Chemistry Paper 2: Blind Spot Part 2

Next is the ever elusive atomic structure and chemical bonding. The points that we often missed about these topics are the shape of orbital, shape of sigma and pi bond, what is instantaneous dipole or permanent dipole.

Orbital is the region where we can find the electron most of the time. Remember that electron moves randomly at a specific area? Orbital is simply the part of that specific area where we it has the highest probability of having the electron.

For now we only need to know the shape of s orbital and the p orbital.



Picture of s orbital



Picture of p orbital which consists of px, py and pz orbital. Note their orientation.

The difference of 1s orbital with 2s orbital is their size. 1s orbital is smaller than 2s orbital and the same goes for p orbital. It is just about their size.

When we draw the electron in the box diagram, take note of the placement of the arrows and their directions.


Move on to the chemical bonding. Important points here are the shape and how to draw sigma and pi bond. Another important point is the understanding between the difference of temporary dipole-dipole and permanent dipole-dipole.


This is the picture of s orbital overlapping another s orbital forming sigma bond (example H-H)



This is the picture of s orbital overlapping p orbital forming sigma bond (example H-Cl)



This is the picture of p orbital overlapping with another p orbital forming sigma bond (example Cl-Cl)

We can also draw a "sausage" (ellipse) for the px px sigma bond.

Pi bond is formed when two p orbitals overlap sideways to produce regions of electron density above and below the axis joining the two nuclear centers. Pi bond is formed when a species forms double bond or triple bond.



This is an example of py orbital forming a pi bond. Notice the shape of the orbital after the pi bond is formed.

Move on to instantaneous dipole or permanent dipole. This is one of three known intermolecular bonds that we have learned (I have absolutely no idea how many are there).

So basically both of these things are due to the effect of electrostatic attraction of the dipole that is created due to 2 different things:
(-) Random movement of electrons for the temporary/instantaneous dipole.

(-) The effect of polarization caused when an element with higher electronegativity bonded with another element of lower electronegativity for the permanent dipole.

The fact that movement of electron is extremely random will sometimes cause the electron to be positioned in such way:


Here we can notice at the second picture had all of its electron positioned at one side of the atom. Then, a millisecond later it would disperse right away. Then a few milliseconds later it would be positioned as the second picture and so on an on (remember, it is moving at speed of light).

When the electrons are positioned as in the second picture, the ion will be polarized for a while and it will produce a dipole with negative side at the left side of the atom (where the electrons gather). So the left side will be positively charged for a while isn't it?


This is what we called as the temporary dipole-dipole/ instantaneous dipole-dipole/ van der Waals force.

This bond is important only when all other bonds are not present, permanent dipole dipole and hydrogen bond. This is important in alkane and monoatomic element such as noble gases.

Take a look at HCl :



Here, we can see that Cl atom, being much electronegative than H, will polarize H and will cause it to become slightly more positive, whereas Cl will become slightly more negative. Thus when there are two HCl molecules come into contact:


The interactions (the vertical line) is the result of the attraction which is the permanent dipole. Basically, if it is a polar molecule, it will have the permanent dipole bond as the stronger intermolecular force.

Hydrogen bond is a special case. As defined in Wikipedia, hydrogen bond results from a permanent dipole force between hydrogen atom(s) bonded to nitrogen , oxygen or fluorine (thus the name "hydrogen bond", which must not be confused with a covalent bond to hydrogen). It is much stronger than the the permanent dipole force since the polarization here is much stronger (electronegativity of F is greater than Cl).

Consider this. Ethanoic acid (vinegar) is able to form a hydrogen bond where as ether such as methoxymethane (CH3OCH3) can't form any hydrogen bond.

Any question, feel free to ask.
All comments will be deeply regarded.

Thank you for reading this.