The history of the Atom

In the Greek times, scientists like Demokritos believed that all matter was made of small molecules which they called atoms. The word atom was derived from the greek 'Atomos' which translates roughly to 'uncuttable' as it was the smallest matter.

In the 19th century, scientist JJ Thomas (after the recent discovery of the electron) made the first well known model of the atom. This model was called 'The Plum Pudding Model' which was the concept that an atom was a ball of positive charge with negative charges evenly distributed throughout (like raisins in a plum pudding). Scientists believed that because solids could not be compressed, it must be because the atoms within it were solid 'balls'.

It was surprisingly hard to find images of plum puddings! (Sharon McCutcheon)

However, in the early 20th century, Ernest Rutherford tested the plum pudding model and discovered that the atom was not arranged the way people had thought. He discovered that the atom was actually made from a small positively charged nucleus which was surrounded by lots of space with electrons in it. He did this using an alpha particle scattering experiment. He placed a thin gold sheet in front of a screen inside a vacuum.  Note Alpha particles are a nucleus of helium (two protons and two neutrons ((basically a positively charged particle)). If the plum pudding model was correct, all the particles would shoot through the gold sheet but to his surprise, only some (well most) of the particles went through. Some of the particles were deflected at different angles and some others bounced straight back.

The outcomes of this experiment showed to Rutherford that due to most of the atoms going through, most of the atom was empty space. The particles that were deflected at large angles showed to him that the atom must have a positive charge because 'like charges' repel each other and the alpha was also positive. Furthermore, alpha particles bouncing straight back showed Rutherford that the nucleus of the atom must be very concentrated as only few particles bounced straight back (which means only a small area was concentratedly charged).

Much like glitter, particles are everywhere(Sarah McCutcheon)

Soon after Rutherford's discovery of the nucleus, Neils Bohr looked further into the way the atom was arranged as a whole. Many scientists of the time knew that the atom had an overall neutral charge and that electrons weren't arranged inside in the nucleus. He suggested a 'solar model' where electrons orbited the nucleus at fixed energy levels like the Earth to the Sun. He wasn't far off the truth here, electrons did orbit the atom however, now we know they are not completely fixed.

The model we have today wasn't too different from Bohr's model, however, Chadwick completed the model we have today due to his discovery of the neutron in 1932. Scientists knew there was still an undiscovered particle because the atomic mass did not make sense. He used a similar method to Rutherford but using Beryllium rather than gold.

So what is the model now?

We now use 'The nuclear model' which is where we know the atom consists of;

• a positively charged nucleus made from protons and neutrons
• surrounding shells of electrons

The proton has a mass of 1 and a charge of 1. The neutron has a mass of 1 and a charge of 0. The electron has a mass of about 1/1836 (very small!) and a charge of -1. The atom, because it has an equal number or protons and electrons, will always have a neutral charge as positive proton charges cancel out the negative electron charges. If an atom has a charge, it becomes an ion.