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24.007 Atoms with only one valence electron make good conductors


Exploring Electronics, Standard 7, 1995
Pages 37-42, atoms with only one valence electron make good conductors


4.1.1 The atom

An atom is the smallest part of a substance or element. It is so small that you cannot see it. If you line ten million of them up side by side they would measure less than one millimetre. The smallest part of a drop of water would be called a molecule of water. This water molecule has three atoms, that is, two hydrogen atoms and one oxygen atom.


4.2 A water molecule

Figure 4.2 A water molecule


The atom consists of a nucleus with electrons orbiting around it. The protons and neutrons are contained in the centre of the nucleus of the atom, and the electrons orbit around the nucleus. The proton is small but very heavy; it is difficult to dislodge from the nucleus. The electron is larger than the proton but is light and moves easily (see figure 4.3).

The electrons and the protons are the particles which have the electrical properties. The electron has a negative electrical charge, and the proton has a positive electrical charge. These charges are equal and opposite. The law of electrical charges states that particles with like charges repel each other, and those with unlike charges attract each other. This is what keeps the electrons in orbit.



Figure 4.3


The nucleus of the atom contains the positive proton, and so has a positive charge that attracts the negative electrons. The centrifugal force of the orbiting electrons counteracts the attraction of the nucleus to keep the electrons orbiting. Since electrons have like charges, they repel each other. This also results in their being spaced equidistant from one another.

Neutrons have no electrical charge; they are neutral. They are sometimes thought of as protons and electrons combined, but they are actually different particles. Usually, atoms have the same number of electrons and protons, and so they are electrically neutral.


(a) Electrical charges

Electrical charges are illustrated in figure 4.4.


4.4 Electrical charges

Figur 4.4 Electrical charges


4.4 Electrical charges (continued)

Figure 4.4 Electrical charges (continued)


(b) Orbital shells

An atom of one element is differentiated from an atom of another element by the number of protons it has in its nucleus. Since a neutral atom has the same number of electrons as protons, an atom with 29 protons should have 29 electrons orbiting around its nucleus. These electrons orbit in groups called a shell. Actually, each electron has its own individual orbit, but certain orbits are grouped together to produce what is called a shell. For convenience, all the electrons in one shell are showb on diagrams as though they follow the same orbit (see figure 4.5).


4.5 Shells of different elements

Figure 4.5 Shells of different elements


(c) The valence shell

The outermost shell of an atom is called the valence shell. The word "valence" is a Greek word meaning "hook". An old chemical theory considered that atoms had hooks that held them to other atoms. Since we know that it is the electrons in the outermost shell that enable atoms to join, the word valence was carried over as the name of the outer shell. Electrons which orbit in the outer shell are also known as valence electrons.



Figure 4.6


You may have noticed in the discussion that although the third shell can hold up to 18 electrons, it only had eight until a fourth shell started (figure 4.6). This is also true of the fourth shell. It will not take on any more than eight until a fifth shell starts, even though the fourth shell can hold up to 32 electrons. This shows that there is another rule: the outer shell of any atom can not hold any more than eight electrons. This rule is important because it shows which atoms make good conductors, insulators, or semiconductors, as you will soon learn.


4.2 Stable and unstable atoms

The tendency of an atom to give up its valence electrons depends on chemical stability. When an atom is stable, it resists giving up electrons, but when it is unstable, it tends to give up electrons. The level of stability is determined by the number of valence electrons, because the atom tries to have its outer or valence shell completely filled.

If the valence shell of an atom is more than half-filled, that atom tends to fill its shell. Therefore, since eight is the largest number of electrons that can be held in the valence shell, elements with five or more valence electrons tend to give up their electrons to empty the valence shell; this would allow the next shell, which is already filled, to be the outermost shell. These atoms make the best electrical conductors.

An atom with eight valence electrons is completely stable, and will resist any sort of activity. These are the inert gases (atomic numbers 2, 10, 18, 36, 54 and 86), which are the best insulators. Atoms with only one valence electron are the best conductors. As you probably have gathered by now, semiconductors have four valence electrons, and are neither good conductors nor good insulators.

Important: Atoms with only one valence electron make good conductors.



For the sake of simplicity, only the valence electrons are shown

Figure 4.7


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