Briefly explain the trends in boiling point and melting point in the alkane series.

(N/A) $(i)$ In alkanes,weak van der Waals forces exist; therefore,at normal temperature,alkanes up to $C_{4}$ are gases,and $C_{5}$ to $C_{13}$ are liquids.
$(ii)$ As the molecular weight of alkanes increases,the van der Waals forces increase,which leads to an increase in both boiling and melting points.
$(iii)$ In branched isomers of alkanes,the molecular shape becomes more spherical,which reduces the surface area available for van der Waals interactions,resulting in a lower boiling point.
$(iv)$ In isomers of alkanes,branching leads to a more spherical shape,which allows for better packing in the crystal lattice,thereby increasing the melting point.