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Atoms and Atomic Structure

For nearly 2500 years it had been suspected that matter did not constitute a continuum – that is, that one could not simply take a piece of matter and divide it up into arbitrarily small pieces.  However, it was not until the early part of the nineteenth century that the scientists of that time became convinced that all matter was composed of objects which later came to be known as atoms.

Atoms are very small – the diameter of the smallest atoms are about 0.00000001 cm.  This is much too small to be seen by the naked eye, or indeed by using a magnifying glass or optical microscope.  In most solids, the atoms are arranged in a regular and symmetric lattice.  In liquids and gases, the arrangement of atoms is much more haphazard.  At the present time, around 120 different types of atom have been discovered.  Materials composed of just one type of atom are called elements.  For example, a block of iron is composed only of iron atoms.  If the iron starts to rust, al layer of iron oxide develops on the surface.  This is a compound – it consists of both iron and oxygen atoms.  

In the early part of the twentieth century, it was discovered that atoms themselves are composed of smaller particles.  In particular, it was found that they consist of a nucleus, around which there rotates a number of electrons.  This can be envisaged by considering our solar system – the sun represents the nucleus and the electrons are represented by the planets rotating around the sun.  The radius of the orbits of the electrons are about the same as the atomic size given above.  The radius of the nucleus is about 100000 times smaller.  This is a vast difference – and indeed it is no exaggeration to state that atoms are basically composed of nothing, in the same way that very little exists between the planets and the sun.  Electrons have the property that they carry a negative electrical charge, whereas the nucleus carries a positive electrical charge.

Further investigations showed that the nucleus itself is composed of two types of smaller particle, called protons and neutrons.  These particles have similar masses (the neutron is slightly heavier), but they differ in that the proton carries an electrical charge (equal in magnitude but opposite in sign to that of the electron), and the neutron does not.  This results in the overall positive charge of the nucleus.  Protons and neutrons differ in other ways as well, but these are not relevant to our discussion.  Indeed, later investigations have shown that protons and neutrons are composed of still smaller particles, called quarks. 

It is a curious property of atoms that the number of electrons rotating around the nucleus is equal to the number of protons in the nucleus.  This indicates that, overall, atoms (and hence matter itself) is electrically neutral.  Individual atoms can acquire electrons from other atoms, and so they gain a negative charge.  Alternatively, they can lose electrons, thus gaining an overall positive charge.  In these circumstances, they are called ions.  Normally, the number of neutrons in the nucleus exceeds the number of protons. 

The 120 or so different types of atom are usually identified in terms of the number protons in the nucleus.  The lightest atom (hydrogen) has only a single proton, whereas the heaviest has 120 protons.  Hydrogen has only a single electron rotating around its nucleus, whereas the heaviest has 120 electrons.  The number of protons in the nucleus is called the atomic number of the atom, and is usually given the symbol Z.  The total number of protons plus the total number of neutrons is called the atomic mass, and this is given the symbol A. 

So there we have it, a very simple picture of the atom.

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