Quantum Phenomena - discrete energy levels for electrons
Q4.
(a) When free electrons collide with atoms in their ground state, the atoms can be excited or ionised.
(i) State what is meant by ground state.
An atom is in its ground state is when its orbital electrons are in their lowest/minimum energy state or most stable state.
(1 mark)
(ii) Explain the difference between excitation and ionisation.
In both cases an electron receives energy by interaction with the incoming electron.
When excitation occurs the orbital electron is promoted to a higher energy level within the atom, but when ionisation occurs the electron receives enough energy to completely leave the atom.
(3 marks)
(b) An atom can also become excited by the absorption of photons. Explain why only photons of certain frequencies cause excitation in a particular atom.
Electrons occupy discrete energy levels, they therefore need to absorb an exact amount of energy to move to a higher level. Only one photon can be absorbed by one electron to make the jump in between levels. The photons need to have a certain frequency to provide this energy as E = hf, meaning that their frequency relates to their energy. The energy levels are the same for all atoms of a particular element therefore the energy of photons absorbed to make the jumps will be the same for all atoms of a particular element.
(4 marks)
(c) The ionisation energy of hydrogen is 13.6eV. Calculate the minimum frequency necessary for a photon to cause the ionisation of a hydrogen atom. Give your answer to an appropriate number of significant figures.
energy = 13.6 × 1.60 × 10–19 = 2.176 × 10–18 J
E =hf
f = E/h = 2.176 × 10–18/6.63 × 10–34 = 3.28 × 1015Hz 3 sfs
(4 marks)
(Total 12 marks)
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