16. Structure of hydrogen atom - Bohr model, Bohr postulates. Arguments for validity of the model, arguments for invalidity of the model. (5p.)

Question 1

Bohr model

Answer 1

Bohr introduced a hydrogen atom model in which the electron circulates about the nucleus (like a planet about the Sun):

ZDJECIE W WORD

Question 2

Bohr postulates

Answer 2

The postulates:

1. The postulate of stationary states

The electrons can orbit nucleus only on stationary orbits, located at fixed distances from the nucleus. They correspond to strictly defined energies of electrons. Electron may stay on an orbit for an arbitrarily long time without radiating out energy.

2. The rule of orbit quantization

The angular momentum of an electron on a stable orbit must be given by the following formula: 𝐿 ! = 𝑚𝑣 ! 𝑟 ! = 𝑛ℎ, i.e. only discrete values of the angular momentum are possible.

3. The frequency postulate

The atom can emit or absorb radiation only when an electron goes from one of its stationary orbits to another. The energy is emitted (or absorbed) in form of photons and it is equal to the difference in energy between these two states.

Question 3

Arguments for invalidity of the Bohr model

Answer 3

Arguments for invalidity of the model:

• The Bohr model predicts the incorrect value for the orbital angular momentum in the ground state of the system („zawsze musi mieć moment pędu, na zerowej nie ma, więc gdzie tu orbita?!“)

• In reality, electrons travel do not travel in a circular orbit (not in the ground state). It is more of an elliptical orbit. (znalezione w internecie, ale tłumacząc dosłownie jego słowa – co w sumie może być bardziej poprawne

– atoms exhibit a reflection symmetry rather than a circular one)

Question 4

Arguments for validity of the Bohr model:

Answer 4

Arguments for validity of the model:

• Bohr managed to predict the value of the Rydberg constant (used in the Balmer formula for spectral lines of the hydrogen atom).