NCEA Level 1 · Year 11 · Science · 2 minutes

How we found out what's inside an atom

Solid ball, plum pudding, nucleus, shells: a hundred years of clues.

Key terms

Electron: A tiny negatively charged particle found by J J Thomson, smaller than an atom.
Nucleus: The tiny, dense, positive centre of an atom that holds almost all of its mass.
Alpha particle: A positive particle fired at thin gold foil in Rutherford's experiment.
Electron shell: A fixed energy level around the nucleus where electrons sit, also called an energy level.
Neutron: A particle found by James Chadwick, located in the nucleus.

Transcript

Everything around you is made of atoms, far too small to see. So how did scientists work out what's inside one? It took a hundred years of clues, and one very surprised New Zealander.

In the early eighteen hundreds, John Dalton proposed that atoms were tiny solid balls that couldn't be split. Every element had its own kind of atom. Simple, and a great start.

Then, in eighteen ninety seven, J J Thomson found something smaller: the electron, with a negative charge. So he pictured a ball of positive charge with electrons dotted through it, later nicknamed the plum pudding model.

Enter Ernest Rutherford, born near Nelson, now on our hundred dollar note. In nineteen oh nine, his team fired positive alpha particles at incredibly thin gold foil. Most went straight through. But a very few bounced right back.

Rutherford said it was like firing an artillery shell at tissue paper, and having it come back and hit you. His explanation: almost all of an atom's mass is packed into a tiny, positive nucleus, which repelled the positive particles. The rest is mostly empty space.

How tiny? If an atom were as big as a rugby field, the nucleus would be no bigger than a pea on the halfway line.

In nineteen thirteen, Niels Bohr, who had worked with Rutherford, added another big idea: electrons sit in fixed shells, or energy levels, around the nucleus. Later, James Chadwick found the neutron, also in the nucleus.

And that's the model you'll use. The first shell holds up to two electrons. For the first twenty elements, the next two shells hold up to eight each. So sodium, with eleven electrons, is two, eight, one. Sodium atoms easily lose that lonely outer electron, so sodium is very reactive.

So, for your exam: One: the gold foil experiment showed a tiny, dense, positive nucleus. Two: electrons sit in shells around it. Three: for the first twenty elements, fill them two, eight, eight. Not bad for a kid from Nelson.

Sources

Every fact in the video, and where it comes from.

1
britannica.com/science/atom
- Atoms are far too small to see; a typical atom is about 0.1 nanometres (10^-10 m) across.
- The nucleus is roughly 10,000 to 100,000 times smaller in diameter than the atom (about 10^-15 m vs 10^-10 m).
2
britannica.com/biography/John-Dalton
- John Dalton proposed his atomic theory in the early 1800s (from about 1803; published in A New System of Chemical Philosophy, 1808): matter is made of tiny indivisible atoms, and atoms of each element are identical to each other and different from other elements.
3
sciencehistory.org/education/scientific-biographies/john-dalton
- John Dalton proposed his atomic theory in the early 1800s (from about 1803; published in A New System of Chemical Philosophy, 1808): matter is made of tiny indivisible atoms, and atoms of each element are identical to each other and different from other elements.
4
britannica.com/biography/J-J-Thomson
- J. J. Thomson discovered the electron, a negatively charged particle much smaller than an atom, in 1897.
5
en.wikipedia.org/wiki/Plum_pudding_model
- Thomson's 1904 model pictured the atom as a sphere of positive charge with electrons embedded in it; it became known as the plum pudding model (a later nickname).
6
nobelprize.org/prizes/chemistry/1908/rutherford/biographical
- Ernest Rutherford was born on 30 August 1871 at Brightwater, near Nelson, New Zealand; he won the 1908 Nobel Prize in Chemistry; his portrait is on the New Zealand $100 note.
7
teara.govt.nz/en/object/36415/fifth-series-of-banknotes-100
- Ernest Rutherford was born on 30 August 1871 at Brightwater, near Nelson, New Zealand; he won the 1908 Nobel Prize in Chemistry; his portrait is on the New Zealand $100 note.
8
en.wikipedia.org/wiki/Geiger–Marsden_experiments
- In 1909 at Manchester, Hans Geiger and Ernest Marsden, working under Rutherford, fired alpha particles (positively charged) at very thin gold foil. Most went straight through; a very small fraction (roughly 1 in 8000 or fewer) were deflected by more than 90 degrees, back towards the source.
- Rutherford said it was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.
9
web.chemteam.info/AtomicStructure/Rutherford-Model.html
- In 1909 at Manchester, Hans Geiger and Ernest Marsden, working under Rutherford, fired alpha particles (positively charged) at very thin gold foil. Most went straight through; a very small fraction (roughly 1 in 8000 or fewer) were deflected by more than 90 degrees, back towards the source.
10
britannica.com/science/Rutherford-model
- In 1911 Rutherford concluded that an atom's positive charge and almost all its mass are concentrated in a tiny central nucleus, and the atom is mostly empty space.
- Alpha particles are positively charged; the rebound of a few was explained by repulsion from a concentrated positive charge (the nucleus).
11
www-archiv.fdm.uni-hamburg.de/b-online/e16/hydrogen.htm
- The nucleus is roughly 10,000 to 100,000 times smaller in diameter than the atom (about 10^-15 m vs 10^-10 m).
12
nobelprize.org/prizes/physics/1922/bohr/biographical
- Niels Bohr worked in Rutherford's Manchester laboratory in 1912 and in 1913 proposed that electrons occupy fixed orbits (shells / energy levels) around the nucleus.
13
britannica.com/science/Bohr-model
- Niels Bohr worked in Rutherford's Manchester laboratory in 1912 and in 1913 proposed that electrons occupy fixed orbits (shells / energy levels) around the nucleus.
14
nobelprize.org/prizes/physics/1935/chadwick/biographical
- James Chadwick, who had studied under Rutherford, discovered the neutron in 1932.
15
savemyexams.com/gcse/chemistry/aqa/18/revision-notes/1-atomic-…
- For the first 20 elements, electron shells fill as 2, 8, 8 (then the 4th shell): e.g. sodium (11 electrons) is 2,8,1; calcium is 2,8,8,2. A single outer electron is easily lost, which makes sodium very reactive.

Image and sound credits

Ernest Rutherford portraits (Auckland Weekly News 1914, 1924, 1928); Rutherford birthplace memorial, Brightwater: Auckland Libraries Heritage Collections, no known copyright restrictions
John Dalton, mezzotint by Charles Turner after James Lonsdale: public domain, via Wikimedia Commons
John Dalton's table of elements and atomic weights: public domain, via Wikimedia Commons
J. J. Thomson, photogravure by Emery Walker: National Library of Wales, public domain, via Wikimedia Commons
Niels Bohr, c. 1922: public domain, via Wikimedia Commons
Physical laboratories staff, University of Manchester, 1910: public domain, via Wikimedia Commons
Members of the United Cooks' Society preparing a monster plum pudding: Wellcome Collection, CC BY 4.0
Map data: Natural Earth (public domain)
Atom models, gold-foil experiment, rugby field and Bohr diagrams: drawn in code
Paper textures: ambientCG (CC0) · Torn paper edges generated with AI
Sound effects: Freesound contributors (CC0)
Music: generated with ACE-Step · Voice: Gemini TTS

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