All matters are made up of minute particles called molecules, which can be further subdivided into atoms.
Elements are composed of molecules containing atoms of one kind only. Mendeleev’s periodic system gives classification of elements. The number of elements is 101 eg. Hydrogen, Carbon. Nitrogen. Oxygen, Sodium. Magnesium, etc.
Compounds are composed of molecules containing atoms of different kinds. The number of compounds is almost infinite.
Atoms of all substances are built up of positive electricity called protons & negative electricity called electrons Neutrons which show no electrical state.
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An atom consists of a central nucleus made up of protons and neutrons and around this nucleus there are numbers of electrons revolving in different orbits. In the normal state the number of protons equals to the number of electrons and the atom as a whole is electrically neutral. Atomic number means, number of protons. eg. Hydrogen 1, Helium 2, Lithium 3, Beryllium 4, Boron 5, Carbon 6, Nitrogen 7, Oxygen 8, Fluorine 9, Neon 10, Sodium 11, Magnesium 12, Aluminum 13, Silicon 14, Phosphorous 15, Sulphur 21, Chlorine 17, Argon 18, Potassium 19, Calcium 20, Scandium 21, Titanium 22, Vanadium 23, Chromium 24, Manganese 25, Iron 26, Cobalt-27, Nicket 28, Copper 29, atomic, weight is the sum of protons & neutrons.
The revolving electrons are held to the nucleus by an attractive force. In conductors they are easily displaced. The electrons in outer most orbits are called free electrons, they can move from an atom to another.
When difference of potential is applied between the ends of a conductor the haphazard movement of charges: causes a steady flow along the conductor and it is this moving stream of electrons which constitutes an electric current.
The electron movement however is impeded by collision with the molecules giving rise to a certain opposition to the flow of currents called resistance.
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The flow of current is from positive to negative. Electrons flow from negative to positive.
In insulators the electrons are firmly held and hence if a potential difference is applied little or no electrons flow occurs.