The smallest unit of all matter are called atom. It carries the same properties of that matter. An atom consist of a central nucleus with protons and neutrons, and electrons outside the nucleus. A proton has positive charge and neutron has no charge, therefore nucleus of an atom is positively charged. The charge of proton is + 1.602×10 +19 and charge of electron is -1.602×10-19, same as proton but in opposite sign, which makes an atom uncharged. Electrons carry a negative charge and rotates in a orbit surrounding the nucleus as it attracted by positively charged nucleus. With different no. of proton and electron makes a charged atom. If proton is greater than electron, atom is positively charged and if electron is greater, we called the atom as negatively charged.
The entire weight of an atom i.e, Atomic weight represents the algebraic sum of number of proton and number of neutron on that atom.
So, mass of atom or atomic weight = No. of proton + No of neutron.
And the atomic number represent the number of proton or number of electron, as no. of proton and no. of electron are same in atom in case of ordinary condition.
Two atoms with same atomic number but different atomic weight is known as isotopes.
ELECTRON: Outer part of an atom consist electron with negligible mass and having negatively charged. It carries equal and opposite charge of a proton. These electrons move round the nucleus with different paths name as orbit by some kinetic energy (by motion) as well as potential energy (charge of nucleus).


    No of electrons in any orbit of an atom are 2 X n 2,where n= no of orbit.
    At 1st orbit= 2X12=2 electrons.
    At 2nd orbit= 2X22= 8 electrons.
    At 3rd orbit= 2X32= 18 electrons, and so on…
    The last orbit cannot have more than 8 electrons.

    VALENCE ELECTRONS: Electrons in the last orbit of an atom which have the higher energy are known as valence electrons.
    In case of metal or conductor the valence electrons of an atom is less than 4.
    FREE ELECTRONS: Valence electrons which have the greater energy are loosely attached to the nucleus. These loosely attached valence electrons are called free electrons.
    Examples: Metal or conductor.
    Free electrons can easily remove from an orbit by applying external energy.

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