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Chemistry deals with varieties of matter. Air, water, rocks, minerals, plants, animals including man, the earth on which we live and other planets and galaxies are some example of matter. Matter exists in innumerable shapes, size colures, odors and posses different properties. Atoms of one element combine with atoms of other elements in a number ways and give a large variety of simple and complex substances. Atoms of different elements behave in different ways

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After going through this unit you will be able to

  • List fundamental particles present in an atom.
  • Differentiate between these fundamental particles.
  • Describe various models of an atom.
  • Draw the schemetic diagram representing the arrangement of electrons around the nucleus.

Structure of Atom

Main points
  • Matter is electrical in nature.
  • Atoms are made up of three fundamental particles –Electrons, Protons and Neutrons.
  • Protons and neutrons are present in a very small nucleus at the centre of the atom .Almost the entire mass of the atom concentrated in the nucleus.
  • Electrons which have negligible mass are present around the nucleus.
  • The arrangement of electrons, protons and neutrons in an atom is explained by a number of models.
  • The electron in an atom revolves around the nucleus in definite energy levels or cells.
  • Atom contains equal number of protons and electrons and is electrically neutral.

Electrical Nature of Matter

Take a balloon and fill it with air blown through your mouth . Now rub the balloon with a woolen , silk or terylene material. Bring it near the wall. What do you observe ? It is attracted by the wall.
Now fill air in two balloons through your mouth .Suspend these balloons with the help of a thread .Bring them nearer to each other .What do you notice?You will see that the balloons come close to each other. Can you tell why is it so? Now, rub these balloons with your coat, silk or terylene shirt .Suspend them with thread and again bring them closer to each other .What do you observe now ? The balloons move apart .Can you cite a few more examples of this kind of phenomena from your daily life experiences?
This phenomenon of attraction and repulsion occurs on accounts of electrical charges presents in matter. Coat, silken shirt, terylene shirt, wall, balloons, comb, water and hair are different kinds of matter. This observations lead us to believe that matter is electrical matter.
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Atomic Particles

We know that matter is electrical in nature. After the discovery of electricity in the nineteenth century, scientists studied the passage of electricity through matter. They found that metals conduct electricity while many other substances (known as non-metals) were non-conductors (insulators). Passage of electricity through gases at low pressure was another interesting observation of scientists at the end of nineteenth century.
If an electric charge is passed between two electrodes sealed in a glass tube containing air at a very low pre pressure (1mm Hg), a glow is seen in between the electrodes. If air in the tube is replaced by any other gas, the glow still persists, but the colour of the glow changes with the nature of the gas.
If similar experiments are performed at even lower pressures (0.001 mm Hg), it is observed that the glow in the tube disappears and instead the glass tube at the end opposite to the cathode starts glowing and emits at greenish light.
It is now known that some invisible rays are formed at the cathode. When these rays strike the glass (discharge) tube, they emit a greenish light. As these rays seem to come out form the cathode, (negative electrode), scientists called these rays as cathode rays.

Model of an Atom

Dalton's Model

File:Dalton.jpg John dolton a danish english proffessor is reffered as the father of modern atomic theories . According to his theory, Atoms are spherical in shape. All elements are made up of atoms Atoms of an element have same mass. Atoms of different element posses different mass. Atoms can neither be created nor be destroyed and it is indivisible.

Thomson's Models

File:200px-J.J Thomson-1-.jpg

Jos eph John Thomson the discoverer of electron proposed a theory on atom model called plum pudding atom model.According to his theory electrons are embedded on uniform positive sphere like Plum pudding.The following figure will give you an idea on his plum pudding atom model.

Bhor's Models

Model of Li Atom

To overcome the objections to the Rutherford model and also to explain the spectral lines of hydrogen, Bohr provided the connection between line spectrum of hydrogen and quantum ideas. He postulated that if an electron revolves around the nucleus in a certain fixed energy level, it does not radiate any energy.
Let us visualize how electrons are arranged around the nucleus in an atom. The electrons from a cloud of negatively charged particles outside the nucleus. In this cloud, the electrons are arranged according to their energy. These energy level are described by number 1,2,3,4……… or by letters K,L,M,N etc. referred to as shells. Smaller values of the principal quantum number indicate that the electrons are in a low energy level. The n=1 energy level is lowest energy level. This corresponds to K-shell. Similarly, the successive higher energy level n=2,n=3 and so on correspond to L,M, shell, etc.

  • The first or the inner most shell (n=1) can take only two elements.
  • The second shell (n=2) can take upto 8 electrons.
  • The next shell (n=3)can take maximum 18 electrons, but if it is the outermost, then it cantake 8 electrons only.

On the basis of experimental observations, Bohr and Busy suggested that:

  • The maximum number of electrons that can be accommodated in an

in an energy level is 2n2 where n is the number of that energy level. K-shell (n=1), 2×12 =2 electrons

L-sheel (n=2),2×22 = 8 electrons

M-sheel (n=3),2×32 =18 electrons

N-sheel (n=4),2×42 =32 electrons

  • The outermost shell of an atom cannot have more than 8 electrons and next to outermost –shell cannot have more than 18 electrons.


An element has atomic number 19 and atomic mass 39 How many these electrons are present in the atom? How are these electrons arranged in the atom?


An atom consists of three fundamental particles – electrons, proton and neutrons. Electrons are negatively charged while protons are positively charged and neutrons carry any charge. Electrons revolve around the nucleus where as protons and neutrons are concentrated in the nucleus .the mass of an atom is due to the mass of protons and neutrons .the atom is electrically neutral showing that the number of electrons and protons are the same . hence the atomic number of an atom is equal to the number of protons in the nucleus and the mass number is equal to the sum of the number of protons and neutrons .

File:Atom Structure 3.png


Points added for a correct answer:  
Points for a wrong answer:
Ignore the questions' coefficients:

1. What is the charge of proton?


2. How many neutron present in Hydrogen atom?


3. who was the discoverer of neutron?


4. The mass Number of an element is 23 and the atomic number is 11,the number of neutrons presents in the nucleus is:


5. An element has atomic number 19 and atomic mass 39,number of electron present in the atom is:


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