# Inside the Atom

> Elements and atoms, how the model of the atom changed over time, and the nucleus and electron cloud.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/atomic-structure/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The gold foil surprise
In 1909, in a lab in Manchester, a student named Ernest Marsden was firing tiny positive particles (alpha particles) at a sheet of gold foil only a few atoms thick. Everyone expected them to zoom straight through, like bullets through paper.

Most did. But about 1 in 8000 bounced back towards him.

His supervisor, Ernest Rutherford, later said it was "as if you fired a 15 inch shell at a piece of tissue paper and it came back and hit you". Something tiny, heavy and positive was hiding inside every atom. They had found the **nucleus**.

**In short:** An atom is the smallest particle of an element. It has a tiny, dense, positive nucleus of protons and neutrons that holds nearly all its mass, with light, negative electrons moving in shells around it, and the atom is mostly empty space.

## Matter, elements and atoms

Everything around you is **matter**: anything that has mass and takes up space.

- An **atom** is the smallest particle of an element that still behaves like that element.
- An **element** is a pure substance made of only one kind of atom. Examples: carbon, oxygen, gold. There are 118 known elements.
- A **compound** is two or more elements **chemically joined** in a fixed ratio, like water ($\mathrm{H_2O}$) or table salt ($\mathrm{NaCl}$).
- A **molecule** is two or more atoms joined together. $\mathrm{O_2}$ is a molecule of an element. $\mathrm{H_2O}$ is a molecule of a compound.

Atoms are tiny. About ten million carbon atoms side by side would span one millimetre.

## Two laws that led to the atom

Around 1800, careful measurements gave chemists two laws that every reaction obeyed.

1. **Law of conservation of mass** (Lavoisier): mass is not gained or lost in a chemical reaction.
2. **Law of definite proportions** (Proust): a compound always contains the same elements in the same proportions by mass. Water is always about 11% hydrogen and 89% oxygen by mass.

In 1803 John Dalton asked: what must matter be like for these laws to be true? His answer was **Dalton's atomic theory**.

| Dalton's idea | Still true today? |
|---|---|
| 1. All matter is made of tiny particles called atoms | Yes |
| 2. Atoms cannot be created, destroyed or split | **No**: atoms contain smaller particles, and nuclear reactions can split them |
| 3. All atoms of one element are identical in mass | **No**: isotopes of an element have different masses |
| 4. In reactions, atoms are rearranged, not created or destroyed | Yes (this explains conservation of mass) |
| 5. Atoms combine in fixed whole number ratios to form compounds | Yes (this explains definite proportions) |

Dalton's theory was not perfect, but it turned atoms from a philosopher's guess into a scientific idea that could be tested.

## How our idea of the atom changed

No one has seen an atom with their eyes. Scientists built **models** from evidence, and changed the model when new evidence came in.

| Year | Scientist | Model | Key evidence |
|---|---|---|---|
| 1803 | John Dalton | Atoms are tiny solid balls that cannot be split. Each element has its own kind of atom. | Elements combine in fixed mass ratios |
| 1897 | J. J. Thomson | **Plum pudding**: a ball of positive charge with negative electrons dotted through it | Discovered the electron |
| 1911 | Ernest Rutherford | **Nuclear model**: a tiny, dense, positive nucleus with electrons around it. The atom is mostly empty space. | Gold foil experiment |
| 1913 | Niels Bohr | **Planetary model**: electrons move in fixed **energy levels** (shells) around the nucleus | Elements give off light of only certain colours |
| 1932 | James Chadwick | Nucleus contains **neutrons** as well as protons | Found a neutral particle in the nucleus |

### Key idea: Why the gold foil result mattered
- **Most** alpha particles went straight through, so the atom is **mostly empty space**.
- **Some** were deflected, so the centre is **positive** (it repelled the positive alpha particles).
- **A very few** bounced back, so the centre is **tiny and very dense**.

## The modern picture

An atom has two parts.

1. The **nucleus**: in the centre, tiny, holds almost all the mass. It contains **protons** (positive) and **neutrons** (no charge).
2. The **electrons**: very light, negative, moving around the nucleus in **shells**.

The nucleus is about 100 000 times smaller than the whole atom. If an atom were the size of a football stadium, the nucleus would be a marble on the centre spot.

*Diagram: Bohr model of a lithium atom: a nucleus of 3 protons and 4 neutrons with 2 electrons in the first shell and 1 in the second*
*A lithium atom in the Bohr model. Not to scale: the real nucleus is far smaller.*

### Watch out: Models are not pictures
The Bohr diagram is a useful **model**, not a photo. Electrons do not really travel in neat circles. We draw them that way because it helps us count electrons and predict how atoms bond.

### Exam tip: Learn the order of the models
Exams love "put these models in order" questions. Remember: **D**alton, **T**homson, **R**utherford, **B**ohr, **C**hadwick. "Dogs Tease Rabbits, Bunnies Chase."

**Mind map**

```text
The atom
  Words
    Element: one kind of atom
    Compound: elements joined
    Molecule: atoms bonded
  Models over time
    Dalton: solid ball
    Thomson: plum pudding
    Rutherford: nucleus
    Bohr: shells
    Chadwick: neutron
  Nucleus
    Protons +
    Neutrons 0
    Tiny, holds the mass
  Electrons
    Negative, very light
    Move in shells
```

### Year 11: From shells to clouds
Bohr's model explained hydrogen well but failed for bigger atoms. In the 1920s, scientists such as Erwin Schrödinger built the **quantum mechanical model**.

In this model, an electron does not have an exact path. Instead there is an **orbital**: a region of space where the electron is likely to be found. Orbitals have shapes. The **s** orbital is a sphere. The **p** orbitals are shaped like dumbbells.

The shells of the Bohr model are still useful. Each shell is split into **subshells** (s, p, d, f), which you will meet in electron configuration.

### Year 12: Evidence from light: emission spectra
When atoms are heated, electrons absorb energy and jump to a higher energy level. When they fall back, they give out light of an exact energy.

$$E = hf$$

where $h = 6.63 \times 10^{-34} \text{ J s}$ (Planck's constant) and $f$ is the frequency. Each jump gives one line of colour, so each element has its own **line emission spectrum**, like a barcode. This is how we know what distant stars are made of, and it was Bohr's main evidence for fixed energy levels.

Flame tests use the same idea: sodium burns yellow-orange, copper burns blue-green, potassium burns lilac.

## Quick summary

- An **atom** is the smallest particle of an element. An **element** has one kind of atom. A **compound** has elements chemically joined.
- Models changed with evidence: Dalton, Thomson, Rutherford, Bohr, Chadwick.
- The gold foil experiment showed the atom is mostly empty space with a tiny, dense, positive nucleus.
- The nucleus holds protons and neutrons and nearly all the mass.
- Electrons are negative, very light, and move in shells around the nucleus.

## Quiz

Try these questions from the chapter quiz.

1. An element is:
   - A. A mixture of two substances
   - B. A pure substance made of only one kind of atom
   - C. Two elements chemically joined
   - D. Any liquid
   - Answer: B. A pure substance made of only one kind of atom. Each element has its own type of atom.

2. Which is a compound?
   - A. $\mathrm{O_2}$
   - B. Gold
   - C. $\mathrm{H_2O}$
   - D. Neon
   - Answer: C. $\mathrm{H_2O}$. Water contains two different elements chemically joined.

3. Who proposed the plum pudding model?
   - A. Dalton
   - B. Thomson
   - C. Rutherford
   - D. Bohr
   - Answer: B. Thomson. Thomson discovered the electron and pictured it dotted through positive charge.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/atomic-structure/#quiz