Physics

If atoms are mostly empty space, why do objects look and feel solid?

The ConversationChemist John Dalton proposed the theory that all matter and objects are made up of particles called atoms, and this is still accepted by the scientific community, almost two centuries later. Each of these atoms is each made up of an incredibly small nucleus and even smaller electrons, which move around at quite a distance from the centre.

If you imagine a table that is a billion times larger, its atoms would be the size of melons. But even so, the nucleus at the centre would still be far too small to see and so would the electrons as they dance around it. So why don’t our fingers just pass through atoms, and why doesn’t light get through the gaps?

To explain why we must look at the electrons. Unfortunately, much of what we are taught at school is simplified – electrons do not orbit the centre of an atom like planets around the sun, like you may have been taught. Instead, think of electrons like a swarm of bees or birds, where the individual motions are too fast to track, but you still see the shape of the overall swarm.

Electrons ‘dance’

In fact, electrons dance – there is no better word for it. But it’s not random dancing – it’s more like ballroom dancing, where they move in set patterns, following steps laid down by a mathematical equation named after Erwin Schrödinger.

Electrons are like a swarm of birds. John Holmes/Wikimedia Commons, CC BY-SA

These patterns can vary – some are slow and gentle, like a waltz whereas some are fast and energetic, like a Charleston. Each electron keeps to the same pattern, but once in a while it may change to another, as long as no other electron is doing that pattern already. No two electrons in an atom can do the same step: this rule is called the Exclusion Principle.

Although electrons never tire, moving up to a faster step does take energy. And when an electron moves down to a slower pattern it loses energy which it gives out. So when energy in the form of light falls on an electron, it can absorb some energy and move up to a higher, faster “dance” pattern. A light beam won’t get far through our table, since the electrons in all the atoms are eager to grab some energy from the light.

After a very short while they lose this gained energy, perhaps as light again. Changes in the patterns of absorbed and reflected light give reflections and colours – so we see the table as solid.

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