Short Answer TypeLifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nano second range. If the radiation source has the duration of 2 ns and the number of photons emitted during the pulse source is 2.5 x 1015, calculate the energy of the source.
Electromagnetic radiation of wavelength 285 nm is just sufficient to ionise the potassium atom. What is the ionization energy of potassium (in kJ mol-1) ?
Long Answer TypeGive essential features of Bohr's model of atom.
2. These circular paths are called orbits, shells energy levels or stationary states in which electron can revolve around the nucleus without emitting radiations. So far as an electron revolves in a certain orbit, its energy remains constant.
3. These different energy levels are designated by numbers 1, 2, 3, 4 etc. or letters K, L, M, N etc. starting from the nucleus. The greater the distance of the energy level from the nucleus more is the energy associated with it.
4. The electrons in an atom can revolve only in those orbits in which the angular momentum (mvr) of the electron is a whole number (n) multiple of a constant 
.
where m = mass of electron,
v = velocity of electron
r = radius of orbit.
h = Planck's constant,
n = whole number
This postulate indicates that the angular momentum of an electron moving in a circular orbit is quantisied. The angular momentum can be
5. When an electron jumps from one stationary state to another, the difference of energy (∆E) between two states (E1 and E2) is emitted or absorbed, as radiation of frequency (v) given by the equation
∆E = E2- E1 = hv
If an electron jumps from higher energy state to a lower energy state, energy is emitted. Energy is absorbed by an electron when it jumps from a lower energy state to a higher energy state.
It is obvious that the electron cannot radiate energy if no energy level is available. That is why atoms do not collapse.
From Bohr model, one can calculate the energy En of an electron in an orbit n. This is given by the expression,
Further, one can also calculate the radius of each circular orbit from the expression
rn = 0.529Å x n2 where n= 1,2,3......
The radius of the first orbit r1, called Bohr’s radius (n = 1) is 0.529A (or 52.9 pm).
Bohr model is also applicable to ions such as He+, Li2+ etc. For such cases,
and 
where Z is the atomic number and has values of 2 and 3 for He+ and Li2+ respectively.
Short Answer TypeThe energy associated with the first orbit in the hydrogen atom is - 2.18 x 10-18J atom-1. What is the energy associated with the fifth orbit?
(ii) Calculate the radius of Bohr’s fifth orbit for the hydrogen atom.
Calculate the energy associated with first orbit of He+. What is the radius of this orbit?
Long Answer TypeHow much energy is required to ionise a H atom if the electron occupies n = 5 orbit? Compare your answer with the ionization enthalpy of H atom (energy required to remove the electron from n = 1 orbit).
Short Answer Type
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