Chapter 3: Bonding

[JWC01]

For calculating the formal charge of O in CO3 I understand how you calculate for the single bond and double bond to get -1 and 0 respectively.
Then you add these together to get -2.
Is this the overall formal charge for O or is it -2/3 as described in the text by -2 x 1/3? And if it is -2/3 shouldn't it add up to the ion's charge which is -2.
Your help would be much appreciated as i'm a little confused on this concept.

[goldstanda3269]

Just so you know: USUALLY, when there is a question like this on the real GAMSAT, they will preface it by reminding you of the rules and then the questions will follow.

Is this the overall formal charge for O or is it -2/3 as described in the text by -2 x 1/3?

It is -2/3 as described in the text.

And if it is -2/3 shouldn't it add up to the ion's charge which is -2.

Yes, and so 3 O atoms x -2/3 = - 2 the overall charge (keep in mind the C has no formal charge).

[JWC01]

Great thanks for your help.

So we don't have to learn the rules for formal charge as they will remind us?
Should we memorise the formula for calculating formal charge?

[goldstanda3269]

So we don't have to learn the rules for formal charge as they will remind us?

That depends on what score you are aiming for. Remember, I used the word "usually". If you are a science student with little arts background then you are aiming for a near perfect science score and so you must do everything to improve in Section 3. On the other hand, an arts student usually needs an average or just above average Section 3 score (presuming excellent Section 1 and 2 scores). An arts student might end up with more confusion by memorizing details regarding unlikely question types.

Should we memorise the formula for calculating formal charge?

Same as above.

It is very important to take one ACER GAMSAT booklet early in your preparation to begin to see how ACER asks questions which will give you a more clear understanding of what is expected of you. If you have not already signed up for the Chemistry Free GAMSAT seminar this weekend, you should consider it:

https://www.facebook.com/events/174821175981225/

[JWC01]

Lovely, thanks for all the info and yes I have signed up for the webinar.

[hyunbb0gam]

Sorry, I have a question in this particular part: 'Similarly, the calculation of the formal charge for O of C-O in the same ion leads to the following: 6-6-1/2(2)=-1'. Isn't it 6-5-1/2(2)? I don't understand how it's 6-6-1/2(2). Thanking you in advance.

[goldstanda3269]

Your question is basically: does the O in C-O (single covalent bond) have 6 non-bonding electrons or 5? The answer is 6 and you can see it drawn out in section ORG 1.5 (at the bottom of page ORG-08 in the 3rd Ed.).

[hyunbb0gam]

I see, thank you

[sting_2694]

Hi, I'm a little confused and I thing I'm missing something. In the calculation given for the O of C-O bond is given as 6-6-1/2(2). why is the the 1/2 multiplied by 2?...Your help would be really appreciated!

[goldstanda3269]

There are 2 answers to your question!

1) we multiply by 1/2 because that is the formula for formal charge that was given in the first column of page CHM-27.

2) The reason behind that number "1/2" is that each atom contributes 1/2 of the electrons to a normal covalent bond.

By the way, the next free online seminar happens to be on Gen Chem, if you are interested: https://www.facebook.com/events/312316635545888/

[olirushworth]

Bonding Chapter Questions - Q&A 1
Question 10 asks which of the supplied compounds are tetrahedral. In the book it states that NH3 and H2O are tetrahedral (although with Trigonal pyramidal and bent geometry respectively).

But NH3 is wrong and then the answer states that TETRAHEDRONS have 4 sigma bonds. Does the question need to ask which of the compounds are tetrahedrons (rather than tetrahedral) or is the info in the book incorrect? As it seems conflicting to me.

[goldstanda3269]

First let's get the nomenclature out of the way. The shape we are discussing is called a tetrahedron (plural: tetrahedra or tetrahedrons) and the adjective for the same word is tetrahedral (as in, this is a tetrahedral molecule thus this molecule forms a tetrahedron).

Table III.A.3.1 has 2 columns for molecules: one describes the arrangement of electrons around the central atom; the other describes the arrangement of other atoms around the central atom (i.e. the latter is the molecular geometry = the geometry of the atoms in the molecule = molecular structure = structure of the compound).

"Question 10 asks which of the supplied compounds are tetrahedral. In the book it states that NH3 and H2O are tetrahedral (although with Trigonal pyramidal and bent geometry respectively). "
- The question is asking about the structure of the compounds and NOT the arrangement of the electrons. The electron arrangement of NH3 and H2O are tetrahedral but not their molecular geometry (structure of the compound which is as you mentioned, trigonal pyramidal and bent geometry, respectively).

"TETRAHEDRONS have 4 sigma bonds."
- True.
- The only way that a molecule can be shaped like a 4 cornered pyramid is if there is 1 atom inside and 1 atom at each of the 4 corners of the pyramid.

[olirushworth]

Ok I think I understand.

When asked about the NH3 as a compound it is Trigonal Planar (because only 3 sigma bonds).

When asked about Electron Configuration in NH3 it is tetrahedral (because 3 bonding pairs of electrons and 1 pair of free electrons = 4 = tetrahedral).

[goldstanda3269]

Exactly!

[goldstanda3269]

δ is the miniscule (lowercase) Greek letter d pronounced 'delta' and it means "a little bit". So δ- is 'a little bit negative' or more formally, 'partially negative', and δ+ is partially positive.

N1+ and O1- do not represent partial charges. N1+ has a formal (actual, not partial) charge of +1 and O1- has a formal charge of -1.

Formal charge is introduced in the GS book in CHM 3.2.

Partial charge is introduced in the book in CHM 3.3 which also refers to ORG 1.5 which you may want to consider reviewing.

Relevant videos for Chapter 3 General Chemistry:

Ionic and Covalent Bonds
Lewis Dot Structures
Multiple Bonds
Resonance
Molecular Polarity (introduces the delta concept)


PS. Note that the majuscule (uppercase) for delta is also used in science and it is written Δ and means 'a change in'.