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## Electron configuration

For people studying chemistry, and spectroscopy within physics, the electron configuration nomenclature will be familiar. For example, the noble gas neon, which is at number 10 in the periodic table, may be written as $1s^{2} \; 2s^{2} \; 2p^{6}$. Titanium, which is at number 22 in the periodic table may be written as $1s^{2} \; 2s^{2} \; 2p^{6} \; 3s^{2} \; 3p^{6} \; 3d^{2} \; 4s^{2}$. What does this mean, and where do the letters come from?

## Energy levels within atoms

It was the Danish physicist Nils Bohr who, in 1913, suggested that electrons can only orbit the nucleus of an atom in certain allowed orbits. Chemists tend to refer to these levels as orbitals or shells. I will explain in a future blog how he calculated what these allowed orbits were, but the number before the s and p in the electron configuration of neon refers to the electron energy level, or what is sometimes called the principal orbit. So, $1s$ means the electrons are in the first or lowest energy level, the $n=1$ energy level as physicists would call it.

## The origin of the s,p,d,f orbital labels

The origin of the letters s,p,d and f in the electron configuration goes back to the origins of spectroscopy. In the period 1872-1880, George Liveing and James Dewar published a series of papers on their observations of the spectral lines of alkali metals.

They classified the lines based on their visual appearance, and came up with the nomenclature s,p,d and f.Their meaning is

1. s – sharp
2. p – principle
3. d – diffuse
4. f – fine (or fundamental)

There are letters beyond this, which just follow the alphabet, so g,h,i etc.

## The superscript after the letter

What does the superscript after the letter refer to? E.g. $1s^{2}$ or $2p^{6}$. We now know from what I have said above that the $1 \text{ in } 1s^{2}$ refers to the electron(s) being in the first orbital, the n=1 energy level. And the $2 \text{ in } 2p^{6}$ tells us the electrons are in the n=2 energy level. But what do the superscripts 2 after the “s” and 6 after the “p” refer to?

These refer to the number of electrons in that particular orbital/shell. So, for neon, there are 2 electrons in the 1s shell, 2 electrons in the 2s shell, and 6 electrons in the 2p shell. This makes a total of $2+2+6=10$ electrons in total, as one would expect given that neon has an atomic number of 10. For titanium we have 2 electrons in the 1s shell, 2 electrons in the 2s shell, 6 electrons in the 2p shell, 2 electrons in the 3s shell, 6 electrons in the 3p shell, 2 electrons in the 3d shell, and finally 2 electrons in the 4s shell. This makes a total of $2+2+6+2+6+2+2=22$, which agrees with the atomic number of titanium.

Why are there only 2 electrons in any of the s-shells, but 6 in the p-shells? I will explain this in a future blog, but it is to do with something called the Pauli exclusion priciple, which states that no two electrons (or, to be more general, no two fermions, and an electron is a fermion) can exist in the same quantum state. I will explain in a future blog how this leads to there being a maximum of two electrons allowed in any s-shell, a maximum of six in any p-shell, etc.

## Visualising all of this

The easiest way to visualise the electron configuration of an element is, of course, with a diagram. Below are the diagrams for neon and titanium. One can see the electron structure, with each n-level (1,2,3 etc) shown separately. But, these diagrams do not show the s-shell and the p-shell in the n=2 level separately, they just show 8 electrons (2 in the s-shell and 6 in the p-shell) in the n=2 level. To get the full story, we also need to have the electron configuration written out, as I did in the first paragraph.

### 8 Responses

1. […] I could spend all day talking about orbitals, however the focus of this guide is the periodic table, so alas I must move on!! (If you would like to know more about electron configuration, check out this great post on The Curious Astronomer) […]

• Thank you! As a non-chemist, I may have got some of the terminology chemists use wrong. One of the stupid things about the educational system in England & Wales is that I have not done any formal chemistry beyond the age of 16, so never did physical chemistry, so never learnt the electron configuration nomenclature in a formal way as a student.

• The terminology looks fine, and the article has been well researched (in particular, I loved the bit about what s,p,d,f stand for!). I can understand how confusing it must have been if you’ve never learnt it before, chemists do have a tendency to get a bit crazy with naming systems …

The only things you may want to add about notation is that when you get to the bigger elements, instead of writing out 1s2 2s2 2p6 … etc at the start, the symbol for the noble gas is used as short hand. So Ti would become [Ar] 4s2 3d6. Also, because the s shell is filled before the d shell, it is written first (i.e. 4s2 3d8). Other than that, great post and an enjoyable read!

• Thank you for that comment. Yes I noticed with heavier elements that they were written as e.g. Ti is [Ar] 4s2 3d6, but I wanted to list things out more explicitly.

My plan is to explain why e.g. the 4s shell is filled before the 3d shell in my blog on why the shells exist in the first place – the Pauli exclusion principle which I hope to post later this week 😉

2. […] angular momentum of the electron in a particular energy level. This is where the s,p,d,f lines that I mentioned in this blog comes into play. So what I have explained above is a first approximation, but perfectly fine for […]

3. […] In this blog, I discussed the “electron configuration” nomenclature which is so loved by chemists (strange people that they are….). Just to remind you, the noble gas neon, which is at number 10 in the periodic table, may be written as . If you add together the superscripts you get , the number of electrons in neutral Helium. Titanium, which is at number 22 in the periodic table may be written as . Again, if you add together the superscripts you get , the number of electrons in neutral Titanium. I explained in the blog that the letters s,p,d and f refer to “sharp, principal, diffuse” and “fine“, as this was how the spectral lines appeared in the 1870s when spectroscopists first started identifying them. […]

4. Thanks, i have challenged by my class-mate about this but now i know something from this blog… Thank you mrs s,p, d, f