writing equations

well_tempered

I see, well I see your going off at a tangent,

The metal Magnesium, groups 2, valency therefore 2, so the hydroxide anions will be -2. Yes or No?

Yes, the sum total of the charge of the hydroxide anions will be -2. What we can't prove is that there are two hydroxide anions. There could easily be one, in which case the OH ion would have a -2 charge. Of course, there isn't one, there's two... but the only way I know to prove that is to use the fact that the charge of OH is -1, which is circular logic. :s-smilie:

Maybe this is why I never got chemistry. :wink:

But I'd be grateful for an answer. Trust me, I'm not thick, you're either not making sense or you're assuming much more chemistry knowledge than I have.

Reply 22

At GCSE level you pretty much need to learn valencies of common ions (not too many though, I can't see phosphate coming up too much). Probably such things as

OH^-

,

SO^{2-}_4

and

CO^{2-}_3

. It's also handy to note that Group I elements form +1 ions and Group II form +2 ions and that the transition metals are confusing.

At A2 I can't remember having to learn too many more, though you do learn oxidation states and more valency stuff (e.g. Cu(I) and Cu(II)).

A higher level than that I don't really know yet. I do know that chemistry gets much more confusing though it is still understandable, just the concepts aren't so basic as "X ion always has Y charge" or some such.

Reply 23

I don't know generalabriety,

I immediately see two things, molecules/atoms like to stable, ie. have no charge and so they form ionic bonds with no overall charge, and also I know that atoms like a full shell of 8, and that's why the noble gases are so stable and 'happy'.

Somewhere in those two concepts, of a balance charge and a full outer shell, the answer lies there, but I can't fathom it now. Does anyone know which 'rule' leads to OH-, having -1 instread of -2 charge?

Reply 24

It's not really such a good idea to use rules to find molecular ions as some are unusual (guessing the structure of

SO^{2-}_4

and trying to calculate its charge is much more difficult than just learning that it is a 2- ion.

If you really want to "work out" what charge hydroxide is, just think of it in an acid dissociation sense:

2H_2O \rightleftarrows H_3O^+ + OH^-

I'd still recommend just learning. It may not be as fun, but it's necessary for chemistry quite a lot of the time.

Reply 25

let's say, to get a -2 charge requires more input of energy, than a -1 charge. Maybe an hydroxide anion of -2 can be created.

Reply 26

It can, but under normal conditions the amounts will be negligible. In the sun you can get such things as

Fe^{26+}

but that's because of the extreme conditions.

Suffice it to say that all the GCSE and A2 level ions are generally fairly well behaved even if it's difficult to explain exactly why they have the charge which they do.

Reply 27

cpchem

14

Argh... people saying "like"... I know that we were taught that "happiness is a full outer shell" and other such crap... but now the idea of an atom being cheery because of it's electronic configuration makes me want to scream.

As far as understanding most of this is concerned, it is, unfortunately, the case that you don't find out why these things happen until A-level and beyond. Mostly because it depends upon quantum mechanics... and those two words scare people very easily.

Reply 28

yeh, we're led to believe the electron would spiral into the nucleus using classical physics, but a lecturer at MIT ducked to answer the question why during a lecture. I hope the guy eventually got his answer later in the course.

why is it crap, have you heard of covalent bonding, and the noble gases. I'm O'level and it's more fun than learning dates in a history lesson.

Reply 29

Kyle_S-C

Suffice it to say that all the GCSE and A2 level ions are generally fairly well behaved even if it's difficult to explain exactly why they have the charge which they do.

Try me. I think the OP's question has been answered ten times over, and they don't seem to be coming back. So how do you determine the charge on an ion?

Reply 30

henryt

17

well_tempered

yeh, we're led to believe the electron would spiral into the nucleus using classical physics, but a lecturer at MIT ducked to answer the question why during a lecture. I hope the guy eventually got his answer later in the course.

why is it crap, have you heard of covalent bonding, and the noble gases. I'm O'level and it's more fun than learning dates in a history lesson.

Isn't it just because the electron must be losing energy as EM radiation when it moves in a circular orbit, which means there's less kinetic energy => moves slower => the distance between electron and atom must decrease and so essentially, because it would ideally (without the loss of energy through light) be a circular orbit, it sprials in? Or have I just misunderstood the post, because if I haven't, I'm seriously worried as to why the lecturer ducked the question...

Reply 31

Remo

henryt

Isn't it just because the electron must be losing energy as EM radiation when it moves in a circular orbit, which means there's less kinetic energy => moves slower => the distance between electron and atom must decrease, and so essentially, because it would ideally (without the loss of energy through light) be a circular orbit, it sprials in?.

yea thats what i would hav thought too. unless i missed a page in the text book. lol.

Reply 32

i'm going to track down the question, couldn't find it yet..

it's amongst this lot:

http://ocw.mit.edu/OcwWeb/Chemistry/5-111Fall-2005/VideoLectures/index.htm

Reply 33

Remo

well_tempered

i'm going to track down the question, couldn't find it yet..

it's amongst this lot:

http://ocw.mit.edu/OcwWeb/Chemistry/5-111Fall-2005/VideoLectures/index.htm

have fun findin it lol. :smile:

Reply 34

lol, right, i put my foot in it

Reply 35

generalebriety

Try me. I think the OP's question has been answered ten times over, and they don't seem to be coming back. So how do you determine the charge on an ion?

I think it's that (for molecular ions) the particular ions formed are the most stable as a result of the lowest energy configuration of electrons filling molecular orbitals. If there are available bonding molecular orbitals then they'll be filled and if it would be more effective to lower the system's energy by losing electrons from antibonding orbitals, then the system does that instead.

Reply 36

Kyle_S-C

I think it's that (for molecular ions) the particular ions formed are the most stable as a result of the lowest energy configuration of electrons filling molecular orbitals. If there are available bonding molecular orbitals then they'll be filled and if it would be more effective to lower the system's energy by losing electrons from antibonding orbitals, then the system does that instead.

Hmm. So if I'm understanding you right, the OH ion has a 1- charge because that third orbital in the 2p subshell of the oxygen atom needs one more electron before it's full?

Reply 37

nota bene

15

generalebriety

Hmm. So if I'm understanding you right, the OH ion has a 1- charge because that third orbital in the 2p subshell of the oxygen atom needs one more electron before it's full?

Yeah that's what I learnt at least, and there is you not taking chemistry :p:

Reply 38

generalebriety

Hmm. So if I'm understanding you right, the OH ion has a 1- charge because that third orbital in the 2p subshell of the oxygen atom needs one more electron before it's full?

I think so, I can't remember to be honest, but you couldn't apply the same logic to deduce the charge of

SO_4^{2-}

or work out the fact that

He_2^+

is stable even though

He_2

doesn't exist. The majority of the ions are much more complicated, though I must confess I probably couldn't work out charges on ions (though I probably should be able to :s-smilie:

). I don't mean to be patronising or anything, but I can see the point in things just been given to us to learn at GCSE level. After all, many people doing chemistry GCSE will then be going on to do english or history of art or some other such completely unrelated subject at degree level.

Reply 39