For which of the following species are the dispersion forces strongest?
a) c4h10 b) c5h12 c) c6h14 d) c7h16 e) c8h18
i have completely no idea how to do this : /

In order to see which species has the strongest dispersion forces, you need to calculate their molar mass, because the higher the molar mass, the stronger the dispersion forces. 
Since E. C8H18 has the highest molar mass, its dispersion forces are also the strongest ones.

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.

Ions gain or lose electrons