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Today in this blog we discuss The Atomic Structure

The Atomic Structure or the structure of an atom, theoretically consisting of a positively charged nucleus surrounded and neutralized by negatively charged electrons revolving in orbits at varying distances from the nucleus, the constitution of the nucleus and the arrangement of the electrons differing with various chemical elements.

Dalton’s Atomic Theory

He is regarded as Father of Chemistry

  1. Every substance is composed of very minute particles, which are indivisible are known as atoms
  2. Atoms of a substance are identical in all respects
  3. Atoms of different substances are different in all respects
  4. When a substance combines with another substance, actually atoms combine with atoms
  5. When atoms combine each other their numbers are in simple ratio.


Therefore an atom is the smallest particle of an element, which can take part in chemical reaction.


Defect of the theory

  1. At present the atom may be further divided into electron, proton and neutron
  2. Atoms of same particles are alike in all respects no doubt but after the discovery of isotopes, the mass of the atoms of same elements is not equal.
  3. Atoms of different elements are different no doubt but in isobar it is found that two atoms of different elements having the equal mass. Eg. Potassium and Argon
  4. Nowadays, it is proved that in a chemical combination electrons are involved and not the atom.



Afterwards, Avogadro gave a new idea about another ultimate particle of a substance is known as molecule.

Molecule is defined as the smallest part of a substance element or compound like H2, and H2O, which can exist in free state but cannot take part in chemical reaction directly.

Molecules are two types

Elementary molecule, which contain atoms of same element like H2, Cl2, O2, O3, I2

Compound molecule, which contain atoms of different elements like H2O, H2SO4, Na2SO4


According to Dalton atom is indivisible but nowadays it is experimentally proved that atom can be subdivided and several particles are obtained after the splitting of an atom.  Rutherford performed an experiment known as scattering of alpha particles and he came to following conclusions.


  1. Most of the place in an atom is empty
  2. The mass of an atom is concentrated to its centre whose volume is negligible-compared to the whole atom
  3. The centre of the atom where the mass is concentrated is positively charged.


The atom consists of three types of particles.

Electron, Proton and Neutron

These three particles are known as fundamental particles because all atoms of different elements contains these particles having the same mass, same charge and same properties.


Rutherford model of an atom

  1. Proton and neutron are concentrated at the centre of an atom which is known as nucleus
  2. An atom is electrically neutral
  3. But the nucleus is positively charged
  4. Electrons are orbiting around nucleus
  5. Electron are negatively charged 1.59 x 10-19 coulomb particles with negligible mass
  6. Protons are positively charged 1.59 x 10+19 coulomb with definite mass 1.674 x 10-24 gm
  7. Neutrons are electrically neutral with definite mass 1.672 x 10-24 gm
  8. Therefore the mass of neutron is slightly greater than that of the proton
  9. Since it is electrically neutral so the number of positive charges are equal to number of negative charges that means number of protons must be equal to number of electrons
  10. The orbits of electrons are named as K, L, M, N, O and P
  11. The maximum number of the electron hold by the definite orbit is governed by the definite formula 2n2

So the first orbit can accommodate maximum number of electrons are 2×12 = 2

  1. Except hydrogen all nucleus of atoms contain neutron

The Rutherford model is as like as the solar system


Atomic number and Mass number

Atomic number

Atomic number of an element is the total number of positive charge present in the nucleus that means the number of protons present in the nucleus.  The chemical properties of the element depend on this number.  It is denoted as Z

Mass number

Mass number is the total number of protons and neutrons present in the particular atom of a substance.  It is represented as A



20Ca40 means calcium atom possesses 20 protons, 20 electrons and 40-20=20 neutrons

Its electronic configuration is Ca = 2+8+8+2



When the atoms of same elements having the same atomic number but having different mass number are called isotopes

Hydrogen isotopes – 1H1, 1H2 and 1H3  – protium, deuterium and tritium



When the atoms of different elements having the different atomic number but having same mass number are called isobars

Ca40 and Ar40



Electronic configuration of inert gases


Intert gas                    At.No.           K          L         M        N         O         P         


Helium                               2               2

Neon                                10               2          8

Argon                               18               2          8          8

Krypton                            36               2          8          18        8

Xenon                              54               2          8          18        18        8

Renon                               86               2          8          18        32        18        8


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Hi viewers My name is Partha Pratim Goswami, (PpG) I am a teacher by profession. I have been teaching biology, chemistry and physics, sometimes mathematics also for last sixteen years. I would like to share my entire experience with all the viewers across the world through YouTube, which I think is an amazing platform to share all my experiences.

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