Why are some elements unreactive?

12 January 2016



Why iridium, platinum and gold are unreactive?


Kat Arney put this question to chemist Ben Pilgrim...

Ben - Thinking back to the periodic table again, these metals are all in the same region of the periodic table, so they're at the bottom right of what we would call the D metals. And, if we think about why something's not reactive, well there's two reasons. One is that it's not favourable for the reaction starting materials to go to products. For example, you might think of a metal reacting with oxygen and forming an oxide and tarnishing and whether that's energetically favourable for that reaction to happen. So that might be one of the reasons. The other one is that it might be favourable for the reaction to happen but it just happens so slowly, there's a sort of barrier for it to start and, with some of these metals, it's a bit of both really.

Kat - So basically, the metals that don't react. It's just really difficult - you need to put too much energy in to get anything out of them?

Ben - Yes. That's one way of thinking about it. Gold does react with some things though. You can dissolve it in aqua regia. This was done in the Second World War.

Kat - That's acid isn't it, basically?

Ben - Yes. It's a concentrated nitrogen hydrochloric acid and it was done in the Second World War to save the gold Nobel Prizes of a couple of German physicists from the hand of the Nazi's. So they were dissolved in aqua regia, hidden in a bottle of acid in a lab, and then after the war, the gold was still there and it was turned back into gold and recast and the medals were re-given to the scientists.


I see that the electronic structure of mercury is very close to that of iridium. Mercury is reactive, usually as a +2 valence atom. Why is mercury's 6s electrons available for bonding while iridium's are not?

What makes the two valence electrons in the 6s orbital un-available for chemical bonding?

The valence electrons are highly energetic due to the strength of the pull of the nucleus.
This actually means their kinetic energy leads to their speeds being relativistic, causing their mass to increase and dropping them lower than is acceptable for bonding.

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