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Dalton's theory of the atom

The word atom comes from 'atomos', an ancient Greek word meaning indivisible. The Greek philosopher Demokritos (460-370 BCE) stated that all matter could be divided and sub-divided into smaller and smaller units until eventually there would be a particle that could not be divided any further. This he called an atom.

There was little change in the understanding of the atom until John Dalton (1766-1844), an English chemist, concluded that:

• all matter is made of atoms, and atoms are indestructible and cannot be broken down into pieces
• all the atoms of a particular element are identical to each other and different from the atoms of other elements

He based his conclusions on experimental work combining gases. He found that if two elements can be combined to form a number of compounds, then the atoms from the first and second element only combine in small, whole number ratios such as 1:1, 1:2, 2:3.

The plum pudding model

After discovering the electron in 1897, J J Thomson proposed that the atom looked like a plum pudding. To explain the two types of static electricity, he suggested that the atom consisted of positive 'dough' with a lot of negative electrons stuck in it. This was consistent with the evidence available at the time:

• solids cannot be squashed, therefore the atoms which make them up must be solid throughout
• rubbing two solids together often results in static charge so there must be something (electrons) on the outsides of atoms which can be transferred as atoms collide

Rutherford and the nucleus

In 1905, Ernest Rutherford did an experiment to test the plum pudding model. His two students, Hans Geiger and Ernest Marsden, directed a beam of alpha particles at a very thin gold leaf suspended in a vacuum.

The scientists observed the following from their experiment:

• most of the alpha particles passed straight through the foil
• a small number of alpha particles were deflected by large angles (>4°) as they passed through the foil
• a very small number of alpha particles came straight back off the foil

Rutherford considered these observations and concluded:

• The fact that most alpha particles went straight through the foil was evidence for the atom being mostly empty space.
• A small number of alpha particles being deflected at large angles suggested that there was a concentration of positive charge in the atom. Like charges repel, so the positive alpha particles were being repelled by positive charges.
• The very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus). The tiny number doing this meant the chance of being on that exact collision course was very small, and so the 'target' being aimed at had to be equally tiny.

Rutherford had discovered the nuclear atom, a small, positively-charged nucleus surrounded by empty space and then a layer of negatively-charged electrons forming the outside of the atom.

Bohr and energy levels

Even though Rutherford had proven the existence of the nucleus, scientists were unsure how electrons fitted into this new model.

In 1913, Niels Bohr revised Rutherford's model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus.

He was thus able to explain that, since particular chemicals burn with certain-coloured flames; the pattern of energy released by electrons in the chemical reaction must be the same for every single atom of that element.

Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom.

Bohr's 'solar system' model of the atom is the way that most people think about atoms today.