27 November 2017

Atomic Structure

  • The scientist Neil Bohr explains the atomic structure. According to Neil an atom consists of positive charged nucleus and negative charged electrons revolve in circular orbit. 
  • The electrons can revolve around the nucleus in certain orbits. 
  • Each orbit consists of certain energy level. As the distance between nucleus and orbit increases, energy of electrons increases. 
  • If the energy is given to an electron, the electron lifts to higher energy level. This is called as excitation.


  • The matter is defined as anything which occupies space and weight such as wood, copper, water, air etc.


  • The matter is made up of very small particles known as molecules.


  • It is smallest into which matter can be divided without release of charged particles. It is smallest unit of matter.


  • The nucleus of an atom is mainly consists of two particles protons and neutrons.
  • The proton has positive charge whereas the neutron has no charge. 
  • The mass of neutron is same as that of proton. Therefore the nucleus has positive charge.

Atomic weight

  • It is sum of weight of protons and weight of neutrons.
        Atomic weight = weight of protons + weight of neutrons
        A = Z + N
  • According to electron theory, the atoms of all elements composed mainly two parts namely nucleus at the centre and electrons surrounding it. 
  • The nuclear symbol consists of three parts, symbol of element, atomic number and atomic mass. Let us try to understand it.

        26Al13 = AAlZ = Number of protons + Number of neutornsAlNumber of protons

  • Mass of aluminium ( A ) is 26
  • Atomic number of aluminium ( Z ) is 13
  • Therefore the number of neutron in the aluminium = A – Z
                                                                                  =  26 – 13
                                                                                  = 13


Extra nucleus ( Electrons )

  • It is outer part of an atom which contains electrons only. 
  • The charge of an electron is equal and opposite to that of charge of proton. 
  • The number of electron is equal to number of proton. 
  • Therefore we can say that an atom is neutral.
        Charge of an electron = 1.602 × 10 – 19 Coulomb
        Mass of an electron = 9.0 × 10 – 19 Kg
        Radius of an electron = 1.9 × 10 – 15 Meter
  • The ratio of charge of an electron to the mass of an electron is 1.77 × 1011 Coulomb per kg. 
  • It means that mass of an electron is very small as compared to its charge. 
  • The electron is greatly affected by electric or magnetic field.

Electrons in orbit

  • The electrons of an atom around nucleus revolve in different orbits. The number of electrons in any orbit is equal to 2n2.
               Where n is orbit number
         Number of electrons in first orbit = 2(1)2 = 2
                                       Second orbit = 2(2)2 = 8
                                        Third orbit = 2(3)2 = 18
  • The number of electron in the last orbit should not greater than 8.
  • Any orbit should not contain more than 18 electrons. 
  • The atomic number of silicon is 14. It means that 2 electrons lies in first orbit, 8 electrons in second orbit and remaining 4 electrons in the third or last orbit.

Atomic number

  • It is defined as the number of proton is equal to number of electrons.
        Atomic number = Number of proton
                                = Number of electrons
  • The atomic number of sodium is eleven ( 11 ). 
  • It means that it has 11 number of proton and 11 numbers of electrons. 
  • It has 2 electrons in first orbit, 8 electrons in second orbit and only one electrons in the last or third orbit. List of atomic number

Energy of an electron

  • Each electron consists of kinetic energy due to its motion and potential energy due to charge of electron. 
  • As the distance between nucleus and electron increases, the energy of an electron also increases. Therefore
  • Energy of 1st orbit electrons < Energy of 2nd orbit electrons
  • Energy of 2nd orbit electrons < Energy of 3rd orbit electrons
  • Energy of 3rd orbit electrons < Energy of 4th orbit electrons
  • It means that electrons of first orbit consists lowest energy whereas the electrons of last orbit have highest energy. 
  • The electrons of last orbit play important role to determine electrical, chemical and physical properties of a material.


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