In the study of electricity, positive and negative electricity were regarded as two types of fluids and that were present in equal quantities in neutral bodies. It was considered that when two bodies are rubbed, this fluid is separated, so that one body obtained an excess of the positive fluid and the other an excess of the negative fluid.
Modern theory of electricity is somewhat similar to this. When Dalton presented his atomic theory about the year 1810, all matter was considered to be composed of some elementary particles called "atoms" these being. Identical for each element, these atoms were regarded as indivisible.
It is now know that atoms are divisible and a number of different kinds of particles, many of which are electrically charged, exist and are formed by the breaking up of the atom. There are three main kinds of elementary particles. They are the electron, the proton and the neutron. The mass of an electron is about 1/1840th of the mass of a hydrogen atom which was considered to be the lightest atom . Also, the electron carries a negative charge of electricity. This charge is considered to be the smallest charge and all other charges are considered as multiples of this elementary charge. The mass of the proton is equal to that of the hydrogen atom, While it carries a charge of positive electricity equal in size to the charge of an electron. neutron has nearly the same mass as a proton but is electrically neutral. The atom is regarded as being built up of these three kinds of particles. The protons and neutrons together form a heavy center called the "nucleus" around which the electrons continuously revolve somewhat like a miniature solar system.
Practically, all the mass of an atom Is concentrated in the nucleus. Because of the protons, the nucleus, the nucleus has a positive charge. But the number of protons in the nucleus is equal to the number of electrons around it. So, the complete atom Is electrically neutral. For instance, let us consider an atom which has three protons in the nucleus, as shown in this image.
The nucleus has positive charge numerically equal to that of three electrons. Since this atom is neutral, three electrons are associated with it, and these electrons revolve in orbits around the nucleus. The presence of neutrons in a nucleus will add to its mass but not affect its electrical charge. It may be mentioned that a nucleus with three protons may contain three or four neutrons. When electrons of an atom are transferred to another atom they both become electrically charged.
For example let us consider two neutral atoms each having three protons in the nucleus and three electrons around the nucleus image . They are both neutral.
Now, suppose that one electron from the left hand atom is transferred to the right hand one , as shown image .
Then , left hand one now has a bigger positive charge on its nucleus than the negative charge around it. So, the charge on the atom is positive and the atom is said to be positively charged . Also, the right hand atom Is negatively charged by its excess electron. When an atom gained or lost electrons, it is referred to as an ion.
In general, atoms of metals tend to lose electrons and become positive ions and atoms of non-metals tend to gain electrons and become negative ions.
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