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Substances whose molecules are formed by atoms of the same type are known as elements, of which there are known to be, at present, 118 ( Table 2.1 ). Some of these are man-made or synthetic. Substances whose molecules are made up of atoms of different types are known as compounds. Hence, water, which is a compound, comprises two hydrogen atoms (H) and one oxygen atom (O), that is H 2O. Similarly, sulphuric acid has two hydrogen, one sulphur and four oxygen atoms: hence, H 2SO4
Molecules are always in a state of rapid motion, but when they are densely packed together this movement is restricted and the substance formed by these molecules is stable (i.e. a solid). When the molecules of a substance are less tightly bound there is much free movement, and such a substance is known as a liquid. When the molecule movement is almost unrestricted the substance can expand and contract in any direction and, of course, is known as a gas .
The atoms which form a molecule are themselves made up of particles known as protons, neutrons and electrons. Protons are said to have a positive ( ve) charge, electrons a negative ( ve) charge and neutrons no charge. Since neutrons play no part in electricity at this level of study, they will be ignored from now on. So what is the relationship between protons and electrons; how do they form an atom? The simplest explanation is to liken an atom to our Solar System, where we have a central star, the Sun, around which are the orbiting planets. In the tiny atom, the protons form the central nucleus and the electrons are the orbiting.
The atomic number (Table 2.1) gives an indication of the number of electrons surrounding the nucleus for each of the known elements. Hence, copper has an atomic number of 29, indicating that it has 29 orbiting electrons. Electrons are arranged in layers or clouds at varying distances from the nucleus (like the rings around Saturn); those nearest the nucleus are more strongly held in place than those farthest away. These distant electrons are easily dislodged from their orbits and hence are free to join those of another atom whose own distant electrons in turn may leave to join other atoms, and so on.
These wandering or random electrons that move about the molecular structure of the material are what makes up electricity. So, then, how do electrons form electricity? If we take two dissimilar metal plates and place them in a chemical solution (known as an electrolyte) a reaction takes place in which electrons from one plate travel across the electrolyte and collect on the other plate.
So one plate has an excess of electrons which makes it more ve than ve, and the other an excess of protons which makes it more ve than ve. What we are describing here, of course, is a simple cell or battery ( Fig. 2.2 ).