Rutherford, Bohr and Beyond: The Nuclear Atom

An experiment that shocked its inventor

In 1909, in a lab in Manchester, England, two researchers working for Ernest Rutherford, Hans Geiger and Ernest Marsden, fired tiny positive particles at a sheet of gold foil thinner than a strand of hair. Everyone expected the particles to zip straight through, because Thomson's plum pudding model spread the atom's positive charge out thinly.

Most did go straight through. But a very few bounced back. Rutherford later said it was almost as surprising as firing an artillery shell at a piece of tissue paper and having it come back and hit you. Something in the atom was small, heavy and powerful.

Remember from the last lesson

Dalton's model (1803) pictured a solid sphere. Thomson's model (1904) put negative electrons inside a sphere of positive charge. Each model explained the evidence of its time. Now new evidence will force bigger changes.

Rutherford's gold foil experiment

Ernest Rutherford was born in New Zealand and did important early research on radioactivity at McGill University in Montréal (1898–1907), work for which he won a Nobel Prize in 1908. He then moved to Manchester, where the gold foil experiment was done.

What they did: a radioactive source shot a beam of positively charged alpha particles at very thin gold foil. A screen around the foil flashed wherever a particle hit, so they could count where the particles went.

What they observed:

1. Most alpha particles passed straight through the foil.
2. A few were deflected (bent) at an angle.
3. Very few (about 1 in several thousand) bounced almost straight back.

What Rutherford concluded (1911)

Each observation led to a conclusion:

• Most went straight through → an atom is mostly empty space.
• A few were deflected → there is a concentrated positive charge that repels the positive alpha particles when they pass close to it.
• Very few bounced back → that positive part is tiny but very dense, containing almost all of the atom's mass. Only a direct hit makes a particle bounce back, and direct hits are rare because the target is so small.

Rutherford called this centre the nucleus. In his nuclear model, the electrons move in the empty space around a tiny positive nucleus. Later, Rutherford named the positive particles in the nucleus protons.

How small is the nucleus? If an atom were as big as a hockey arena, its nucleus would be smaller than a pea sitting at centre ice.

Bohr: electrons in energy levels (1913)

Rutherford's model had a problem. Physics predicted that electrons circling the nucleus should lose energy and spiral into it, so atoms should collapse. They don't.

The Danish physicist Niels Bohr used evidence from light. When an element is heated or has electricity passed through it, it gives off light. Passed through a prism or a spectroscope, this light is not a full rainbow. It is a set of separate bright lines of specific colours, called a line spectrum. Every element has its own pattern, like a fingerprint.

Bohr explained this:

• Electrons can only be in certain energy levels (also called shells or orbits) around the nucleus. They cannot be in between.
• When an atom absorbs energy, an electron jumps up to a higher level.
• When it falls back down, it gives off that exact amount of energy as light of one specific colour.
• Since the levels are fixed, only certain colours appear: a line spectrum.

This is why fireworks have different colours: compounds of different elements give off different colours of light.

Chadwick: the neutron (1932)

There was still a puzzle. Atoms had more mass than their protons and electrons could account for. A helium atom has 2 protons but about 4 times the mass of a hydrogen atom, which has 1 proton. In 1932, the British physicist James Chadwick found a particle in the nucleus with no charge and about the same mass as a proton: the neutron. This completed the model you will use in this course: protons and neutrons in the nucleus, electrons in energy levels around it. It is called the Bohr-Rutherford model.

Today: the quantum mechanical model

In the 1920s, scientists including Erwin Schrödinger showed that we cannot know an electron's exact path. Instead, the quantum mechanical model describes regions around the nucleus where an electron is likely to be found, often drawn as fuzzy electron clouds. You will study this model in Grade 11 and 12 chemistry. For Grade 9, the Bohr-Rutherford model is the right tool: it explains a great deal and is easy to draw.