Chemistry help!! Test tomorrow

partyatmygaff

I've got a test tomorrow on Molecular structure and I am completely and utterly lost. I know what each type of structure is and its bond angle but I can't understand why.
Like how do you tell if its tetrahedral and has a double bond etc?

ayumi

wot book do you use?
if you have chemistry live or the revision book it would help you

partyatmygaff

I have chemistry live and tbh I think its terrible, Its layout is a mess and It doesn't explain molecular structure at all, just 4 examples and no actual text about it.

ayumi

try these
http://www.cem.msu.edu/~reusch/VirtualText/intro3.htm
http://www.docbrown.info/page20/irishOLHLchemistry.htm
http://www.sip.ie/sip069/Documents/chemistrypage.html

partyatmygaff

Anything is better than that book.

Thanks

Making It Bad

Chemistry Live! is one of the better books imo...

Anyway to find it's structure first you have to find out the number of electrons in the outer shell in each atom bonding. To do this use your periodic table (group 1= 1 electron in outer shell, group 7= 7 electrons in outer shell etc.) . Remember it is only the outer energy shell which is involved in bonding.

Take Methane for example. It has the chemical formulae CH4. Carbon has 4 electrons in outer shell, it needs to gain 4 more electrons to have a full outer shell. Each hydrogen atom provides one of these electrons. Together they form a covalent bond.

Now, electrons carry a relative charge of -1. Like charges repel each other, different charges attract. The electrons thus try to separate themselves as much as possible. The largest possible angle they can make between them is 109.5°. Remember that the real world isn't 2d but is 3d, if life were 2d the angle would be 360÷4 as there are 4 electrons.

We represent this in 2d on diagram as follows (ignore the 108.7pm):


The weird lines are supposed to show it "standing" out of the page, so in 3d it would look like this:



The black represents carbon, the white hydrogen. The bit joining them is the "bond angle".

partyatmygaff

Making It Bad wrote: »

Chemistry Live! is one of the better books imo...

Anyway to find it's structure first you have to find out the number of electrons in the outer shell in each atom bonding. To do this use your periodic table (group 1= 1 electron in outer shell, group 7= 7 electrons in outer shell etc.) . Remember it is only the outer energy shell which is involved in bonding.

Take Methane for example. It has the chemical formulae CH4. Carbon has 4 electrons in outer shell, it needs to gain 4 more electrons to have a full outer shell. Each hydrogen atom provides one of these electrons. Together they form a covalent bond.

Now, electrons carry a relative charge of -1. Like charges repel each other, different charges attract. The electrons thus try to separate themselves as much as possible. The largest possible angle they can make between them is 109.5°. Remember that the real world isn't 2d but is 3d, if life were 2d the angle would be 360÷4 as there are 4 electrons.

We represent this in 2d on diagram as follows (ignore the 108.7pm):


The weird lines are supposed to show it "standing" out of the page, so in 3d it would look like this:



The black represents carbon, the white hydrogen. The bit joining them is the "bond angle".

Well I do understand all of that, the part I don't get is how would I know if its trigonal planar, linear, v-shaped, tetrahedral e.t.c.

Completely lost on that, the actual idea I understand its just I have no idea why certain compounds have a certain molecular structure.

AddictedToYou

I've got a test on that tomorrow too, along with the chapter on Ionisation Energy and Electronegativity.

That table isn't much use really, you'd be better going through the examples. Basically you have to draw out the bond. Lone pairs (i.e. two electrons not bonded to anything) stay together and bond pairs (i.e. one electron from one atom and another from another bonded together) stay together. It's hard to explain without diagrams really. Our teacher did it on the board so I just learned from that. If I type those noted up sometime I'll pass 'em on to you.

Good luck tomorrow!:D

Making It Bad

partyatmygaff wrote: »

Well I do understand all of that, the part I don't get is how would I know if its trigonal planar, linear, v-shaped, tetrahedral e.t.c.

Completely lost on that, the actual idea I understand its just I have no idea why certain compounds have a certain molecular structure.

The reason why atoms bond is in order to become more stable. The octet rule states that atoms are in their most stable state when they have the electronic configuration of the nearest noble gas to them in the periodic table. For example, oxygen will try achieve the electronic configuration of neon. Hydrogen will try achieve the electronic configuration of helium. So in the molecule water, H2O , two hydrogen atoms bond with one oxygen atom in order to form a H2O molecule. Oxygen needs to gain 2 electrons in order to have the same electronic configuration as neon. Each hydrogen atom provides one of these electrons. In return the oxygen atom shares one of its 6 electrons with each one of the hydrogen atoms (it is thus sharing two electrons as clear from the diagram), they thus both now have the same electronic configuration as their nearest noble gas.

jumpguy

We've literally JUST finished this and it's a pretty simple concept if you can get your head around it.


Okay, I'll try and explain what I know as best I can, and I'm assuming you know abit about bond pairs and lone pairs already. Note that I say "CENTRAL atom". Don't forget the 2 other atoms bonded onto that.
When reading this, look at the diagrams of the specific covalent shaped (linear, trigonal planar, etc).

Central atom: The atom you have least of and central in the bonding, in H2O, O (Oxygen) is the central atom.
Bond pair: 2 electrons shared between atoms.
Lone pair: 2 electrons in an atom (always central atom as far as I know) not shared.


Remember: Bond pairs repel more than lone pairs. A lone pair is 2 'unbonded' electrons (not involved in any bond). Because a lone pair is 2 lone electrons, it has a stronger negative charge than bond pairs.

If the central atom has 2 electrons on it's outer shell (and is covalent) it will have a linear bond. This is because the 2 electrons repel each other as far as possible. 2 Bond Pairs.

If the central atom has 3 electrons on it's outer shell (and is covalent) it is trigonal planar. The bond angle is 120 Degrees. This is because the 3 electrons repel each other to be as far as possible. The farthest they can be away from each other is 120 Degrees. 3 bond pairs.

If the central atom has 5 electrons on it's outer shell (and is covalent) it is Pyramidal. This consists of one lone pair and 3 bond pairs. The lone pair has a stronger negative charge (because it is 2 lone electrons) and therefore push the bond angle the 109 Degrees.

If the central has 6 electrons on it's outer shell (and is covalent) it is V-Shaped Planar. This consists of 2 bond pairs and 2 lone pairs. The 2 lone pairs have an even more powerful negative charge and push the two bond pairs down to an angle of 105 Degrees.



I've explained it as best I can! I know all shapes aren't included but hopefully you'll get the picture from the ones I have. Lone pairs repel more than bond pairs. This pushs the 2 bond pairs down and decreases their bond angle. The more lone pairs, the more the bond angle is decreased.


How do I know if it's trigonal planar, tetrahedral, etc? Look at the outer shell of the central atom. If it has 2 electrons on it's outer shell it's linear (2 bond pairs), 3 electrons on it's outer shell trigonal planar (3 bond pairs), 4 electrons tetrahedral (4 bond pairs), 5 electrons pyramidal (3 bond pairs, 1 lone pair), 6 electrons (2 bond pairs, 2 lone pairs) V-Shaped planar.


We have Chemistry Live! too, but our teacher works with notes. It's far, far better working with notes IMO, the book has much unneeded information. However, I sometimes use it if I'm unsure or I want to test myself (even though some the Q's are not on the course).

Making It Bad

Jumpguy explained it fairly well but I am extremely bored (mid term tomorrow!!), so I drew you a little diagram on MS paint, kinda ****ty but it may help (doubtful).

jumpguy

Well partyatmygaff how'd you get on?

partyatmygaff

Been busy with the mid-term!

I think I got a high B / Low A

Thrilled with it, thanks everyone