# Which is more electronegative: hydrogen or astatine?

Which is more electronegative - hydrogen or astatine? I'm a bit confused because my teacher taught me that for an atom to be electronegative it has to be on the top of the periodic table and/or on the right. But hydrogen is on the top-left and astatine is on the bottom-right of the periodic table.

How does one determine which is more electronegative?

• Check out an electronegativity chart. A few of them say that Astatine is more electronegative. – Pritt Balagopal Jun 1 '17 at 7:05
• @PrittBalagopal I agree with you that looking up a chart is the best option, but I believe the OP is looking for a theoretical approach if they were asked such a question in exam (where such EN charts are generally not allowed). – Gaurang Tandon Feb 23 '18 at 2:09
• Astatine does not quite exist, so why bother. As for hydrogen, it is not on the top-left. It is on the top-everywhere. – Ivan Neretin Mar 2 '18 at 8:11
• As alluded to by @IvanNeretin, the placement of hydrogen in the periodic table is a bit of a special and controversial case. There is indeed a decent argument to be made that it should really be a wide bar extending all the way across the top row, from above lithium to above fluorine. – Ilmari Karonen Mar 2 '18 at 8:24

An interesting way to determine which of $\ce{A}$ or $\ce{B}$ is more electronegative is to take their binary compound $\ce{A_xB_y}$ and observe its dissociation. Note that it can be $$\ce{A_xB_y -> xA^y+ + yB^x-} \tag{1}\label{a}$$ or $$\ce{A_xB_y -> yB^x+ + xA^y-} \tag{2}\label{b}$$

Reaction $\ref{a}$ indicates that $\ce{A}$ is less electronegative than $\ce{B}$, because in the dissociation of $\ce{A_xB_y}$, $\ce{A}$ carries the positive charge, while $\ce{B}$ carries the negative charge. Reaction $\ref{b}$ instead indicates that, for similar reasons, $\ce{A}$ is more electronegative than $\ce{B}$ 1

Going by the same logic, we may tend to observe a hydrohalo acid in this case, specifically, the hydroastatic acid/hydrogen astatide. Quoting from its Wikipedia page,

This chemical compound can dissolve in water to form hydroastatic acid, which exhibits properties very similar to the other four binary acids, and is in fact the strongest among them. However, it is limited in use due to its ready decomposition into elemental hydrogen and astatine (my emphasis) Because the atoms have a nearly equal electronegativity, and as the $\ce{At+}$ ion has been observed, dissociation could easily result in the hydrogen carrying the negative charge. (my emphasis) Thus, a hydrogen astatide sample can undergo the following reaction: $$\ce{2HAt -> H+ + At- + H- + At+ -> H2 + At2}$$

which also explains the spontaneous decomposition into elemental hydrogen gas and the astatine precipitate.

If you also have a look at their electronegativities, both have a Pauling electronegativity $2.2$, further complicating the comparison.

Therefore, based on these evidences, I can say, without the least hesitation, that theoretical comparison of the electronegativities of hydrogen and astatine is not possible.

[1]: a shrewd observer would notice that the apt way to write the second reaction's reactant would be $\ce{B_yA_x}$ instead, since $\ce{A}$ is more electronegative