9th chemistry -Discovery of the neutron - Ruther ford Model of Atom

DISCOVERY OF THE NUCLEUS

J.J.Thomson’s atomic theory explained electrical neutrality of atoms, it could not reveal the presence of nucleus in an atom, which was later in 1909 proposed by Ernest Rutherford

A stream of alpha particles was made to pass through a thin gold foil of about  4x10-5 cm thickness. Most of the alpha particles did go through the foil in a straight line. Some alpha particles were deflected through an average angle of 90o .Rarely the path of 1 in 20,000 alpha particles scored a direct hit on the nucleus and returned by an angle of 180o.

He concluded that there is a heavy positive charge occupying small volume, at the centre of an atom. The atom consisting of a small, dense, positively charged nucleus containing most of the mass of the atom with the electrons in the space outside the nucleus .The moving electrons occupy most of the volume of the atom. The electrons must be moving very rapidly in the space around the nucleus.

He state a moving electron should accelerate and continuously lose energy. Due to the loss of energy, path of electron may reduce and finally the electron should fall into nucleus.

This is not true

If it happens so, atom becomes unstable. But atoms are stable. Hence Rutherford’s theory was unable to explain the stability of atom.

Niel’s Bohr modified Rutherford’s atom model and put forth the following postulates.

• In atoms, the electrons revolve around the nucleus in stationary circular paths. These paths are called

• orbits or shells or energy levels. 

• As long as electrons revolve in the same orbit, it does not lose or gain energy. 

• The circular orbits are numbered as 1, 2, 3, 4 or designated as K, L, M, N shells. These numbers are referred to as principal quantum numbers (n). 

• Smaller the size of orbit, smaller is the energy of the orbit. 

• As we move away from nucleus, energy of orbit is constantly increasing. 

• Maximum number of electrons that can be accommodated in an energy level (n) is given by 2n2 

• When an electron absorbs energy, it jumps from lower energy level to higher energy level. 

• When an electron returns from higher energy level to lower energy level, it gives off energy.

The fundamental particles of an atom are,

Protons: They are positively charged particles. They are present inside the  nucleus.

Electrons: They are negatively  charged particles. They revolve around the nucleus in circular orbits.

Neutrons: They are neutral particles.  They are present inside the nucleus.

Protons are positively charged.  Protons repel each other because of their like-charges. Hence, more than one proton cannot be packed in a small volume to form a stable nucleus unless  neutrons are present.

Neutrons reduce the repulsive force  between positively charged protons and contribute to the force that holds the particles in the nucleus together.

Atomic number

Atom as a whole is electrically neutral. It is due to the presence of equal number of protons and electrons. This number is referred to as atomic number.

Atomic number  =  The number of protons in the nucleus

Atomic number  = The number of electrons revolving around the nucleus.

Mass number : the mass of an atom  entirely depends on the mass of nucleus.

The mass of the lightest atom, hydrogen  has been chosen as the unit of mass.

Since the nucleus of an atom contains protons and neutrons, mass number (A) is defined

as, the sum of the number of protons and neutrons in the nucleus of an atom

Mass Number (A) = Number of protons + Number of neutrons

Representation of Atomic number and  Mass number

Superscript represents mass number.      Subscript represents atomic number.

For example, Atomic number of nitrogen is 7. Mass number of nitrogen is 14.

Representation:  

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