Material: Elaine Wilson (Homerton College Cambridge)
Richard Feynman summed up how important the model of the atom is when he said
‘If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms, little particles that move around in perpetual motion, attracting each other when they are little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied’.
The word atom comes the Greek word atmos meaning indivisible particle, and the Greek Philosophers are credited with the first atomic theory. We now know that atoms can in fact be broken down into sub-atomic particles. A more precise definition might be that elements can be sub - divided into atoms and that although further sub - division can take place the identity of the original element will be lost.
At KS3 pupils are introduced to a model of atomic structure which Ernest Rutherford developed in the late 19th century when his team probed the interior of the atom. The Rutherford ‘solar system’ model is now widely used particularly at GCSE and can help explain why chemical reactions take place when electrons move from one shell to another.
The model suggests that atoms are made of smaller subatomic particles. In the middle of every atom is the nucleus. The nucleus contains two types of subatomic particles, protons and neutrons. The protons have a positive electrical charge and the neutrons have no electrical charge. A third type of subatomic particle, electrons, move around the nucleus. The electrons have a negative electrical charge. An atom usually contains an equal number of positively charged protons and negatively charged electrons. This makes the atom itself electrically neutral. The electrons exist at different energy levels, called shells, around the nucleus. The shell can only accommodate a limited number of electrons as shown in table one.
Atoms of different substances have different numbers of protons in their nucleus. The number of protons in the nucleus is called the atomic number and the total number of protons and neutrons is called the mass number.
Atoms of the same element with different numbers of neutrons are called isotopes. So all the isotopes of an atom have the same atomic number but a different mass number. Isotopes have different physical properties but their chemical properties are the same.
Pupils should be made aware that this a model of the how the atom is arranged and that electrons do not exist as dots and crosses in two dimensions. Pupils should also be aware that other more complex models exist and they may encounter these later in their learning.
1 What do you think you would see if you magnified a pile of sulphur powder as much as possible?
2 Describe the properties of individual iron atoms.
Metal atoms joining