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The Danish Niels Bohr was no stranger to science from an early age. His father was a professor of physiology (human anatomy) and his brother was a distinguished mathematicien. After getting his Ph.D from the university of Copenhagen, he was invited to work with Rutherford at Cambridge.
Bohr distinguished himself by his intellect and amazing imagination. He was brave enough to accept Max Planck’s idea that energy came in little packets calles quanta (hence quantummechanics) and he applied this to Rutherford’s model of the atom.
What did Bohr do?
Bohr thought of the electrons as orbiting not in any random ‘altitude’ above the nucleus, but in pre-determined altitudes called shells. An electron could move up or down to an open spot in another shell but it could not inhabit an altitude between the shells. His ideas remained theoretical until two scientists (Franck and Hertz) produced measurements that corresponded with the theoretical calculations that Bohr did. They initially thought their measurements were caused by the ionisation of the atoms but hearing Bohr’s explanation, they were convinced they had inadvertently proven his ideas.
Bohr’s model of the atom (1922)
Most of the mass of an atom is in the central nucleus that contains protons.
The electrons are arranged in definite shells or energy levels and orbit the nucleus.
The electron shells are a long way from the nucleus.
The shells are filled from the innermost shell to the outer shells.
Electrons determine the reactivity of the atom.
Atoms with full shells are not very reactive.
Chemists not happy
While Bohr‘s model of the atom could explain the spectrum of the hydrogen atom, chemists didn‘t think it would explain the reactivity of the other chemical elements.
His theory was based on incomplete physical data and mathematical calculations.
Many chemists favoured Lewis’ theory. The American’s “Octet theory” was based on real
chemical data. Lewis proposed that the fixed nucleus was surrounded by cubic shaped electron shells. The electrons were fixed in the corner positions.
Even though we now know that the shells aren’t cuboid, (nor are they spherical), Lewis’s octet theory is still used to explain chemical reactions and structures. You will encounter this theory in the upper classes together with the Lewis Structures.
Shells and CSI
Bohr was able to prove that electrons are indeed distributed in shells and that each of those shells has a different energy level; The higher the shell, the more energy the electron need to possess to be in it. Thanks to these shells, we can explain fluorescence (black light). Fluorescence is used at parties and bank notes but if you watch CSI or any other police show, you’ll see them use UV light to make blood and other traces visible even after they have been cleaned by the killer.
You should know that each particle of light (photon) has a set amount of energy and that amount of energy determines the colour.
When a photon hits an atom while it has the right amount of energy to lift an electron to a higher shell, the energy of the photon is used up to do this.
When the electron later drops back down to the lower shell, it must release that amount of energy again. This results in a photon with exactly the same amount of energy and colour.
What we know thanks to Bohr is that the electron can lose a little bit of energy before it falls back to a lower shell. When the electron that eventually drops back down, the amount of energy that he has to release is less.
Because the photon has less energy, we have to look a little bit more to the left of the spectrum to determine the colour and that is where the secret of fluorescence lies. Certain substances can absorb invisible UV-light and return visible light. This make the substance appear to produce light on its own.
The amount of energy an atom needs to lift an electron, the amount of energy the electron is able to lose before falling back down and with that, whether the substance fluoresces is determined not only by the atom itself, it is also determined by the molecule the atoms is part of at that moment. With all molecules, that includes the large molecules that make up living tissue, you could calculate whether the substance is able to transform invisible UV-light into visible light. If that’s the case, you can use a UV-lamp to make the substance glow which makes it really easy to spot.
Too bad for the CSI-fans, blood does not fluoresce with UV-light. To find blood, other tricks are available. Urine does fluoresce which makes the picture on top of the page a different “crime scene”.
Assignments:
21. Explain the difference between the models of Bohr and Rutherford.
22. Explain why fluorescence can’t turn infrared light in to visible light.
23. Explain why fluorescence from UV to blue is a lot more useful then fluorescence from blue to yellow.
Elektronenconfiguratie
Bohr’s model says:
Electrons are found in different shells of energy levels around the nucleus.
When one shell is full, the next shells starts to fill up. This is called electron configuration.
Hydrogen has one proton in the nucleus and one electron in the first shell.
Lithium has three protons and three electrons. In the first shell, only two electrons fit so the third electron in placed in the second shell.
the electron configuration is 2,1.
The electron shells are designated by a capital letter and a value for the letter n.
The letter K (n=1) is for the first shell which can contain two electrons.
The letter L (n=2) is the letter for the second shell. It may contain a maximum of 8 electrons.
The third, fourth and fifth shell are called the M (n=3), N (n=4) and O (n=5) shell. (and after that P, Q, R. etc.). These shells may contain a maximum of 2n2 electrons.
So in shell N, the 4th shell so n=4, you can fit a maximum of 2∙42 = 2∙16 = 32 electrons.
Even during Bohr’s lifetime, his model was adapted to explain new observations. It was discovered that the shells are divided up into sub-shells. These sub-shells were given the lower case letters s, p, d, f and g. The shell with n=3 has 2 sub-shells, n=4 has 3 etc. For elements larger than Argon, the order in which the (sub-)shells are filled up becomes a bit weird. The first sub-shell of the fourth shell is filled before the last sub-shell of the third shell. The electron configuration of Calcium is 2,8,8,2. Scandium had 2,8,9,2 and Titanium has 2,8,10,2.
Note: The capital letters for the shells aren’t used that often, we now just use the number. The lower case letters for the sub-shells are used widely.
Assignments:
24. Draw an atom of magnesium according to Bohr.
25. What is the electron configuration of this atom?
26. Explain the difference between the models of Bohr and Rutherford.
27. The chemists liked Lewis’s theory better and we now know they were wrong but can you explain whether their argument was a good one?
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