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24.008 Electrons are normally constituents of atoms


Malcolm Plant
Electronics, 1992
Pages 15-18, electrons are normally constituents of atoms


2.1 Electrons in atoms

Section 1.8 explained that electronics began with the discovery that cathode rays were actually beams of negatively charged particles called electrons. Nowadays, cathode rays are widely used to ‘draw’ information on television and radar screens. In these devices, electrons are temporarily ‘free’ as they move through the vacuum inside the television or radar tube. But electrons are normally constituents of atoms.

Atoms are extremely small ‘bits’ of material – millions of them lie side by side across the diameter of the dot at the end of this sentence. All atoms have a nucleus. Fig 2.1 shows a simple model of an atom in which atomic particles called protons and neutrons have their home in the nucleus. Electrons, however, make up an ‘electron cloud’ outside the nucleus. The nucleus is very small compared with the overall size of an atom – say the size of an orange compared with the vast volume of an English cathedral. Using this model of an atom, you can imagine the electrons to be flies in the cathedral. Most of the mass of an atom lies in nucleus. In fact, the neutron and proton have about equal masses whereas the electron has a mass about 2000 times smaller than either particle.


2.1 A simple model of an atom

Fig. 2.1 A simple model of an atom


From the point of view of electronics, the two most important properties of an electron are its electrical charge and its small mass. Its electrical charge means that it can be moved by an electric field, as in a telephone wire. Its small mass means that the path of a beam of electrons can be rapidly bent as in a television tube. An electron carries a negative charge and the proton an equal positive charge. Since these charges are opposite, protons and electrons attract each other. It is this attraction that keeps electrons in the electron cloud surrounding the nucleus of an atom. The neutron carries no electrical charge, i.e. it is neutral, and it does not have any part to play in making electrons stay in the electronic cloud.


2.2 Atomic structure

Hydrogen and oxygen are two very common substances since their atoms go to make up that very useful liquid called water. Fig. 2.2 shows that a hydrogen atom has the simplest structure of all atoms, since it has just one proton in its nucleus and one electron in the space surrounding the nucleus. It is this single proton that tells us the atom is hydrogen since it is the number of protons in the nucleus of an atom which determines the physical and chemical properties of that atom. The electrical charge on a proton is equal and opposite to the charge on an electron, making the normal hydrogen atom electrically neutral.

An oxygen atom has a more complicated structure as shown in Fig. 2.3. Its nucleus is made up of eight protons and eight neutrons. Thus an electrically neutral oxygen atom has eight electrons in the space surrounding the nucleus. There are more than 100 different atoms in the universe, all made from the three main fundamental particles, neutrons, protons and electrons. The table opposite summarises the atomic structure of just a few of these atoms.


2.2 A hydrogen atom

Fig. 2.2 A hydrogen atom



2.3 An oxygen atom

Fig. 2.3 An oxygen atom


Neutrons combine with protons to make up the nuclei of all atoms, but they do not carry an electrical charge. In spite of the neutron’s zero charge, neutrons and protons do attract each other strongly when they are close together in the nucleus of an atom. When this force is overcome, as is the case when uranium atoms are ‘split’ in nuclear reactors and atomic bombs, an enormous amount of energy is released. However, electrons are held much more weakly to atoms, and it is on this weakness that electronics is based.


Some information about atoms


2.3 Conductors, insulators and semiconductors

The reason why some materials, such as copper, are good electrical conductors is that they contain ‘free’ electrons which are quite weakly bound to the nuclei of the atoms of the material. These electrons can be moved easily by connecting the material across a battery. Copper and aluminium are good electrical conductors and are used in electronics to allow electrons to flow easily between one device and another. Electrons are more strongly attracted to their parent nuclei in electrical insulators, which therefore do not have any free electrons. Thus electrical insulators such as glass, polythene and mica are used to resist the flow of electrons between electronic devices.


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