Programme 17: Atomic Structure

Background Information

Atomic Structure

People have always wondered what matter or ‘stuff’ is made of. Even 2500 years ago ancient Greeks discussed this question. One of these Greeks, called Democritus, was particularly interested in thinking about materials. He had the idea that everything in the world was made up of tiny bits that were too small to be broken up any further. His word for these bits was ‘atomos’, which is Greek for uncuttable. Democritus thought that everything was made up of atoms joined together in different combinations. Democritus’ ideas were written down, but because there were no printing presses at the time, very few people got to hear about them. They were copied by hand less and less and then eventually they were completely forgotten about.

In 1417 a copy of an old Roman poem referring to Democritus and atoms was discovered in an attic. From then on people began to discuss his ideas again and the invention of the printing press in 1454 allowed many more people to hear about them.

In 1808 an English schoolteacher called John Dalton used Democritus’ ideas to put forward the first real atomic theory. From his experiments Dalton was able to show that elements were made up of different atoms. The atoms in each element were different and the thing that made them different was their mass. Dalton proved that an atom of oxygen was 16 times heavier than an atom of hydrogen.

As technology improved, scientists were able to find out more about atoms. In the 1890s one of Democritus’ ideas about atoms was proved wrong. Whilst working on cathode rays, the scientist J J Thompson discovered that atoms themselves were made up of other tiny particles and that atoms could be split up. The particle that Thompson discovered was called an electron. Electrons have very little mass compared to an atom and a negative charge. Soon after, other parts of an atom were discovered.

In 1910 Ernest Rutherford discovered a second particle in atoms and called it the proton. By 1932 James Chadwick had found a third type of sub-atomic particle called the neutron. Rutherford’s other great contribution to understanding atoms was to suggest how these three types of sub-atomic particle are arranged inside an atom. He said that an atom was a bit like our solar system. Protons and neutrons were at the centre in a part called the nucleus. The electrons moved in orbits around the nucleus.

Particle	Charge	Relative mass
Electron	-1	0
Proton	+1	1
Neutron	0	1

Atomic Number and Mass Number

Hydrogen is the smallest and simplest atom. It contains 1 proton in its nucleus and 1 electron. Atoms differ in the number of protons they have in their nucleus. The number of protons is called the atomic number or sometimes the proton number.

The mass number is the total number of protons and neutrons in the atom. All the mass in an atom is concentrated in the nucleus. A carbon atom has 6 protons and 6 neutrons. It is written like this:

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Electron Structure

Each atom has enough electrons surrounding the nucleus to make the atom electrically neutral. The electrons move around the nucleus in distinct shells or levels with differing energies. Each energy level can only hold a fixed number of electrons. To help us to picture the electrons the electron shells are often drawn as circles surrounding the nucleus. The electrons closest to the nucleus are bound to it with the greatest energy.

The first energy level can hold two electrons and this level is filled first. The next level can hold eight electrons. The third level can hold a maximum of 18 electrons. When eight electrons are in this shell it becomes more stable and lower in energy, and then the next two electrons enter the fourth shell. After this the next ten electrons go back and fill up the third shell. Because GCSE specifications only require students to know the electron structure for the first 20 elements the electron arrangement is often shown as a maximum of 2.8.8. But electrons with potassium have the structure 2.8.8.1 and with calcium it is 2.8.8.2. The arrangement of electrons in an atom is sometimes called the electron configuration.

When atoms are arranged in order of increasing atomic number in the periodic table patterns appear in the arrangement of electrons. Elements in the same group in the periodic table have the same number of electrons in their outer shell. The exception to this is Group 0 which all have full outer shells. This clearly shows that the properties of an element are related to its electron arrangement. As you move across a horizontal row in the periodic table (or period) an electron shell fills up one electron at a time.