[charco]

FACT: the stronger the interparticular forces the higher the melting point as more energy is needed to separate the particles.

In metals the smallest particles are ions and they are held together by delocalised valence electrons (from the outer shell).

1. The greater the charge density of the positive ion (number of positives/volume) the greater the power of attraction of the ion.
2. The larger the number of valence electrons the more electron 'glue' there is to be attracted to the ions.

lithium has a small volume and only one valence elctron delocalised into the structure
beryllium has a smaller volume and has two valence electrons (per atom) delocalised into the structure
Consequently beryllium has a stronger structure than lithium

lithium has a small volume and only one valence elctron delocalised into the structure
sodium has a larger volume and only one valence elctron delocalised into the structure

The sodium ions have a lower charge density i.e. they are not so attracting as lithium
Consequently lithium has a stronger structure than sodium and a higher melting point

Hope this helps