Atoms

Question 1

Alpha scattering experiment observations

Answer 1

1. Most of the alpha particles (99.86%) remain undeviated
2. Some alpha particles (0.14%) are deflected at small angles (of the order 1 degree or less)
3. Very few alpha particles were scattered through angle 90 degree (1 in 8000) or 180 degree (1 in 10^6)

Question 2

Conclusions of rutherford’s model

Answer 2

• Atom: most of the atom is empty
• nucleus: at the centre of the atom, there is a positively charged core
• Motion of electrons around the nucleus: Electrons surrounding the nucleus revolve around it with very high speed in circular paths
• Size of atom and nucleus:
  r_n = 10^-14 or 10^-15 m
  r_a = 10^-10 m

Question 3

Distance of closest approach

Answer 3

The minimum distance b/w the centre of the nucleus and the alpha particle before it gets deflected back through 180⁰

Question 4

Drawbacks of Rutherford’s model

Answer 4

1. Regarding stability of the atom
2. Regarding explanation of line spectrum

Question 5

Drawbacks of Rutherford’s model: stability of atom

Answer 5

1. Electrons revolving around the nucleus have centripetal accln
2. Accelerated charged particles radiate energy in the form of EM waves
3. Due to continuous loss of energy of the elctrons, they will move closer to the nucleus and fall into it
4. The atom cannot remain stable

Question 6

Drawbacks of Rutherford’s model: Explanation of line spectrum

Answer 6

In rutherford’s model, electrons can revolve in any orbit hence according to classical theory, the electrons are emitting energy, which should give a continuous spectrum but in actual experiments, it is a line spectrum

Question 7

Bohr’s atomic model - 1st assumption

Answer 7

Electrons will revolve in circular orbit around the nucleus. The necessary centripetal force is given by electrostatic force of attraction

mv²/r = e²/4πε_o

Question 8

Bohr’s atomic model - 2nd assumption

Answer 8

Electrons can only revolve in those orbits in which the angular momentum of the elctron will be equal to an integral multiple of h/2π

Question 9

Bohr’s atomic model - 3rd assumption

Answer 9

When electron will jump from orbit of higher energy (excited state) to lower energy (ground state) it emits the energy in the form of photons whose energy is equal to the difference b/w energy levels of those orbits

h𝜈 = E_f - E_i

Question 10

Why are there more emission transitions than absorption transitions

Answer 10

1. Emission takes places from higher to lower energy level
2. Absorption takes place from lower to higher energy level
3. so emission is easier