Atomic structure recap from gcse

Early ideas

The idea of atoms is not a new one! The Greeks philosopher Democritus suggested over 2500 years ago that matter was made up of solid tiny balls called atoms. Democritus thought of atoms as unbreakable spheres. These ideas about atoms changed little for thousands of years and it is really only in the last 120 years or so that any real insight was made into the nature and structure of atoms. One of the first pioneers on this journey was John Dalton.
Dalton was born in 1766 in the Lake District, England. He was interested in the sciences, in particular chemistry, physics and meteorology. Dalton is best known for his atomic theory and his gas laws, which you will probably meet in your physics course. According to Dalton's atomic theory:

All substances are made up of atoms and the atoms of any one element are all identical.
Atoms of different elements are different and have different properties.
Atoms of different elements have different masses.
Compounds are formed when atoms of more than one element join. The atoms in any one compound are always present in a fixed ratio.

Dalton was on the right track but ultimately he lacked the technology and resources to further his work. He also made an attempt at calculating the masses of some elements, though his calculations were not always correct.

The discovery of the electron

Sir John Joseph Thomson was professor of physics at Cambridge University in 1884. Thomson is credited with the discovery of the electron. The idea that atoms were not made of indestructible solid balls had been around for around 30 years or so and the idea of electrons had been around since 1874 when G.J Stoney suggested the name to explain some observations in his work. But Thompson is credited with providing proof of its existence.
Now gases do not normally conduct electricity, however Thompson was investigating how their conductivity changed at very low pressures when very high voltages were applied across them. He set up an experiment similar to the one shown below. It basically consists of a glass tube filled with gas at very low pressure, with a fluorescent screen at one end. Half way up the tube were 2 charged plates, one positively charged and one negatively charged. At the other end was a piece of metal which was connected to the negative terminal of a power supply, this was the cathode. A metal disc with a slit in it was connected to the positive terminal of the power supply. A diagram of his apparatus is shown below.

The plum pudding model of the atom

plum pudding model What Thompson observed while running the experiment was a green glow from the fluorescent screen where a "beam" or "ray" (shown as blue dots in the diagram) coming from the cathode had struck it. He noticed that the "ray" was deflected or bent away from the negatively charged plate. This told him that the "ray" had a negative charge. Since these rays were coming from the cathode he called them cathode rays!. Calculations showed that these cathode rays were over 1840 lighter than a hydrogen atom and that no matter which gas was placed in the tube or if a different metal was used as the cathode they were always produced.

Thompson realised that these "corpuscles", as he called them had a negative charge and they were present in all atoms. So atoms were not solid indestructible balls after all. The name corpuscles was later changed to the electron.

Thompson developed a new model of the atom to explain his observations. Thompson's model of the atom is often called the plum pudding model. His idea of what an atom looks like is a sphere of positive charge in which are embedded the electrons. A bit like a plum pudding or chocolate chip cookie. Thompson's calculations had shown that the electron was 1/1840^th the mass of a hydrogen atom. The hydrogen atom must therefore contain 1840 electrons.

One of Thompson's students at Cambridge University, was the brilliant scientist Ernest Ruthford. In 1911 Rutherford with help from two other research scientists, Hans Geiger and Ernest Marsden conducted a now very famous experiment which lead to the idea that atoms contain a nucleus. An outline of the experiment is shown in the diagram below.

A sample of radioactive radium metal was placed inside a lead block with a hole at one end. Radium emits alpha particles. Alpha particles are large heavy slow moving particles with a charge of 2+, they consist of 2 protons and 2 neutrons, similar to the nucleus of a helium atom. Alpha particles are only able to travel a few centimetres in air so the whole experiment was carried out in a sealed container in which the air was sucked out by a vacuum pump. Next the alpha particles which were emitted by the radium travelled towards a very thin sheet of gold foil.

Rutherford was able to detect the alpha particles as they emitted a glow when they struck the fluorescent screen shown in the image below. From the image you can see that some of the alpha particles travelled straight through the gold foil but some were deflected through small angles and some even bounced stright back, as if they had hit something solid and heavy!

Rutherford discovers the nucleus

According to Thompson's plum pudding model of the atom the positive charge of the pudding was spread out over the whole atom and it would not have been able to stop a heavy bullet like projectile such as a heavy positively charged alpha particle. So Rutherford would have expected all the alpha particles to pass straight through the gold foil. Indeed most of the alpha particles did but some were deflected and some bounced straight back. This was not what Rutherford was expecting at all. This should have been impossible based on the plum pudding model!

The nuclear atom

Since most of the alpha particles passed straight through and only a few were deflected through large angles, it was concluded that the atom consisted of mostly empty space but with a tiny area positive of charge located at the centre - the nucleus! The alpha particles that were deflected were those that had come close to the nucleus , since the alpha particles have a positive charge the closer they got to the positively charged nucleus the greater the angle through which they would have been deflected (this is shown in the image opposite).

The fact that very few alpha particles, about 1 in 10 0000 were deflected straight back means that the chance of any hitting the nucleus must have been small, so the nucleus must be tiny. Rutherford was also able to calculate the size of the nucleus from his experimental work, he calculated that the nucleus was about 1/10 000th the size of the atom.

The next step

The results from Rutherford's gold foil experiment proved beyond any doubt that Thompson's Plum Pudding model was wrong. However it still left some questions unanswered. The main problem were the electrons. In Rutherford's model the electrons were simply spinning around the nucleus. Since they had a negative charge and the protons inside the nucleus had a positive charge, what was there to stop the electrons crashing into the nucleus? The answer to this problem was provided by Neils Bohr and led to the idea of electron shells or energy levels in atoms.

Key Points

The plum pudding model considered atoms as spheres of positive charge with negatively charged electrons spread through the positively charged sphere.
Rutherford's famous gold foil experiment where alpha particles were fired at a thin sheet of gold foil resulted in the discovery of the nucleus and proved that Thompson's plum pudding model was wrong.