Bohr's Theory Of Hydrogen Atom

Question 1

Why was ‘gold’ foil used in Rutherford’s experiment?

Answer 1

Gold consists of heavy atoms which could produce much larger deflections in the path of the alpha particles. It is also very malleable and can be beaten into a thin sheet, to ensure a ‘single’ collision.

Question 2

What is the significance of the distance of closest of approach?

Answer 2

It leads to an estimation of the nuclear radius.

Question 3

Fro the scattering of alpha particles, only the nucleus of the atom is responsible and not the electrons. Why?

Answer 3

Electrons are very light compared to alpha particles and due to conservation of momentum cannot scatter alpha particles.

Question 4

Is the probability of scattering alpha particles at angles greater than 90o predicted by Thomson’s model less or greater than Rutherford’s?

Answer 4

It is less, because in Thomson’s model, the positive charge is spread out all over the atom which in Rutherford’s it is concentrated in a very small place.

Question 5

What is the meaning of: ‘angular momentum of an orbiting electron is quantised’?

Answer 5

Electrons can revolve only in certain discrete orbits of definite radii.

Question 6

Calculate the angular momentum of an electron in the third Bohr orbit of hydrogen atom.

Answer 6

Using J=nh/2pi,

Arrive at the answer 3.15*10^-34 Js

Question 7

What is Bohr’s frequency condition?

Answer 7

f=(E2-E1)/h

Question 8

Explain the meaning of ‘excited atom’.

Answer 8

When a normal atom gets appropriate energy from outside, it is raised from its lowest-energy state to some high-energy state. Then it can be called an excited atom.

Question 9

The ionisation potential of hydrogen is 13.6 eV. Will it be more or less for helium?

Answer 9

More, because it will require more energy to excite an electron enough so that it can be released from the pull of the nucleus.

Question 10

State the transition that emits the first line of the Balmer series of spectral lines of hydrogen.

Answer 10

n=3 —> n=2

Question 11

If the hydrogen atom goes from the 3rd to the 2nd excited state, what kind of radiation is emitted?

Answer 11

Visible light, as it is part of the Balmer series.

Question 12

On passing visible light through hydrogen, lines of which series are absorbed most and why?

Answer 12

The Lyman series lines are absorbed most because maximum absorption starts in the ground state when n=1.

Question 13

Only certain definite lines are found in the atomic spectrum of substances. Why?

Answer 13

Atom has only certain definite energy states and so only certain definite frequencies can be emitted by electrons in the atom.

Question 14

What is the radius of the second orbit of a hydrogen atom if it’s first radius is r?

Answer 14

Using radius=n^2(h^2epsilon)/(pimZ*e^2),

Get the answer 4r.

Question 15

How can the energy of a Bohr orbit be calculated? Calculate for the third of the first is -27.2 eV.

Answer 15

Use the formulae E=-Z^2*Rhc/n^2

The answer is -3.02 eV.

Question 16

What should be the minimum energy for three lines to be observed because of an exiting electron in the hydrogen spectrum?

Answer 16

Calculate the energy required for n=1 and n=3 and find the positive difference. The value reached should be 12.1 eV.

Question 17

What is the distance of closest approach?

Answer 17

This is the distance that the alpha particle is at when it stops momentarily just before being deflected.

Question 18

Total energy of an electron orbiting around the nucleus of an atom is always negative. What is the significance of this?

Answer 18

This means that the electron is bound to the nucleus in the sense that the energy supplied to it for its ejection is maximum negative in the lowest orbit making it most tightly bound.

Question 19

What is ‘excitation potential’?

Answer 19

The minimum accelerating potential required to energise an electron, which on collision can excite an atom is called the ‘excitation potential’.

Question 20

How the elements be identified by the study of their line spectra?

Answer 20

Energy levels of each atom are definite, and different from the energy levels of other elements. Hence their spectra have certain frequencies which are different from the other elements.

Question 21

The electron in a hydrogen atom is in the third exited state (n=4). What is the maximum number of photons that may be emitted in passing of the electron to the ground state? What is the minimum number?

Answer 21

Max=3

Min=1

Question 22

How are the various lines of the Lyman series formed?

Answer 22

According to Bohr’s theory, when an atom gets excited and travels to a high level from its ground state in the case of Hydrogen, it remains their for 10^-8 seconds before returning back to its ground state, which it come to directly or through the other shells. Then, it releases photons and thus waves which form the spectral lines present in the Lyman absorption series.

Question 23

Calculate the excitation energy of n=3 level of the H+ ion.

Answer 23

Using deltaE = -Z^2 * Rc (delta 1-n^2),
Obtain the answer 48.4 eV.

Can also use E=-Z^2 *13.6/n^2 eV