Atoms Flashcards
graph of scattering angle with no. of α particles scattered
kind of like inversely proportional
Significance of α particle experiment
Most of them passed undeflected
Most space of atom is empty
Few deflected through 90 & 180
All +ve charge & the mass of atom is concentrated in a very small region called nucleus
Size of nucleus is 1/10^15m & that of atom is 1/10^10m
Since gold nucleus is much heavier than α particle, gold nucleus remains at rest
Impact parameter
Perpendicular distance of initial velocity vector of α particle from center of nucleus
For large impact parameter, α particle is
almost undeflected
For head on collision, impact parameter is
0
Rutherford’s postulates
Entire +ve charge & mass of atom is concentrated at very small volume at the center called nucleus
Amount of +ve charge is equal to -ve charge
Atom is neutral
E’s are continuously revolving around the nucleus in circular orbits
Force of attraction provides necessary centripetal force
Limitations of Rutherford’s atomic model
- an accelerating particle emits radiation as EM waves. So energy of the electron show gradually decrease leading to decrease in radius of e’ orbit
- Electron follows a spiral path & finally fall into the nucleus so atom isn’t stable
- As e’ spiral inwards, their angular velocity & their freq change continuously they would emit continuous spectrum instead of actually observed line spectrum
Bohr’s atomic model
- In a H atom, an e’ revolves in stable orbits called stationary orbits
- These orbits are those for which angular momentum is integral multiple of h/2pi
As long as e’ is revolving in stationary orbit, it doesn’t radiate energy. When e’ make a transition from higher energy to lower, a photon is emitted, having energy equal to energy diff b/n initial & final states
radius of orbit r =
n²h²ε/Ze²pi m
Are stationary orbits equally spaced?
No
Speed _______ as we move away from the nucleus
decreases
Potential energy U =
-kZe²/r
Kinetic energy K =
kZe²/2r
E =
-mZ²e⁴/8ε²n²h²
-ve sign in E indicates that?
e’ is bound to nucleus so energy is needed to be done to separate e’ from nucleus
Rydberg const
1.1 x 10⁷ per metre
For lyman series, n2 =
2,3,4,….
Lyman series lies in
UV region
Balmer region lies in
1st 4 lines in visible region and later in UV
For alpha H, n1 & n2 =
2; 3
For beta H, n1 & n2 =
2; 4
Paschen and Pfund lies in
IR region
Excitation energy of atom
Energy required by an e’ to jump from the ground state to any one of excited states
Ionisation energy
Energy required to knock an e’ completely out of atom
Limitations of Bohr’s model
- Capable only to H like atoms
- Couldn’t explain fine structure of spectral lines
- Doesn’t explain why only circular orbits are chosen where elliptical orbits are possible
- Doesn’t tell anything about relative intensities of various spectral lines
- Fails to explain further splitting of spectral lines in M.F & E.F
Zeeman effect
Splitting of spectral lines in M.F
Stark effect
Splitting of spectral lines in E.F
Alpha particle has _________ penetrating power
least
Lyman in ___ range
UV
Balmer in ____ range
visible
Paschen, Brackett, Pfund in ___ range
Infra Red
