Bohr Model

Question 1

Who introduced a theory to explain the atomic spectrum?

Answer 1

Neils Bohr

Question 2

What are the initial problems associated with Bohr model? (3)

Answer 2

1) e are expected to slow down gradually.
2) Why would an e move in an orbit around the nucleus?
3) nu is + an e is -, shouldn’t they attract? So e would spiral inward and collide with the nu?

Question 3

What are Bohr’s Postulates? (2)

With explanations of how they solve the issues of Bohr model

Answer 3

1) Energy level postulate
e don’t radiate E when staying in one orbit, so they don’t slow down.
2) Transition between E levels postulate
When e moves from one orbit to another;
- towards nu : radiates E
- away from nu : absorbs E
This way centrifugal F = centripetal F, therefore the e will remain stable in the new orbit.

Question 4

What atoms can Bohr model be successfully applied to?

Answer 4

H or ions with one e. (He+, Li2+, Be3+)

Question 5

Why was Bohr model not a successful atomic model? (6 problems)

Answer 5

1) Violates Heisenberg’s uncertainty principle (considers e to have both a known radius and orbit)
2) Incorrect values for GS angular momentum
3) Poor prediction of spectra of larger atoms
4) Doesn’t predict relative intensities
5) Doesn’t explain hyperfine structure in atoms
6) Doesn’t explain Zeeman & Stark effects

Question 6

What are the refinements to Bohr theory? (5)

Answer 6

1) The nucleus remains stationary; except for rotating on its own axis
2) Would be correct if the mass of the nucleus is infinite. (but ratio is 1:1836)
3) Since the nu oscillates slightly, the mass of the e, m, is replaced by reduced mass, u.
4) u = mM/(m+M)

Question 7

Draw the Bohr model diagram of H atom and the E level diagram.

Answer 7

Bohr orbits - distance between orbits increases with n
E level - distance between levels decrease with n
Spectral lines - Lyman(UV), Balmer(V), Paschen(IR)

Question 8

What is an orbit?

Answer 8

An orbit is a fixed path along which electrons revolve around a nucleus.

Question 9

What are the ni and nf values of the first line of the following series?
And what region of the EMS can they be found in?
1) Lyman
2) Balmer
3) Paschen
4) Pfund

Answer 9

1) ni=2, nf=1, UV
2) ni=3, nf=2, Visible
3) ni=4, nf=3, IR
4) ni=5, nf=4, IR