic Model

The model of the atom has changed over time as new evidence was discovered:

Date	Scientist/Model	Description
Before 1897	Dalton's model	Atoms are tiny solid spheres that cannot be divided
1897	Thomson's plum pudding model	Atom is a ball of positive charge with electrons embedded in it (like plums in a pudding)
1909	Rutherford's alpha scattering experiment	Showed the atom is mostly empty space with a small, dense, positive nucleus
1913	Bohr's model	Electrons orbit the nucleus at specific distances in fixed energy levels (shells)
Modern	Quantum model	Electrons exist in probability clouds (orbitals) rather than fixed orbits

Rutherford's Alpha Scattering Experiment

Rutherford fired alpha particles at thin gold foil and observed:

• Most alpha particles passed straight through → the atom is mostly empty space.
• Some were deflected at large angles → there is a small, dense, positive nucleus that repels the positive alpha particles.
• Very few bounced straight back → the nucleus is very small but contains most of the atom's mass.

Exam Tip: AQA commonly asks you to describe the evidence from alpha scattering and explain how it led to the nuclear model. Make sure you link each observation to a conclusion about atomic structure.

---

Common Misconceptions

Misconception	Correction
Isotopes are different elements	Isotopes are the same element — they have the same number of protons but different numbers of neutrons
Electrons are found inside the nucleus	Electrons orbit outside the nucleus in shells
The nucleus makes up most of the atom's volume	The nucleus is tiny — most of the atom is empty space
The atomic model has always been the same	The model has been revised many times as new evidence was discovered

---

Summary

• Atoms have a small, dense, positive nucleus (protons + neutrons) surrounded by electrons in shells.
• Atomic number = number of protons; mass number = protons + neutrons.
• Isotopes = atoms of the same element with different numbers of neutrons.
• Some isotopes are radioactive (unstable).
• The atomic model developed from Dalton → Thomson → Rutherford → Bohr → quantum model.
• Rutherford's alpha scattering experiment provided evidence for the nuclear model.

---

Worked Example 1 — Counting Subatomic Particles

An atom is represented as $^{35}_{17}\text{Cl}$1735​Cl. How many protons, neutrons and electrons does it have (assuming it is a neutral atom)?

• Atomic number $Z = 17$Z=17 → 17 protons.
• Mass number $A = 35$A=35 → neutrons = 35 − 17 = 18 neutrons.
• Neutral atom → 17 electrons.

---

Worked Example 2 — An Ion

The fluoride ion $^{19}_{9}\text{F}^-$919​F− has an extra electron. How many of each subatomic particle does it contain?