AS Energy Levels & Spectra, Summarised
Rutherford’s model of the atom proposed a dense, tiny nucleus
at the centre containing all the positive charge and nearly all
the mass, surrounded by “orbiting” electrons. But physicists
knew that orbiting objects are accelerating, and when you
accelerate electrons they give off EM waves and lose energy. If
this was happening, they would spiral into the nucleus. But
they don’t...
Nils Bohr applied quantum theory to the Hydrogen atom and
proposed that electrons do not obey classical rules, but that they
can only exist with certain discrete energies (“energy levels”).
If electrons gain enough energy they can jump up to a higher energy
level, and when they drop back down they release energy.
Sometimes this is in the form of EM radiation (a photon).
Atomic electrons in a gas can gain energy if an electric current is passed through it. Electrons from
the cathode are accelerated by th e electric field and collide with the gas atoms. When a free
electron collides with a gas atoms what happens depends on how much kinetic energy the electron
has.
If the incident electron has less KE than the energy gap to the next
atomic level, the atomic electron cannot absorb its energy and the
electron just bounces off (is “scattered”).
If the KE of the incident electron is greater than the gap between
energy levels, some of its energy is transferred to the atomic electron
which is promoted up one or more levels (into an “excited state”).
Later it will drop back down to its lower level, emitting a photon. The
incident electron leaves with less KE.
If the KE of the incident electron is greater than the negative energy
of the atomic electron’s orbit it will be completely knocked off the
atom and become free. The atom has been ionised. Any excess
energy appears as the KE
of the two electrons.
We define an ionised electron to have an energy of 0.
Because there is an attractive force between the
electrons and the nucleus, the energies of all bound
states are therefore negative (you have to put energy
in to remove the electron from the atom). Energy levels
are often measured in eVs (1eV=1.6x10-19J).
Negative energies
E1
When an electron drops down to a lower
energy level it releases the energy as a photon.
E2
The frequency of the photon is given by the
relation E=hf=E1-E2 (remember c=f too).
Electrons can make their way back to the
ground state by any combination of level
transitions. In a sample of a gas, photons with energies corresponding to all the
possible transitions will be seen. In the example on the right, there are six
frequencies of photon emitted from the sample, corresponding to the six possible
transitions between the four levels.
This explains the line emission spectra we see when gases are
excited. The narrow lines are produced by de-exciting atomic
electrons and have frequencies corresponding to the energy gaps
between levels. Many allowed transitions produce photons
outside the visible part of the spectrum (UV and IR). The
characteristic spectra of elements give us a way of identifying
which are present in a sample (Helium was discovered by looking
at the spectrum of sunlight).
Another way for an atomic electron to be excited to a higher energy
level is through absorbing a photon. In this case the photon energy
must exactly correspond to the difference between the two energy
levels to be
absorbed.
Shining white
light through a sample of cold gas gives rise to
absorption spectra, where black lines appear
in the continuum at particular frequencies
corresponding to electrons being promoted to higher energy levels. These should exactly match lines
seen in the emission spectrum. The emission spectrum for a substance usually shows more lines
than the absorption spectrum because absorption tends to promote electrons from the ground
state, while emission can involve transitions between any pair of states.
Fluorescence is the emission of visible light by a substance when it has absorbed ultraviolet
radiation. It is used in fluorescent tube lamps, which are 8 times more efficient than incandescent
lamps.




The tube contains mercury vapour which is excited
(through collisions and ionisation) by passing a current
through it.
The mercury atoms de-excite, emitting much energy as
UV photons (plus other frequencies)
The UV photons are absorbed by the fluorescent coating
on the inside of the tube, exciting the atoms
The coating atoms de-excite through more than one
transition, emitting visible photons.
UV photon
absorbed
visible photon
emitted