Question

Which of the following would be a polar molecule? Why?

a) `O_2`
b) `HCl`
c) `CH_4`
d) `F_2`

Answer 1

HCl - see below

Explanation:

Determining molecular polarity requires 2 things: Bond Polarity and Geometry. Once you have those, you have to figure out if you have a net dipole moment. I think of this kind of like vector addition.

`O_2`
Both atoms of oxygen are strongly electronegative. This is like one really strong person pulling a rope a clone of themselves. They are both super strong and pull super hard, and the rope doesn't move. If nothing moves, its nonpolar.

`HCl`
H has an electronegativity of 2.1, whereas Cl has an electronegativity of 3.0. The difference is 0.9. This is like a strong adult playing a tug-of-war with a school kid. The school kid can still hold the rope, but the adult will pull the kid across the floor. The entire system will move, and so this is Polar.

`CH_4`
The bonds in C-H are nonpolar to begin with (2.5 and 2.1 electronegativity, respectively). Also, the bond angles are all 109.5 degrees, so even if there was a pull, it would be symmetric and cancel out.

`F_2`
Same reasoning as above for `O_2`