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2. The World of Atoms 2.1. The structure of the atom

For many centuries people believed that matter has a continuous form. The first person who suspected the discrete structure of matter was Democritus, a Greek philosopher living in 460 - 370 B.C. He defined an 'atom' as the smallest, indivisible portion of matter. He claimed that every substance is madę up of a large number of specific atoms.

Today we know that Democritus was right about the atomie structure of matter, but that he was wrong about the indivisibility of an atom. Ali atoms consist of three kinds of elementary particles: protons, neutrons and electrons. The protons and neutrons form the nucleus of the atom, while the electrons move around the nucleus in a complex way. Let us now look at the properties of elementary particles.

Particie	Mass (kg)	Relative mass	Relative charge	Symbol
Electron	9.109.10'^31	5.45x10'^4	-1	e
Proton	1.673.10^27	1	+1	P
Neutron	1.675x10^27	1	0	n

An electron and a proton each carries an electric charge of equal magnitude but of opposite sign. By convention, an electron is assigned a -1 sign (negative), a proton a +1 sign (positive). As atoms are electrically neutral, the number of protons must equal the number of electrons in each atom.

A proton and a neutron have an almost identical mass. An electron, though, is about 1830 times lighter. Since the nucleus is madę up exclusively of protons and neutrons, the mass of an atom is concentrated in its nucleus. The nucleus is very smali, about 100 000 times smaller than the whole atom. Therefore, an atom is mostly empty space, with a massive but smali nucleus in the centre and a diffuse, spherical cloud of electrons around it.

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