# Homework Help: Bigger the molecule the more polarizable

1. Nov 20, 2013

### Qube

1. The problem statement, all variables and given/known data

Which is less polarizable in each of the following groups?

1) H2CCH2 or H3CCH3

2) CH4 or C2H6

3) Na+ or Na

4) Sb3- or S2-

2. Relevant equations

Polarizability is affected by the number of electrons, the distance of the electrons from the nuclear charge, and molecular orientation, although we're not considering this factor. So basically the bigger the molecule the more polarizable.

3. The attempt at a solution

1) I think it's H2CCH2 rather than H3CCH3 because the latter has more electrons attached to the carbons, and the C-H bond is non-polar. Having more electrons will only make the molecule bigger and more polarizable. This, however, is wrong according to the key, and this makes no sense, as the line of reasoning I'm using works with all the other molecules.

Also the former has a double bond between its central carbons and double bonds are shorter than single bonds and that would make the former more compact than the latter.

2) CH4 is less polarizable. CH4 and the other hydrocarbon are both non-polar, but the latter is bigger, so it's easier to be distorted. Correct according to the key.

3) Cations are smaller than their respective anions and also neutral species. So Na+ is less polarizable. Size matters. Correct again.

4) Sb3- and S2- - there's no comparison. Sb is rows below S in the periodic table. S2- is way smaller than Sb3- so therefore S2- is less polarizable. Again, correct. Size matters.

Is the key wrong on the first one? Or am I missing a factor here?

2. Nov 22, 2013

### Staff: Mentor

3. Nov 22, 2013

### Qube

Polarizability refers to the effort needed to distort a molecule's electron cloud.

4. Nov 22, 2013

### Ygggdrasil

How might the physical size of the electron cloud affect a molecule's polarizability?

5. Nov 22, 2013

### Qube

A bigger cloud is more polarizable.

By this logic, of the choices in the first pair:

1) H2CCH2 or H3CCH3

H2CCH2 should be less polarizable than H3CCH3.

6. Nov 22, 2013

### Ygggdrasil

How do you think the polarizability of sigma bonds and pi bonds compare?

7. Nov 22, 2013

### Qube

Sigma bonds are single bonds and sigma bonds are also stronger bonds due to the side-to-side (spooning) overlap. Pi bonds are weaker.

H2CCH2 is composed of sigma and pi bonds while H3CCH3 is composed entirely of sigma bonds. H2CCH2 has a sigma and a pi bond connecting the double-bonded central carbon atoms.

I'm supposing that the sigma/pi bond factor here explains why the two molecules don't follow the trend you'd expect them to looking at size alone, right?

8. Nov 24, 2013

### Ygggdrasil

In general, the polarizability of electrons is not due to the number of electrons in an element, but in how tightly held those electrons are to the nuclei. Large atoms like iodine are polarizable not because they contain many electrons but because the outer electrons are much farther away from the nucleus than the electrons in an element like fluorine, so it's easier for external electric forces to influence the position of those electrons.

In a sigma bond, the electrons are tightly held in the space between the two nuclei involved in a bond. The electrons in a pi bond, however, lie outside of the internuclear axis, are on average farther away from the nuclei, are less tightly held by the nuclei, and are therefore more polarizable.