Energy levels of atoms

(or why shoving a red hot poker where the sun doesn’t shine is an experiment in quantum physics)

In my previous artlicle, we had a brief look at spectroscopy and how atoms of different elements absorb and  emit light at different frequencies but why is this?

The spectroscopic ‘signatures’ of different elements

If we consider the Bohr model of an atom, we can see that electrons orbit the nucleus and that these are  arranged in distinct shells; you may remember from school that each shell contains a maximum number of  electrons and these shells are further broken down in to sub-shells. From the nucleus, the first shell is s  which has a maximum of 2 electrons, p has a maximum of 6, d has a maximum of 10 and so on as we move away  from the nucleus. Just for interest, s stands for Sharp, p for Principal and d for Diffuse.

So what.

Well, if an atom absorbs energy, be it light, heat or whatever, then the energy must be converted in to  something – the law of conservation of energy states that energy can never be created or destroyed but only  converted. So where does it go?
The energy absorbed shifts an outlying electron to the next sub shell so in effect, the energy (regardless  of its form – heat, light etc.) can be thought of being converted in to gravitational energy acting on the  electron. Gravitational potential energy can be demonstrated by lifting a heavy box and putting it on to a  shelf. The energy expended in lifting the box is simply stored as gravitational potential energy in the box  owing to its greater height. If it were to fall, that energy is converted in to kinetic energy (moving  energy) and so will make a louder thud when it hits the floor than it would have done from its original  height, hence, the conversion path will be gravitational potential energy → kinetic energy → sound energy.
We are now applying energy to an atom and have lifted an electron to the next sub-shell. If we continue to  add energy, further electrons will be moved. Now, atoms are fairly inert in most instances so if we stop  supplying energy to the atom, it will revert to what is known as its Ground State – it’s normal, natural  energy level. It does this by allowing the electrons to drop back to their original sub-shells. As they do,  they ping off the accumulated energy as photons – the equivalent of dropping the afforementioned box from a  high shelf and making a noise as it hits the ground. These photons may be in the visible or non-visible part  of the EM spectrum and this is what gives us the spectral lines shown above. This is how by simply observing  the light from a distant planet, astronomers can work out which elements exist in its atmosphere and from  that, consider the odds of life there.

But what about the red hot poker?

Max Planck discovered the concept of the quantisation of energy – the supply of energy to atoms in small  packets, Nils Bohr took this a step further and found that atoms possessed discrete energy levels and this  tied up with electrons moving between their sub-shells. Below is a table showing their different energy  levels. This shows how much energy is required to move an electron between sub-shells.

The energy levels are given as electron volts where 1 electron volt = 1.602 x10-19 joules.

Should the energy level reach that of ionisation then that electron is pinged from the atom in to free space  and the atom then becomes an ion and, owing to the mismatch of the number of electrons to its number of  protons, is positively charged – atoms are always neutral in charge as their number of electrons matches the  number of protons. When the ion has the energy removed from it then as it is positively charged, it will  grab a free electron from the surroundings (it is positively charged and electrons are negatively charged)  through electrostatic attraction and becomes an atom once again.

What about the bloody poker?

OK, we have a poker that at room temperature, looks silver/grey. This is because it is reflecting that  colour of light only and absorbing the rest. If we now heat it until it is red hot then remove the heat, we  see it cool down. Why is it red? The heat energy that we have applied to the poker has caused electrons to  move in to higher sub-shells. The atoms are desperate to move back to the ground state so some drop down and  release photons as they do. Energy is still being applied so they move back up again. The wavelength of  these emitted photons is in the red part of the spectrum and so it appears red. Once the heat is removed,  the poker releases its stored energy and although it is cooling down gradually, it’s emitted ‘redness’  reduces in increments that match the energy levels for the material it is constructed from though we cannot  detect that with the eye.
 

© Rat Catcher 2017