Electrons...

[gorillaRN]

Hey, I'm taking my MCAT in January and and reviewing chemisty (which has been about 5 years since I took it). I am reading about electrons and just still have trouble understanding the pi bonds and the octet rule. I get all the other aspects seemingly but just seem to have trouble grasping these.

I know that the periods on the periodic table show different levels on the electron scale (2, 8, 18, ect..) but I just don't get how some can be octets and some can't.

In general does anyone have a good way of describing electrons as a whole picture, because I just seem to be having a hard time grasping the concept.

Thank you everyone, BTW, I am using Exam Krackers to do this prep and it seems to be good, but sometimes I feel like I'm missing out on something (maybe it is just my long absence from chemisty).

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[Caesar]

Moved to Study Q + A for ya.

 

[xanthomondo]

Ok, obviously, this question has the potential for a HUGE response: there are entire courses dedicated to electrons and bonding theory. This is kind of a quick and dirty explanation (all of the details won't be 100% accurate but it should give you a good way to visualize things). You don't need the theories behind this for the MCAT (I don't think).

As for electron bonds, (we'll stick with carbon for this), but the orbitals of carbon are 1s (which has 2 electrons) 2s (has 2 electrons) and 2p (which has 2 electrons as well). These orbitals look like: (1s orbital is also just a sphere, it's smaller and is usually forgotten about when dealing with this)

Here there are two electrons in the 2s guy (not really useful for understanding bonds, but it's nice to remember for the octect later) and there is 1 electron occupying two p orbitals. Imagine two of the right hand picture coming together, below. This represents to carbon atoms coming into contact with each other to bond.

While this is more idealized, who cares lol. The 2Px (pink) orbitals will come together to make the sigma bond, remember there can only be 1 sigma bond between atoms. The 2Pz orbitals will come together as well. Notice the difference here, the pink ones bump end to end while the green have two areas that they can overlap. This is the main difference between the sigma and pi bonds: there are two parts of the orbitals that overlap in pi bonds. A crude picture below (this ignores hybridized orbitals with hydrogen because we only care about sigma and pi bonding):

Things to remember: the pi bond has two LOBES (upper and lower). The molecule above has one pi bond and one sigma bond. If you really want to get crazy technical and play with hybridized orbitals, ethene looks like this:

From the microsoft paint picture above you can see that the 2Py orbitals can also form a pi bond in another plane, which is how ethyne forms. That will look like:

So a triple bond has 1 sigma bond and 2 pi bonds. You always need a sigma bond before you get a pi bond. Also, there are 4 lobes total of pi bonds here, since each pi bond requires 2 lobes.

 

[gorillaRN]

Thanks alot, that really helps. If someone could just explain the orbitals and SPDF to me I sure would appreciate it as well. I have the concept just need a little assistance fully grasping it. Please don't think ignorant of me, I'm just old hehe.

 

[BlackSails]

Thanks alot, that really helps. If someone could just explain the orbitals and SPDF to me I sure would appreciate it as well. I have the concept just need a little assistance fully grasping it. Please don't think ignorant of me, I'm just old hehe.

The idea of an "orbital" comes from the Bohr model of the atom, where the electrons moved in nice circular orbits.

In modern chemistry though, we know that the electron is "smeared out" in a 3d space. So Bohr's orbits become what we call orbitals.

SPDF and just (german?) words for symmetries or something. I doubt that you will need to know anything f orbitals on the mcat.

S orbitals are Spherical. They have no angular component. The number of nodes is equal to n-1.

P orbitals exist in groups of 3. Each one has two pairs of lobes.

D orbitals exist in groups of 5. They are demented. Seriously, I dont know what you need to know about them for the MCAT. I think the only time they come up is in questions on transition metals.


Because of the pauli exclusion principle, each orbital can hold 2 electrons. So each s can hold 2, each p can hold 6 and each d can hold 10.