Introduction

Our understanding of the physical world has grown at an incredible rate in the last 200 years. The key to the advances made in chemistry has been our growing knowledge about atoms. In this unit we will look at some of the early historical discoveries that helped us build up a useful 'working model' of the atom.

The unit Evidence for Particles showed how the ancient Greeks had ideas about particles and atoms. But it wasn't until the start of the nineteenth century that a theory of atoms became linked to strong experimental evidence. It was then that an English scientist called John Dalton put forward his ideas about atoms.


From his experiments and observations, he suggested that atoms were like tiny, hard balls. Each chemical

element

An element is a substance made from only one type of atom. An element cannot be broken down into any simpler substances.element had its own atoms that differed from others in mass. Dalton believed that atoms were the fundamental building blocks of nature and could not be split. In chemical reactions, the atoms would rearrange themselves and combine with other atoms in new ways.


In many ways, Dalton's ideas are still useful today. For example, they help us to understand elements, compounds, and molecules. (See the unit Chemical Shorthand.)

We can also visualize chemical reactions. Look at the molecules reacting below:

At the end of the nineteenth century, a scientist called J.J. Thomson discovered the electron. This is a tiny negatively charged particle that is much, much smaller than any

atom

An atom is the smallest particle of an element that can still be defined as that element.atom. When he discovered the

electron

Electrons are tiny, negatively charged particles that orbit the nucleus of an atom in energy levels (or shells).electron, Thomson was experimenting by applying high voltages to gases at low pressure. He noticed an interesting effect. This is shown in the experiment in Fig.2 below:



Thomson did experiments on the beams of particles in his tube. They were attracted to a positive charge, so Thomson correctly concluded that they must be negatively charged themselves. Other experiments showed that it would take about 2000 electrons to weigh the same as the lightest atom, hydrogen. He called the tiny, negatively charged particles electrons.

But where had these tiny particles come from? Since they were so small, Thomson suggested that they could only have come from inside atoms. So Dalton's idea of the indestructible atom had to be revised.

Thomson proposed a different model for the atom. He said that the tiny negatively charged electrons must be embedded in a cloud of positive charge (after all, atoms themselves carry no overall charge, so the charges must balance out). Thomson imagined the electrons as the bits of plum in a plum pudding (rather like currants spread through a Christmas pudding – but with lots more space in between).

The next development came about 10 years later. Two of Ernest Rutherford's students, Hans Geiger and Ernest Marsden, were doing an experiment at Manchester University with radiation. They were using the dense, positively charged particles (called alpha particles) as 'bullets' to fire at a very thin piece of gold foil. They expected the particles to barge their way straight through the gold atoms unimpeded by the diffuse positive charge spread throughout the atom that Thomson's model described. However, they got a big surprise. Look at their experiment below:


In 1911, Ernest Rutherford interpreted these results and suggested a new model for the atom. He said that Thomson's model could not be right. The positive charge must be concentrated in a tiny volume at the centre of the atom, otherwise the heavy alpha particles fired at the foil could never be repelled back towards their source. On this model, the electrons orbited around the dense nucleus (centre of the atom).

Here is Rutherford's model:

The next important development came in 1914 when Danish physicist Niels Bohr revised the model again. It had been known for some time that the light given out when atoms were heated always had specific amounts of energy, but no one had been able to explain this. Bohr suggested that the electrons must be orbiting the

nucleus

The nucleus is the centre of an atom, containing protons and neutrons.nucleus in certain fixed energy levels (or shells). The energy must be given out when 'excited' electrons fall from a high energy level to a low one.

Summary


Our ideas about the nature of atoms have progressed over the last two centuries (and continue to develop today).

John Dalton introduced a new form of the ancient Greek idea of atoms at the beginning of the nineteenth century.

In 1897, J.J. Thomson discovered the electron and suggested the 'plum pudding' model of the atom.

In 1911, Rutherford suggested that electrons orbit the atomic nucleus like planets round the Sun.

In 1914, Bohr modified Rutherford's model by introducing the idea of energy levels.

We can think of the atom as a positively charged nucleus with negatively charged electrons orbiting the nucleus in energy levels (or shells).