Energy Levels and Spectra

What are Energy Levels?

In an atom, electrons around a central nucleus can only have particular energy values. These values are called ‘energy levels’. The electrons cannot sit half way between one level and the next and must either absorb or emit a specific amount of energy to move to or from an energy level. This specific amount of energy must be equal to the difference between the energy levels for the electron to ‘jump’. When energy is absorbed by an electron, it becomes excited into a higher energy level, and when an electron falls to a lower level, energy is given out in the form of a photon (packet of energy). However, electrons do not like sitting at higher energy levels and after becoming excited, will drop back to the lower energy state, which means that they have to lose energy. We can calculate the difference between the energy levels by using the equation E = hf. For example, if we was finding the difference between energy level 1 and energy level 2, we would do hf = E(1) – E(2). Electron energies are expressed in electronvolts (eV), which is the kinetic energy acquired by an electron when accelerated through a potential difference of 1 volt.

Ionisation

In the ground state, the electrons are not excited and have the least amount of energy. Yet, electrons which sit at higher energy levels take less energy to remove as they are closer to the highest level and therefore require less energy to become ionised. The minimum amount of energy to release an electron from the ground state is known as the first or principal of ionisation energy.

'''Absorption and Emission Spectrum'''

Each element can be identified by the photons that it absorbs or emits from the electrons moving between energy levels. Absorption Spectrum – Shining a white light through vapour of an element. The spectrum of the light that has passed through the vapour should be a perfect spectrum apart from a few gaps.

Emission Spectrum – Excite the electrons in a vapour. Collect the photons that are emitted when the electrons drop from the higher energy levels down to the lower levels. This will produce a series of coloured lines on a mainly back background. This shows that light is only emitted at certain frequencies relating to certain ‘jumps’.