Structure Of Atom

Structure Of Atom

Proton 

• Credit for the discovery of  Proton goes to E.Goldstein.
• Protons are positively charged.
• The mass of a proton is taken as one unit each.

Electrons 

• Discovered by J.J. Thomson in Cathode Ray Tube Experiment.
• Electrons are negatively charged.
• The charge of an electron was measured by R.Millikan in Oil drop experiment.
• The mass of an electron is about 1/2000 times the mass of a hydrogen atom. 

Neutrons

• Discovered by J. Chadwick
• It has no charge and a mass nearly equal to that of a proton.
• The mass of a neutron is taken as one unit each.

Atomic nucleus 

• Discovered by Rutherford
• Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus.The fast moving alpha (α)-particles (doubly charged helium ions) were made to fall on a thin gold foil.
• The mass of an atom is therefore given by the sum of the masses of protons and neutrons present in the nucleus.
• The mass of an electron is about 1/2000 times the mass of a hydrogen atom. The mass of a proton and a neutron is taken as one unit each.

Valency

• The electrons present in the outermost shell of an atom are known as the valence electrons.
• The atoms of elements, outermost-shell having a completely filled outermost shell show little chemical activity.Their combining capacity or valency is zero.
• An outermost shell which had eight electrons was said to possess an octet. Atoms would thus react, so as to achieve an octet in the outermost shell.
• The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us the valency.

Atomic Number

• The atomic number is defined as the total number of protons present in the nucleus of an atom.It is denoted by "Z".

Mass number

• The mass of an atom resides in its nucleus.The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom.Since they are present in the nucleus of an atom. So, protons and neutrons are also called nucleons. 
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Isotopes

• Atoms which have the same atomic number but different mass numbers. 
• It has three atomic species, namely

1.  Protium  
2. Deuterium  or 
3.  Tritium or T 

• The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively.
• The chemical properties of isotopes are similar but their physical properties are different.But some isotopes have special properties which find them useful in various fields. 
• Some of them are:

(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii) An isotope of cobalt is used in the treatment of cancer.
(iii) An isotope of iodine is used in the treatment of goitre.

Isobars

• Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.

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