Topic Menu

Cyber-chess a chess instruction site

Quantum Phenomena - discrete energy levels for electrons

Q3.

 

(a) Line spectra were observed before they could be explained by theory. We now know that photons of characteristic frequency are emitted when the vapour of an element is bombarded by energetic electrons.

Here we are told that the electrons are given energy by bombarding electrons so we are dealing with an emission spectrum, not an absorption spectum. You have to be careful that you do not just 'regurgitate' your notes when answering questions like this - think carefully about what is asked. If the image had not been there would you have told the examiners bot both types of spectra? If so you need to plan answers more carefully.

The spectrum of the light emitted contains lines, each of a definite wavelength.

Explain how

• the bombarding electrons cause the atoms of the vapour to emit photons

• the existence of a spectrum consisting of lines of a definite frequency supports the view that atoms have discrete energy levels.

The high energy electrons bombard atoms of vapour, interact with atomic orbital electrons and give energy to to those electrons. If an orbital electron recieves enough energy to move to a higher energy and there is a vacant state for it, it will be 'prmoted' to the higher energy level. Such an electron is called an 'excited electron'.

Excited electrons move down to lower energy levels, losing energy by emitting photons of energy specific to the energy difference between the energy levels. Photons have energy that is calculated by E=hf, therefore photons of characteristic frequencies are emitted from atoms of a particular element. Therefore only discrete lines are observed on the spectrum.

(6 marks)

(b) The ionisation energy of a hydrogen atom is 13.6 eV.

(i) State what is meant by the ionisation energy of hydrogen.

The ionisation energy of hydrogen is the energy required to (completely) remove a (ground state/lowest energy level ) electron from atom/hydrogen

(2 marks)

(ii) Express the ionisation energy of hydrogen in joules, giving your answer to an appropriate number of significant figures.

You are given that it is 13.6eV - you just have to convert eV to joules. Rememeber a joule is a coulomb-volt. Therefre you have to multiply by the charge on an electron to get joules

13.6 × 1.6 × 10–19

= 2.18 × 10–18 J

Answer given to 3sf

(3 marks)

(Total 11 marks)