h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: chemical bonding for A level - A87721743
Author: nicki - U871201

I would welcome any feedback on this entry.

Thanks.

Hi nicki, I think you'll need to say what the elements actually are. I'm not into chemistry, and although I know what NaCl is, or O2, I have no idea what CH4, KBr, MgO stand for (even though O know that Mg is magnesium and O is oxygen.

Personally, I think the Title is unfortunate, too. I wondered how students bond chemically for their A levels.

I think you could mention who Pauling and van der Waal were.

Oh, and you don't tell us why the atoms join other atoms.

Thanks Bel - I have edited it now.

Hi nicki

I did take A-level Chemistry, but my in depth knowledge has evaporated over time and neglect. I promise to give it a thorough review, though, and quite likely so seek out some other more chemically active members.



Just out of interest, and in case I forget to mention it later, we were talking about how geckos stick to walls, and Van der Waals had cropped up there, so maybe it could be used as a practical demonstration of this force used so effectively in nature:

http://www.pnas.org/content/99/19/12252.abstract

I think this is a very good subject for the Guide nicki - thanks

here are a few Entries about Chemistry, to look through in case any are suitable for you to add as more links.

http://wsogmm.h2g2.com/dna/h2g2/alabaster/Search?searchstring=chemistry&showapproved=1&searchtype=article&dosearch=Search+the+Guide


and of course this category list too:

http://wsogmm.h2g2.com/dna/h2g2/alabaster/C56

I think this is a useful article - the other h2g2 entries on chemical bonding (A1011899 and A1011871) go into the maths behind the quantum mechanics of bonding, whereas this is more accessible. I have a few comments on the chemistry in the entry. Some of the generalisations (that atoms in bonds end up with a noble gas configuration, ionic lattices have the rock salt crystal structure, etc.) might be part of the A Level syllabus (I didn't do A Levels so I don't know), but there are plenty of exceptions to all of these. I think it's important to point out that the chemistry is often a lot more complex than these simple cases, even if you don't go into the details. I'll highlight the bits I mean, along with a couple of typos that I spotted:

"All other atoms will bond with others to form this octet and to become stable."
This is broadly true for light elements in the p-block (oxygen, nitrogen, etc.) and it's also true of alkali and alkaline earth metal when they lose electrons to form ionic compounds. The situation is often a bit more complicated though, especially for the transition metals. You don't mention any transition metals later in the entry, so maybe A Level Chemistry doesn't cover more complicated types of bonding? I still think it's worth clarifying that sentence, though, because it implies that *all* bonds give atoms 8 electrons in the outer shell. You could also link to A593561, which introduces atomic energy levels.

"Na -------> Na+ + e-"
Some of the other chemistry entries use the ASCII code → for arrows. I think it looks a bit neater than using hyphens.

"Giant ionic lattices form from ionic compounds."
This seems the wrong way round to me. Wouldn't ionic compounds form giant lattices?

"A giant ionic lattice comprises hundreds of thousands of ions depending upon the sixe of the crystal."
Typo: sixe --> size
Also, although hundreds of thousands sounds like a lot, a crystal which is actually visible (like a salt crystal) is going to contain much more than that! Avogadro's number is 6x10^23, after all. That's what, sextillions of ions?

"The arrangement is characteristic of all ionic compounds – each positive ion is surrounded by 6 negative ions and each negative ion is surrounded by 6 positive ions."
That's only possible if there are an equal number of positive and negative ions. So it's true for your example, NaCl, but not for CaCl2, which you mention later. There are also some lattices with equal numbers of cations and anions that *don't* have the same structure as NaCl. For example, each Cs atom in CsCl is coordinated to eight Cl atoms, and vice versa. If you don't want to go into details, it might be better to say something like "This arrangement is a common one for ionic compounds." You could add a link to A49292724, the entry on salt, which has some info on its chemistry.

"Therefore giant structures don’t form like they do in ionic compounds."
Except in cases like diamond or crystalline silicon, where the atoms can't satisfy the octet rule by bonding in pairs. The bonding is still covalent and directional, it just forms lattices rather than small molecules. Other non-metal elements like phosphorus and sulphur are solids at room temperature too (despite having the same valence electron arrangement as nitrogen and oxygen), but that's because of trade-offs between bond energies and entropy. I'm not sure you need to cover the thermodynamics of that here, but it's probably worth changing this sentence because there are important examples of covalent lattices.

"We usually write a dative covalent bond as A --->; B"
If A is donating a pair of electrons to B, this should be A:--->B

"This is the type of bonding which occurs in metal atoms only."
This should be "between metal atoms" rather than "in metal atoms".

"Polar bonds are formed when the electronegativity of the molecules is different."
Should this be "electronegativity of the atoms"?

"van der Waal’s forces"
There's no apostrophe in van der Waals, but "van der Waal" needs an "s" at the end when you mention his name later.

"2. the attractive force between molecules if increased"
if --> is

Sorry, this list has got rather long. Most of these changes are just small fixes, though. And sorry if I'm being overly pedantic here! I just think it's worth being as accurate as possible even if the aim is not to go into a lot of technical detail.

Thanks Aurora! Really appreciate that sort of review for a piece like this. I've not got the knowledge, and my time is pulled in many directions too.

I think this Entry will most certainly be useful, and we need all the support we can get to help nicki get this into a finished state


Lanzababy

Thanks for all the input. I haven't abandoned it, I just have a few things to do in RL but will have a look at it next week.

Slightly odd perhaps to have a small amount of Ice (i finished doing chem AS last year, then dropped it) stuff. More/Less detail perhaps

Oh... and if you're mentioning Ice... A57241226 Some Cold, Hard Facts About Ice

Hope RL stuff is going ok - if youre on my timetable, you're on mocks

Just catching up here, nudging it up the list, to show that it's not being overlooked by the reviewers. All of nicki's Entries are going to be very useful as Guide Entries, and hope that her RL gives her some time soon to come back to these PR submissions.

Hi

I will read this bit by bit and comment bit by bit.

First of all right at the start of your Entry you should explain to people where the electrons get into it all. You just assume that they know but I'm sure many don't.

When you explain about the different types of bonding you should probably say how many electrons each of the atoms has in the outer hull at the start of the process.
Maybe it's worth getting the periodic table into it all too. We can add a picture of at least part of it (because of the size) when this is finished so people see what you mean.

As you mention crystals you may also mention that the shape of a crystal always mirrors the shape of it's ionic compounds.

is hull another word for shell/orbital?

Or this...
A lonely hydrogen ion sees a helium atom and walks up to him and scolds,
"Hey, you shouldn't practice polygomy on those poor electrons, it's not
attractive you know?" And the helium atom laughs and says, "Well, at least
i get to keep them!"

Sorry, I never did Chemistry in English. I mean orbital, yes.

I would also mention what a metal etc actually is and where you find them on the periodic table.

You should explain the metal bonding more clearly.

'The outer shell electrons are delocalised – they are free to move around the metal.'
I at first thought you're talking about a single atom while you talk about several bound together already.
Also explain why this is so (here the atomic table gets into it again I think?). I can't really remember, was it because they had too many?

A bit further up you could mention that NaCl is usual salt that you use for cooking. It gives people something they know... maybe.

All over the place, for complex values of chemistry, and that would require explaining that hydrogen, is indeed, a metal

Hmm, i must have flicked off to quickly, i posted that comment ages ago

Well, yes. Would it be too much for this Entry to explain it? I find it important. Or at least mention all the stuff about how many electrons in the outer orbit. It would explain it all more clearly and make things less mysterious.

I learnt it for my AS chemistry (thats the 1st half of the A-level course, but it covers most bonding stuff)

It should be in there for a strict interpretation of the course, but would make it harder for someone to try and read the entry off a fresh start, which a smart reader could currently do

It would really work if we had a matching set of GCSE stuff to support the various A-level bits - does the writer have a year or so free to cover all the science syllabi?

Hmmm. I thought it would make things *easier* for someone who knows nothing about it. No idea who's right.