1. Atom, as a whole, is electrically neutral and therefore contains equal

amount of positive and negative charges.

Ans


2. In Thomson’s model, an atom is a spherical cloud of positive charges

with electrons embedded in it.

Ans


3. In Rutherford’s model, most of the mass of the atom and all its positive

charge are concentrated in a tiny nucleus (typically one by ten thousand

the size of an atom), and the electrons revolve around it.

Ans


4. Rutherford nuclear model has two main difficulties in explaining the

structure of atom: (a) It predicts that atoms are unstable because the

accelerated electrons revolving around the nucleus must spiral into

the nucleus. This contradicts the stability of matter. (b) It cannot

explain the characteristic line spectra of atoms of different elements.

Ans


5. Atoms of most of the elements are stable and emit characteristic

spectrum. The spectrum consists of a set of isolated parallel lines

termed as line spectrum. It provides useful information about the

atomic structure.

Ans


6. To explain the line spectra emitted by atoms, as well as the stability

of atoms, Niel’s Bohr proposed a model for hydrogenic (single elctron)

atoms. He introduced three postulates and laid the foundations of

quantum mechanics:

(a) In a hydrogen atom, an electron revolves in certain stable orbits

(called stationary orbits) without the emission of radiant energy.

(b) The stationary orbits are those for which the angular momentum

is some integral multiple of h/2p. (Bohr’s quantisation condition.)

That is L = nh/2p, where n is an integer called the principal

quantum number.

(c) The third postulate states that an electron might make a transition

from one of its specified non-radiating orbits to another of lower

energy. When it does so, a photon is emitted having energy equal

to the energy difference between the initial and final states. The

frequency (n) of the emitted photon is then given by

hn = Ei

 – Ef

An atom absorbs radiation of the same frequency the atom emits,

in which case the electron is transferred to an orbit with a higher

value of n.

Ei

 + hn = Ef

Ans


7. As a result of the quantisation condition of angular momentum, the

electron orbits the nucleus at only specific radii. For a hydrogen atom

it is given by

r

n

m

h

e

n =

2 2

0

2 2

4

π

πε

The total energy is also quantised:

4

2 2 2 8 0

n

me E

n h ε

= −

 = –13.6 eV/n

2

The n = 1 state is called ground state. In hydrogen atom the ground

state energy is –13.6 eV. Higher values of n correspond to excited

states (n > 1). Atoms are excited to these higher states by collisions

with other atoms or electrons or by absorption of a photon of right

frequency.

Ans


8. de Broglie’s hypothesis that electrons have a wavelength l = h/mv gave

an explanation for Bohr’s quantised orbits by bringing in the waveparticle duality. The orbits correspond to circular standing waves in

which the circumference of the orbit equals a whole number of

wavelengths.

Ans


9. Bohr’s model is applicable only to hydrogenic (single electron) atoms.

It cannot be extended to even two electron atoms such as helium.

This model is also unable to explain for the relative intensities of the

frequencies emitted even by hydrogenic atoms.

Ans


1. Both the Thomson’s as well as the Rutherford’s models constitute an
unstable system. Thomson’s model is unstable electrostatically, while
Rutherford’s model is unstable because of electromagnetic radiation
of orbiting electrons.

Ans


2. What made Bohr quantise angular momentum (second postulate) and
not some other quantity? Note, h has dimensions of angular
momentum, and for circular orbits, angular momentum is a very
relevant quantity. The second postulate is then so natural!

Ans


3. The orbital picture in Bohr’s model of the hydrogen atom was
inconsistent with the uncertainty principle. It was replaced by modern
quantum mechanics in which Bohr’s orbits are regions where the
electron may be found with large probability.

Ans


4. Unlike the situation in the solar system, where planet-planet
gravitational forces are very small as compared to the gravitational
force of the sun on each planet (because the mass of the sun is so
much greater than the mass of any of the planets), the electron-electron
electric force interaction is comparable in magnitude to the electronnucleus electrical force, because the charges and distances are of the
same order of magnitude. This is the reason why the Bohr’s model
with its planet-like electron is not applicable to many electron atoms.

Ans


5. Bohr laid the foundation of the quantum theory by postulating specific
orbits in which electrons do not radiate. Bohr’s model include only
one quantum number n. The new theory called quantum mechanics
supportes Bohr’s postulate. However in quantum mechanics (more
generally accepted), a given energy level may not correspond to just
one quantum state. For example, a state is characterised by four
quantum numbers (n, l, m, and s), but for a pure Coulomb potential
(as in hydrogen atom) the energy depends only on n.
6. In Bohr model, contrary to ordinary classical expectation, the frequency
of revolution of an electron in its orbit is not connected to the frequency
of spectral line. The later is the difference between two orbital energies
divided by h. For transitions between large quantum numbers (n to n
– 1, n very large), however, the two coincide as expected.

Ans


7. Bohr’s semiclassical model based on some aspects of classical physics
and some aspects of modern physics also does not provide a true picture
of the simplest hydrogenic atoms. The true picture is quantum
mechanical affair which differs from Bohr model in a number of
fundamental ways. But then if the Bohr model is not strictly correct,
why do we bother about it? The reasons which make Bohr’s model
still useful are:

Ans

(i) The model is based on just three postulates but accounts for almost
all the general features of the hydrogen spectrum.

(ii) The model incorporates many of the concepts we have learnt in
classical physics.

(iii) The model demonstrates how a theoretical physicist occasionally
must quite literally ignore certain problems of approach in hopes
of being able to make some predictions. If the predictions of the
theory or model agree with experiment, a theoretician then must
somehow hope to explain away or rationalise the problems that
were ignored along the way.


Post ID: DABP007236