May 12, 2021

I, Science

The science magazine of Imperial College

For something so small, it’s not shy of a big theory. In fact, the atom and its activity have a long history in the world of theoretical science, punctuated by the ideas of many a perplexed physicist.

The birth of atomic theory takes us way, way back to ancient Greek times. Here, Democritus (460–370 BC), a philosopher pondering the makeup of the physical world, speculated that all matter consisted of minute little particles that moved around in an infinite space. He called these particles ‘atoms’, meaning ‘indivisible’. However, a lack of evidence meant his idea went largely unnoticed and it took almost two millennia before the credibility of the concept was restored.

The man responsible for its revival was John Dalton (1766–1844), an English chemist with a penchant for gaseous mixtures. He took the idea of tiny, indestructible particles one step further by suggesting that each one had a certain size, mass and way of behaving that was determined by what kind of element they were. Dalton’s theory, which he put forward in a lecture to the Royal Institution in 1803, added a dash of logic and rationality to Democritus’ philosophy of matter.

Towards the end of the same century, English physicist Sir J. J. Thomson (1856–1940) debunked the notion of atoms being indivisible when he discovered an electric current could be used to break atoms down into parts – tiny, negatively charged particles known as ‘electrons’. In 1904, he illustrated his thinking with the infamous ‘plum-pudding’ model, in which electrons (the plums) floated in a mass of positive charge (the pudding).

A few years later, English chemist and physicist Ernest Rutherford (1871-1937) put Thomson’s ideas to the test using some gold foil. If the plum pudding model were right, alpha particles (speedy particles with a positive charge) fired at foil should all pass through it. But when Rutherford bombarded his piece of foil with these particles, a few bounced back. From this, he concluded that the positive charge inside an atom must be squeezed together into one small place, creating a force strong enough to cause it to repel. Rutherford called this spot the ‘nucleus’.

This year marks 100 years since Rutherford’s paper on atomic structure was published. Although some parts were later discovered to be inaccurate, his work remains instrumental to atomic theory; not only did it disprove Thomson’s pudding model, but it provided the groundwork for fellow physicist, Danish-born Niels Bohr (1885–1962), who suggested that the nucleus is orbited by electrons that hop between different energy levels. Bohr’s model is still taught in schools today and his work has sparked numerous mathematical approaches to atomic theory, such as quantum mechanics.

Although no theory has been proven with absolute certainty, each idea has filled one more piece in the atomic puzzle. No longer a whimsical philosophical concept, our atomic knowledge is fundamental to both physics and chemistry. It is allowing scientists to better understand the origins of the universe, such as at CERN’S Large Hadron Collider. All of this is a rather mighty feat, especially considering atoms are too tiny for us to even see.