March 14, 2012

Atomic Structure - II Three Marks

1. Define hybridisation.
The intermixing of the orbitals of an atom having nearly the same energy to give exactly equivalent orbitals with same energy, identical shapes and symmetrical orientations in space.
2. Distinguish between a particle and a wave.
Occupies a well-defined position in space i.e. a particle is localized in space.
Spread out in space i.e. a wave is delocalized in space.
When a particular space is occupied by one particle, the same space cannot be occupied simultaneously by any other particle and hence particles do not interfere.
Two or more waves can coexist in the same region of space and hence interfere.
When a number of particles are
present in a given region of space, their
total value is equal to their sum i.e. it is neither less nor more.
When a number of waves are present in a given region of space, due to interference, the resultant wave can be larger or smaller than the individual waves i.e. interference may be constructive or destructive.
3. Explain bond order.
Half the difference between the number of electrons in bonding molecular orbitals (Nb) and the number of electrons in antibonding molecular orbitals (Na) i.e,
Bond Order = ½ (Nb - Na )
4. State Heisenberg’s uncertainty principle.
1.It is impossible to measure simultaneously both the position and velocity (or momentum) of a microscopic particle with absolute accuracy or certainty.
2.Mathematically, it can be defined as
Δx . Δp h / 4π
Δx  = Uncertainty in the position of the particle.
Δp = Uncertainty in the momentum of the particle.

Δx . mΔv ≥ h / 4π
Δx = Uncertainty in the position of the particle.
Δv = Uncertainty in the velocity of the particle.
  m = Mass of the particle.
5. What are the conditions for effective hydrogen bonding?
1.High electronegativity of the atom bonded to hydrogen so that bond is sufficiently polar.
2.Small size of the atom bonded to hydrogen so that it is able to attract the bonding electron pair effectively.
6. What is the significance of negative electronic energy?
The energy of an electron at infinity is arbitrarily assumed to be zero. This state is called zero-energy state. When an electron moves and comes under the influence of nucleus, it does some work and spends its energy in this process. Thus, the energy of the electron decreases and it becomes less than zero ie., it acquires a negative value.
7. Why is He2 not formed?
1.The electronic configuration of helium atom (Z = 2) in the ground state is 1s2.
2. He2 molecule has 4 electrons.
3.The electronic configuration of He2 molecule is He2 : (σ1s)2 (σ*1s)2.
Here, Nb = 2 and Na = 2
4.Bond order = Nb - Na / 2 = 2 - 2 / 2 = 0
As the bond order for He2 comes out to be zero, this molecule does not exist.
8. Define orbital
The region of space around the nucleus within which the probability of finding an electron of given energy is maximum


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