A is for Atom

By Siobhan Fairgreaves

Never trust an atom- they make up everything!

Scanning Tunnelling Microscopy can give us incredible images like this one of a piece of graphite, showing how its atoms fit together like Lego. Luckily, treading on atoms isn’t like treading on actual Lego, or walking would be incredibly painful | Image: Frank Trixler

Terrible jokes are finished, for now…


In this post, we will look at the basic structure of the atom. But first, what are atoms?

Atoms are the building blocks of the world- think of Lego. If the whole world was made of Lego an atom is that tiny single square block. Imagine how many of those tiny blocks would be needed to build the whole world and all the people, animals and stuff inside it… That’s a lot of Lego, and there are a lot of atoms. A single grain of sand contains millions of these tiny particles.

For a long time atoms were thought to be the smallest piece of the puzzle. Then in 1897 a scientist named J.J. Thomson identified an even smaller particle which helps to make an atom.

Cheer up J.J., you’ve revolutionised modern physics! | Image: Benjamin Crowell

Thomson made the discovery when he was experimenting with mysterious beams of particles called cathode rays. When firing cathode rays at hydrogen atoms, he measured how the path of the beams changed as they interacted with the atoms. Thomson realised that the cathode rays were made of tiny, negatively charged particles – around 1/2000th the size of a hydrogen atom. He named the particles ‘corpuscles’, but we know them today as electrons. But Thomson’s discovery doesn’t tell the whole story about what we came to know about the atomic structure.

In fact, we have another scientist to thank for that. In 1909, New Zealander physicist Ernest Rutherford fired some positively charged radioactive particles through a sheet of gold atoms, and measured the different paths they took. He was testing out J.J. Thomson’s ‘plum pudding’ model, which proposed that atoms were made up of electrons sitting happily inside a positive sphere, holding them together.

Thanks to Ernest Rutherford, scientists can now study atoms without constantly thinking about delicious desserts | Image: Library of Congress

Rutherford noticed that most of the particles passed straight through the atoms, but a tiny proportion were deflected back. That meant that instead of being plum puddings, each atom was made up of a small positive nucleus, surrounded by orbiting electrons, with lots of empty space between them. And so, the basic model for  an atom was born!

A typical illustration of an atom will show a ball in the middle surrounded by orbits- but what is going on in there? The ball in the middle is the nucleus which Rutherford discovered, and inside the nucleus are protons and neutrons. The things whizzing round the outside are the electrons. Protons, neutrons and electrons are known as sub-atomic particles, now that’s an impressive dinner party phrase.

Electrons in different energy levels form a cloud of negative charge around the nucleus| Image: Mets501

It might look like electrons are in a messy, complicated cloud but they are actually very precisely arranged. Around each nucleus are different shells, or energy levels, which have space for a different number of electrons.


The very first energy level around the nucleus can only hold 2 electrons. In an atom of the element Helium both of the spaces in the first energy level are filled by an electron. So, using Helium as an example, what else is in there? To work that out you should know that it is really important for an atom to be balanced. Each electron carries a negative charge so to balance Helium we now need two positive charges. Fortunately, protons have a positive charge each. So we’ve got two negative electrons whizzing around the outside, two positive protons snug in the nucleus, the charges are balanced. So, are we finished?

It might seem fun to try and chop a uranium nucleus in half, but it’s not actually a very good idea… | Image: Scienities

Almost, don’t forget the third component, neutrons. Luckily, they are- you guessed it- neutral, so it’s okay for the number of them to vary between different forms of the same element.  Most of the time, Helium has two neutrons and with two of everything it is nicely balanced and known as stable.

Helium is a nice example with small numbers but not all elements are quite so compact. Take Uranium, for example, there are a lot more protons (92!) and electrons involved to try and get Uranium to balance.

That’s a very basic introduction to atoms- and for sticking with it, you’ve earned yourself another terrible joke! What a treat…

A neutron walked into a bar and asked for a drink.

“How much?” asked the neutron.

The bartender replied, “For you, no charge!”

Hopefully, that joke will make a bit more sense now you know your stuff about subatomic particles and their charges.

Until next time!


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