1.Describe Pauli's exclusion principle with a suitable example.

Pauli's exclusion principle states that it is impossible for any two electrons in a given atom to trace all the four quantum numbers identical that is in an atom, two electrons can have maximum three quantum numbers (n,l and m) the same and the fourth (s) will definitely be having a different value. thus if s = +½ for one electron, s should be equal to -½  for the other electron. The two electrons in the same orbital should have opposite spins(↑↓)

Uses of the principle

Pauli's exclusion principle is helpful in determining the maximum number of electrons that a main energy level can trace.
Example: Consider the 'K' shell  when two combinations are possible for the principle quantum number n =1 that is

(1) n = 1, l = 0, m = 0, s = +½ (1^st electron)

(2) n = 1, l = 0, m = 0, s = -½ (2^nd electron)
This means, in K shell there is only one sub shell corresponding to l = 0 value contains only two electrons with opposite spins.

2.What is Rutherford's -ray scattering experiment? what are its conclusions.

Rutherford conducted a scattering experiment to find out the arrangement of electrons and protons. He bombarded a thin gold foil with a stream of fast moving positively charged -particles from a radiative source, when the particles where passed through thin sheets of metal foil. He concluded that,

(1) Most of the space in the atom is empty as most of the - particles passed through the foil.

(2) A few positively charged - particles are deflected. The deflection must be due to enormous repulsive force showing that the positive charge of the atom showing that the positive charge of the atom is not spread throughout the atom as Thomson had thought. The positive charge has to be concentrated in very small volume that repelled and deflected .This portion was called nucleus by Rutherford.

(b) The model developed by Rutherford showed that the volume occupied by the nucleus is negligibly small as compared to the total volume of the atom.
 
(c) The nucleus is surrounded by electrons that move around the nucleus with very high speed in circular paths called orbits. This model  of atom (electrons revolve around the nucleus) resembles the solar system (like planets around the sun).

(d) The model showed that the number of electrons in an atom is equal to the number of protons in it. Thus the total positive charge of the nucleus exactly balances the total negative charge in the atom making it electrically neutral. The number of protons in an atom is called its atomic number.

(e) Electrons and the nucleus are held together by electrostatic forces of attraction.

3.What are the defects of Rutherford's model of atom?

1.The model facted to account for the existence of atomic spectra.

2. If it be correct, the electron should spiral into the nucleus and the atom should collapse. But atoms are stable.

4. Explain Hund's rule of maximum multiplicity.

 Hund's rule of maximum multiplicity states that in filling of p,,d or f orbitals, as many unpaired electrons as possible are placed before pairing of electrons with opposite spin is allowed otherwise pairing of electrons in the orbitals belonging to the same sub shell (p,d or f) does not takes place until each orbital belonging to that sub shell has got one electron each that is singly occupied.

5.Briefly explain the electronic configuration of elements according to Aufbau's principle.

According to Arufbau's principle, the electrons in the ground state of an atom are filled in orbitals in increasing order of their energies. The order of increase of energy of orbitals can be calculated from the (n +l) rule explained below.

1. The lower the value of (n+l) for an orbital, the lower will be its energy. This means that between 3d and 4s the 4s (n+l = 4+0=4) will fill before 3d (n+l = 3+2 =5)

2.If two orbitals have the same (n+l) the one with lower n will be filled first. Thus, between 2p(n+l = 2+1 = 3) and 3s (n+l = 3+0=3) 2p will fill before 3s.

The order in which the energies of the orbitals increase and hence the order in which the orbitals are filled it as follows:

1s< 2s < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p

6.State Heisenberg's uncertainty principle.

Heisenberg's uncertainty principle states that it is impossible to determine simultaneously both the position and momentum of an object with certainty. It acquires significance when applied in elementary particles.

7.How many neutrons and protons are there in the following nuclei?

8.Write the complete symbol for the atom with the given atomic number (z) and atomic mass (A). (i) z = 17, A = 35  (ii) z = 92, A = 233     (iii) z = 4, A = 9.

9.What are the defects of Bohr's atomic model?

Bohr's atomic model could not explain,

(i) the atomic spectra of higher elements like He, Li, Be, B, C... which have more than an electron.

(ii) the spectral lines in pressure of magnetic and electric field.

(iii) The quantisalian of angular momentum.

(iv) the formation of chemical bonds.

10.Explain the four quantum numbers briefly.

Quantum numbers are a set of four values that define the energy state of an electron in an atom. Quantum number values are designated as n, l, m and s.

The four quantum numbers are :

1. Principal quantum number (n)

2. Azimuthal quantum number (l)

3. Magnetic quantum number (m)

1. Principal quantum number (n)

a. It was proposed by Neel's Bohr.

b. It is denoted by the letter 'n'

c. It indicates the size and energy of a stationary orbit.

d. Its values are 1, 2, 3, 4.... and these are designated as K, L, M, N....

2. Azimuthal quantum number (l)

a. It was proposed by sommer feld

b. It is denoted by the letter (l)

c. It indicates the shape of the sub shell.

d. Its values are 0, 1, 2, 3, 4 and these are designated as s, p, d, f, g etc.

3. Magnetic quantum number (m)

a. It was proposed by Laude

b. It is denoted by the letter (m)

c. It indicates the orientation of the orbitals in the presence of magnetic field

d. Its value depends on the value of 'l'.

e. For a given 'l', m can have (2l + 1) values.

4.Spin quantum number (s)

a. It was proposed by Uhlenbeck and Goldsmith.

b. It is denoted by the letter 's'

c. It indicates the direction of spin of electrons

d. Its values are +½ and -½

11.What are the postulates of Bohr's model of atoms?

The main postulates of Bohr's theory are,

(1) Electrons are moving with high velocity around the nucleus in specified paths called orbits or shells.

(2) Each stationary orbit corresponds, to a definite energy level.

(3) When electron jump from higher to lower energy orbit, the difference in energy is emitted as radiation.

(4) The angular momentum of the electron revolving in a stationary orbit is equal to integral multiples of   .