# Syntax and Typography of Atomic Orbitals

For mhchem (i.e. the \ce command you use here), I want to improve the support for atomic orbitals. (Reason: I see it quite often used here.) But only a few things about orbitals are mentioned in IUPAC documents. Many more seem to be "unwritten law". As I am not a chemist I need your help.

## 1

The Green Book p. 32 defines the letters s, p, d, f, g, h, i, k ... (omitting j). They have to be typeset upright.

Question a: How far does this "alphabet" reasonably go? (Wikipedia goes up to i).

## 2

On the same Greek Book page, electron configurations are explained, e.g. $\mathrm{(1s)^2 (2s)^2 (2p)^1}$.

Many other sources write it without parenthesis: $\mathrm{[Kr]\mskip2mu4d^{10}\mskip2mu5s^1\mskip2mu5p^3\mskip2mu5d^4}$

Question a: What values can the left-side number take? (At Wikipedia, I see 1 to 7.) Can it be 2-digit?

Question b: What values can the right-side exponent take? (At Wikipedia, I see 1 to 10.)

## 3

I think I did not come across the configuration with one-electron orbitals (in Greek). $\mathrm{(1\sigma)^2 (2\sigma)^2 (3\sigma)^2 (1\pi)^1}$. (The Greek letters should be upright, but that's a drawback of MathJax.)

Would this be useful? Or not needed?

## 4

The examples introduce another notation, "electronic states" $\mathrm{\cdot\cdot\cdot (2a_1)^2 (1b_2)^2 (3a_1)^2}$

a: What values could the letter take? b: The left-side number? c: The subscripts? d: The right-side exponent?

## 5

From Wikipedia, I learned that there are notations IUPAC does not mention: $\mathrm{p_x}$, $\mathrm{p_y}$ and $\mathrm{p_z}$. (All subscripts should be upright, because they are labels, no variables.)

But it does not end there. Here and here I saw: $\mathrm{d_{xy}, d_{xz}, d_{yx}, d_{x^2{-}y^2}, d_{z^2}}$ This was confirmed by this book.

There are alternative styles here ($\mathrm{{p_x}^1}$). and here ($\mathrm{p_{x1}}$).

a: I guess the $\mathrm{d_{z^2}}$ notation (with index z-square) is correct?

Oh, I just found this book that also includes $\mathrm{4f_{x(x^2{-}3y^2)}}$ and $\mathrm{4f_{y(3x^2{-}y^2)}}$.

b: Are there other indexes possible than x, y, z?

c: Can it be just squares? Or are other index-superscripts possible?

d: What about the 3 in the last examples? Could this also be any other numer?

(You see, I just look at the typography of things and don't know their meaning.)

e: Could there be any other forms of indexes?

f: Typographically, what does this dash stand for? Is it a hyphen or a "transition between a higher energy state and a lower energy state"?

## 6

Finally, there is orbital hybridization, which will lead to notations like $\mathrm{sp, 2sp, sp^2, 2sp^3, s^{0.5}p^3, s{-}s, sp^3{-}sp^3}$.

a: Can the superscript be any other value than 2 and 3? Can the "base" be any other thing than s, p and sp?

b: Is $\mathrm{2sp^3}$, is it $\mathrm{2s}$ and $\mathrm{p^3}$ or is sp one ... thing with a 2 before and a 3 in superscript.

c: Is it a hyphen or a "transition between a higher energy state and a lower energy state"?

• Wow, tempted to flag as too broad. Feb 17, 2017 at 21:05
• Question 1 remember there are also the corresponding term symbols en.wikipedia.org/wiki/Term_symbol
– MaxW
Feb 17, 2017 at 21:27
• @Mithoron Yes, it got rather long. However, I assume that there is a high correlation between the answers (e.g. "left-side number can be ..., everywhere") and a comprehensive answer would be less than 10 lines. Feb 20, 2017 at 22:23

Question 1. The Green Book p. 32 defines the letters s, p, d, f, g, h, i, k ... (omitting j). They have to be typeset upright.

How far does this "alphabet" reasonably go? (Wikipedia goes up to i).

1. Section 2.6.3 of that version of the Green Book shows the term symbols (Wikipedia) which are capital letters S,P,D,F,G,H,I,K,... and used to indicate the electronic configuration of the valence electrons and for excited states.

2. The lower case letters are used for the azimuthal quantum number (ℓ) which is one of the four quantum numbers for an electron in an atom. You can typeset $99.9~\%$ of chemistry of even more using only the letters s, p, d and f, corresponding to the left-hand main group elements, the right-hand main group elements, the transition metals and the lanthanides/actinides, respectively. Everything above f is and will be for quite some time not relevant to chemistry outside a very small circle that would know how to deal with stuff.

Question 2. On the same Greek Book page, electron configurations are explained, e.g. $\mathrm{(1s)^2 (2s)^2 (2p)^1}$.

Many other sources write it without parenthesis: $\mathrm{[Kr]\mskip2mu4d^{10}\mskip2mu5s^1\mskip2mu5p^3\mskip2mu5d^4}$

Question a: What values can the left-side number take? (At Wikipedia, I see 1 to 7.) Can it be 2-digit?

Question b: What values can the right-side exponent take? (At Wikipedia, I see 1 to 10.)

1. This has two sub-questions.

1. By a similar reasoning as presented in the answer to 1, I think 8 will be the limit until the not-so-close future. But there is nothing intrinsically preventing it from becoming two-digit at some distant point.

2. The superscript can take values of up to $4l+2$, where $l$ can be considered ‘computer science counting’ of the letter sequence s, p, d, …; so for s $l = 0$ and thus $4l+2=2$; likewise for p ($l=2$) the value is maximum $4l+2=6$ etc. Note that $4l+2$ is actually $2(2l+1)$: each subshell gives up to $2l+1$ orientation possibilities that each can be populated with two electrons.

Question 3. I think I did not come across the configuration with one-electron orbitals (in Greek). $\mathrm{(1\sigma)^2 (2\sigma)^2 (3\sigma)^2 (1\pi)^1}$. (The Greek letters should be upright, but that's a drawback of MathJax.)

Would this be useful? Or not needed?

1. Yes, this is useful, especially for discussing molecular orbitals. The Greek letters you want to be aware of are σ, π and δ. Maybe a fourth will be added soon corresponding to f orbitals (υ?) but I don’t see that coming anytime soon.

Question 4. The examples introduce another notation, "electronic states" $\mathrm{\cdot\cdot\cdot (2a_1)^2 (1b_2)^2 (3a_1)^2}$

a: What values could the letter take? b: The left-side number? c: The subscripts? d: The right-side exponent?

1. I’m going to answer that as one.

This notation uses irreducible representations to define orbitals. The bit between the initial number and the superscript can be any of the symbols that appear in Little Bobby Tables (courtesy of Orthocresol). They each represent a type of orbital with a certain symmetry.

Orbitals of the same irreducible representation (i.e. the same centre) are numbered from lowest to highest energy — so in principle the sky is the limit. Practically, however, you will be hard-pressed finding three-digit examples.

The exponent can at maximum be $2n$, where $n$ is the number in the first column of the respective character table (the E column). This number corresponds to a certain symmetry’s degeneracy (i.e. so many orbitals of the same energy) and each orbital can be populated by two electrons.

Question 5. From Wikipedia, I learned that there are notations IUPAC does not mention: $\mathrm{p_x}$, $\mathrm{p_y}$ and $\mathrm{p_z}$. (All subscripts should be upright, because they are labels, no variables.)

But it does not end there. Here and here I saw: $\mathrm{d_{xy}, d_{xz}, d_{yx}, d_{x^2{-}y^2}, d_{z^2}}$ This was confirmed by this book. There are alternative styles here ($\mathrm{{p_x}^1}$). and here ($\mathrm{p_{x1}}$).

a: I guess the $\mathrm{d_{z^2}}$ notation (with index z-square) is correct?

Oh, I just found this book that also includes $\mathrm{4f_{x(x^2{-}3y^2)}}$ and $\mathrm{4f_{y(3x^2{-}y^2)}}$.

b: Are there other indexes possible than x, y, z?

c: Can it be just squares? Or are other index-superscripts possible?

d: What about the 3 in the last examples? Could this also be any other numer?

e: Could there be any other forms of indexes?

f: Typographically, what does this dash stand for? Is it a hyphen or a "transition between a higher energy state and a lower energy state"?

It was my assumption that the axis directions should be italicised but I don’t know. To discuss this, please go to the break-out discussion for this question.

p orbitals will always have $x$, $y$ or $z$. d orbitals will have one of the five you noted ($\mathrm{d}_{z^2}$ is correct — as I said, I am not sure on the upright versus italic question but I would have assumed italics). The f orbitals themselves can be labelled by various different polynomials in $x$, $y$, $z$, and $r$. See these two web pages: 1, 2. In general the subscript is just a polynomial.

Which symbols and exponents can appear here is fixed already by the results of the hydrogen-like orbitals — again, for most of chemistry d orbitals are sufficient and you can cover $99.9~\%$ by including f orbitals.

The ‘dash’ is a mathematical minus-sign operator. Yes, it truly is $x^2$ minus $y^2$.

Question 6. Finally, there is orbital hybridization, which will lead to notations like $\mathrm{sp, 2sp, sp^2, 2sp^3, s^{0.5}p^3, s{-}s, sp^3{-}sp^3}$.

a: Can the superscript be any other value than 2 and 3? Can the "base" be any other thing than s, p and sp?

b: Is $\mathrm{2sp^3}$, is it $\mathrm{2s}$ and $\mathrm{p^3}$ or is sp one ... thing with a 2 before and a 3 in superscript.

c: Is it a hyphen or a "transition between a higher energy state and a lower energy state"?

1. Sub questions.

1. Yes. You have $0.5$ as a superscript in your own examples. But, as posts by Martin and Ron somewhere point out, in $\mathrm{sp}^x$ the superscript $x$ can, in principle, be any number.

2. $\mathrm{2\,sp^3}$ is $\mathrm{\{2\}\{sp^3\}}$ — and there could potentially be another superscript making it $\mathrm{(2sp^3)^2}$.

3. I don’t know what that hyphen is supposed to be.

• More question than a comment: do we use italics at all? I see sometimes s,p... italicized, and frankly, I use it often, too, because I found texts more readable when orbital notations pop up.
– Greg
Feb 21, 2017 at 1:13
• @Greg The orbital designation letters should be in upright type as they are not variables. So that bit at least is clear.
– Jan
Feb 21, 2017 at 1:15
• Great! I went through it step by step. At question 4, I am stuck. These tables surpass my brain capacity. Starting with the fact that everything is captical letters, there. Is it the left-most column you are referring to? So, the middle part could be anything like $\mathrm{e_1, d_{4h}, a_2''}$? Feb 21, 2017 at 9:26
• @mhchem Sort the answers by "active", and ignore the last two posts on "direct product tables" and "tables of descent in symmetry" - those are different things. Apart from those two posts, it can be anything in the left-most column, except for the header row. So, $D_\mathrm{4h}$ does not count, but $\mathrm{e_1}$ and $\mathrm{a_2''}$ are both possible. If you are referring to orbitals, then the capital letters will become small letters. The capital letters in the tables are referring to irreducible representations, which are sets of matrices describing the behaviour of the orbitals. Feb 21, 2017 at 12:25
• For question 1: if you take the orbital letters up to i you'll be able to handle ~100% of chemistry -- computational heavy-element chemistry needs to go higher than a lot of chemistry because they need to add on higher-level functions to allow molecular orbitals to be more finely shaped. Feb 21, 2017 at 17:23