Consider the H2+ ion. (e) Suppose that the ion is excited
by lig...

Question

Consider the H2+ ion. (e) Suppose that the ion is excited 
by light so that an electron moves from a lower-energy to 
a higher-energy MO. Would you expect the excited-state 
H2+ ion to be stable or to fall apart?

Accepted Answer

hey everyone in this example, we're asked whether our helium plus one ion was excited by light to move an electron from lower to higher energy molecular orbital. Would the excited state be stable? So what we want to think of is recalling upon our concept of bond order. And we should recall that we can find the bond order of our eye on here by taking one half and multiplying in brackets. The difference between our number of electrons in our bonding molecular orbital subtracted from our number of electrons in our anti bonding molecular orbital. So we're going to end the brackets here. And what we want to do is figure out bond order for our given atom of the helium plus one cat ion. So what we're going to begin with is our helium plus one cat ion and finding total valence electrons. So we would recall that for one atom of helium we have two valence electrons but in this case we have two atoms of helium because we have this subscript of two here. So what we're going to do is have to times two electrons and that's going to give us a total of four electrons. However, we also have this plus one chart here meaning we're going to have, we're going to recall that a plus one Catalan charge means we lose one electron. And so this is going to leave us with a total of three electrons total. So we should recall that helium is located across the first period of the periodic table. And so to draw out its orbital diagram, we're going to start out with the sigma one s bonding molecular orbital where we will fill in our first two electrons here. So we have one in the upward spin and then one electron in the downward spin. And then our next orbital we would move up to is the anti bonding which we recall is sigma astrid one. S And then here's where we will place our third electron, It can be in any directions been. So now we can go ahead and calculate our bond order. And what we would say is that we have one half, multiplied by the number of electrons in our bonding molecular orbital. So we have two electrons in our bonding molecular orbital. So we'll plug that in and then minus and sorry, we should keep that, we should make this blue so that it matches our formula above. So we have two electrons in the bonding molecular orbital will plug that in us. Two electrons subtracted from the number of electrons that are anti bonding molecular orbital. We have one electron in our anti bonding molecular orbital. So we subtract one electron. And so what we would get for bond order is one half times one, which is just going to give us a bond order of one half. Now, according to the prompt, we're going to be again moving our electron by exciting it from a lower to a higher energy molecular orbital. And so right now if we excite, sorry if we excite one electron we would take one of our electrons here. So we would just maybe take one of These electrons here. This downward spin one. And we would just move it up to our higher energy level which would be our sigma asterisk one. S anti bonding molecular orbital, meaning it would now be placed in a higher energy level where it's now excited to our anti bonding molecular orbital which has higher energy. And this will change our bond order so that we now have one half multiplied by now. We only just have one electron in our bonding molecular orbital minus. Now we would have two electrons in our anti bonding molecular orbital here. So we would say here two electrons, anti bonding molecular orbital. And so this would give us now one half one minus two which is going to be negative one times one half. And that's going to give us a bond order of negative one half. And so because we now have this bond order of negative one half, we would say therefore negative bond order, it's actually not possible. Thus exciting one electron to the sigma. Anti bonding one s molecular orbital means no bond and molecule falls apart and so therefore the excited state of the helium or plus one cat eye on is unstable. And this would be our final answer to complete this example. It's going to be an unstable excited state or in its excited state due to the fact that we would have now a negative bond order, as we shown, which was negative one half. So unstable is our final answer. I hope that everything I explained was clear. If you have any questions, please leave them down below and I will see everyone in the next practice video.

Consider the H2+ ion. (e) Suppose that the ion is excited by light so that an electron moves from a lower-energy to a higher-energy MO. Would you expect the excited-state H2+ ion to be stable or to fall apart?

Watch next