Cambridge International AS Level Chemistry
are two types of nucleon: protons and neutrons. Atoms of different elements have different numbers of protons. Outside the nucleus, particles called electrons move around in regions of space called orbitals (see page 37). Chemists often find it convenient to use a model of the atom in which electrons move around the nucleus in electron shells. Each shell is a certain distance from the nucleus at its own particular energy level (see page 37). In a neutral atom, the number of electrons is equal to the number of protons. A simple model of a carbon atom is shown in Figure 2.3.
Atoms are tiny, but the nucleus of an atom is far tinier still. If the diameter of an atom were the size of a football stadium, the nucleus would only be the size of a pea. This means that most of the atom is empty space! Electrons are even smaller than protons and neutrons.
electron
nucleus
electron shells (energy levels)
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neutron
proton
Figure 2.4 Ernest Rutherford (left) and Hans Geiger (right) using their alpha-particle apparatus. Interpretation of the results led to Rutherford proposing the nuclear model for atoms.
Figure 2.3 A model of a carbon atom. This model is not very accurate but it is useful for understanding what happens to the electrons during chemical reactions.
EXPERIMENTS WITH SUBATOMIC PARTICLES We can deduce the electric charge of subatomic particles by showing how beams of electrons, protons and neutrons behave in electric fields. If we fire a beam of electrons past electrically charged plates, the electrons are deflected (bent) away from the negative plate and towards the positive plate (Figure 2.5a). This shows us that the electrons are negatively charged. A cathode-ray tube (Figure 2.5b) can be used to produce beams of electrons. At one end of the tube
b
–
cathode
– electron beam
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fluorescent screen with scale
cathode rays charged plates (anode)
magnets causing electromagnetic field
a
+
beam deflected downwards
Figure 2.5 a The beam of electrons is deflected away from a negatively charged plate and towards a positively charged plate. b The electron beam in a cathode-ray tube is deflected (bent) by an electromagnetic field. The direction of the deflection shows us that the electron is negatively charged.