Structure of Atom

Atoms are made up of three particles

  1. Electrons
  2. Protons
  3. Neutrons


Electrons are discovered by JJ Thomson in 1897, protons are discovered by E. Goldstein and neutrons are discovered by Chadwick in 1932. Electrons are negatively charged whereas protons are positively charged and neutrons are charge less.

protons and neutrons are present in a small nucleus at the center of the Atom whereas electrons are present around the nucleus. The absolute mass of electron is 9×10-28 gram and charge on it is 1.6×10-19 Coulomb of negatively charge. Whereas as mass of Proton is 1.6× 10-24 gram and charge on it is 1.6×10-19 Coulomb of positively charge. Neutron have mass which is equal to mass of Proton and it is charge less.

Atomic number

The number of protons in one Atom of an element is known as the atomic number of that element. It is denoted by Z. Atomic number of an element is equal to the number of electrons only in neutral Atom and not in an ion.

Mass number / atomic mass

The sum of numbers of protons and neutrons present in one Atom of an element is known as mass number/ atomic mass. It is denoted by A, A= Z+ number of neutrons.

Arrangement of electrons in one Atom: electrons are arranged according to their potential energy in different energy levels of shells. Shells are denoted by the latter’s K, L, M, N, N, O, P. First energy level is K shell , 2nd is L, 3rd one is M and so on. The maximum number of electrons which can be present in any energy level of the Atom is given by 2n2 (where n=number of energy level) therefore
K    n = 1    2n2    2×12 = 2 electrons
L    n = 2    2n2    2×2 2 = 8 electrons
So on
M have 18 electrons
N have 32 electrons

Valance electrons

The electrons which take part in chemical reactions are called valence electrons. They presents in the outermost shell of an atom. We can find the number of valance electrons by write down the electronic configuration of elements by using its atomic number.

Valancy of elements

Valancy is known as capacity of an Atom to form chemical bonds. For eg. Carbon has 4 valancy , hydrogen has 1 .

It is the number of electrons loose or gain by an atom to full its valance shell. For eg. Chlorine have 7 valance electrons in valance shell so it is easy to take 1 electrons from another element atom instead of loosing 7 electrons to another element. Therefore valancy of chlorine is 1. Another example is Sodium has 1 valance electrons in valance shell so that it will loose 1 electron easily then gaining 7 electrons from its valance shell therefore valancy of Sodium is 1.

Chemical Bonding

The force that holds any two atoms to form a new substance is called chemical bonding. There are two type of bonds.

  1. Electrovalent Bond: When a bond is formed by complete transfer of electrons is known as ionic bond
  2. Covalent Bond: When a bond is formed by the mutual sharing of electrons is called covalent bonds. Size of bond is smaller


Nuclear reaction

It is the reaction in which nuclear nucleus is bombarded with neutrons to produce other products with high energy produce in a very short time. It is discovered by Rutherford. It is of two types: Nuclear fission and Nuclear fusion.

  1. Nuclear Fission: In this a large nucleus is splits into two smaller nucleus and produce large amount of energy, For eg. 
    236 U 92 + 1 n 0 → 84 KR 36 + 144 Ba 56 + 0 n 1 + 210 MeV
  2. Nuclear Fusion: It is a reaction in which two or more atomic nucleus come very close band then collide at a very high speed and join to form a new nucleus.
    1 H 3 + 1 H 2 → 2 H 0 n 1 + energy


Nuclear Energy

Energy produced by nuclear fission or fusion is called nuclear energy. In nuclear reactions there is loss of mass this mass is converted into high energy.


Elements having same atomic number but different atomic mass. Eg. Oxygen have 8 atomic number but have different atomic mass (16,17,18)


Elements having the same atomic mass but different atomic number. Eg. Argon, potassium and calcium have same mass that is 40 but all have different atomic number (18,19,20 respectively)


Elements having same number of neutrons.

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