Discovering chemistry
Discovering chemistry

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Discovering chemistry

1 Atoms

The idea that everything that you can see is an assembly of tiny particles called atoms is chemistry’s greatest contribution to science.

But this isn’t a new idea.

In fact an early proposal came from the Greek philosopher Democritus (460 – 370BC), often known as the ‘laughing philosopher’ as he was always keen to promote the importance of being cheerful, although other accounts suggest he was laughing at and ridiculing the misfortunes of others and refer to him as the ‘mocker’.

Democritus proposed that no matter how many times you cut something in half, and half again, and half again, eventually you will reach a point when you cannot divide it up any further. This led him to propose that everything is built up from tiny individual particles. In fact atom in Greek translates as ‘not’ and ‘cut’. Democritus along with another philosopher of the time Epicurus (340-270BC) believed in a world made of tiny, hard atoms in endless motion.

It wasn’t until the 19th century, and the work of English chemist, physicist and meteorologist John Dalton that atoms became part of modern science; even in 1900 there were eminent scientists who did not believe that atoms are real. However, between 1900 and 1920 phenomena as varied as the motion of pollen grains in water, diffusion in liquids, radioactivity and the diffraction of X-rays by crystals, all gave similar values for the sizes of atoms. The merging of evidence from such different directions pretty much destroyed any serious opposition to the existence of atoms.

The final nail in the coffin of the doubters was the emergence of instruments capable of producing images of atoms and molecules. This was realised in the 1980s, with the invention of the Scanning Tunnelling Microscope (STM). Importantly, atomic sizes measured using this technique agreed with those obtained from the earlier experiments.

Figure 1 shows a ring of 48 iron atoms on a copper surface observed by STM.

Described image
Figure 1 An image of iron atoms on a copper surface observed by STM

Maybe not what you thought an atom might look like, but the wavelike crests and troughs may be explained with reference to quantum mechanics and the wave-like properties of electrons, a topic you will touch on later in this module.

So, you’ve seen that atoms are so small they need sophisticated microscopes in order to ‘see’ them.

But how small they?

As a rough indication, the diameters of atoms range between 1 x 10-10 to 5 x 10-10m

However, despite being originally thought to be indivisible, atoms are built up from a number of smaller components.

If you look inside the atom you will find at its centre the nucleus, this is positively charged and contains nearly all of its mass. Around the nucleus move much tinier negatively charged particles called electrons. The positive charge of the nucleus is balanced by the negative charge of the electrons, so overall an atom carries no electrical charge. The electrons themselves circle huge distances from the nucleus, moving at speeds close to the speed of light.

So, in fact most of an atom is empty space!

Returning to the nucleus, its positive charge is provided by positively charged particles called protons. These are very tightly packed in the nucleus, and your immediate thought might be they would repel one another. However, although we won’t go into details here, there exists a force which holds the nucleus together – a sort of atomic ‘superglue’ ensuring this doesn’t happen.

The number of protons in its nucleus, is used to define a unique atomic number for each atom.

For an atom, how is atomic number related to the number of electrons?

Answer

As atoms are neutral, the number of positive charges has to equal the number of negative charges, so the number of electrons = number of protons. Thus atomic number must also equal the number of electrons.

So far, you have focussed on the submicroscopic, invisible (to the naked eye) world of an atom.

How does this translate into something you can actually see?

This question will be answered in the next section.

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