Question

why does the melting point of metals increase across peri
3, from sodium (na) to aluminium (al)?
increased number of
valence electrons
contributed to the \sea\
attraction force between
nuclei and free electron
increases
increased space
between the valence
electrons in the \sea\
size of atoms gets
smaller

Explanation:

Metals have metallic bonding where valence electrons form a "sea". From Na to Al, the number of valence electrons contributed to this "sea" increases (Na has 1, Mg has 2, Al has 3). More valence electrons mean stronger metallic bonding. Stronger metallic bonding requires more energy to break, so melting point increases. Also, as we move from Na to Al, the nuclear charge increases (proton number increases), and the atomic radius (size of atoms) decreases (due to increased nuclear pull on electrons). A smaller atomic radius and more valence electrons lead to a stronger attraction force between nuclei and free electrons (metallic bond strength increases).

Answer:

Increased number of valence electrons contributed to the "sea" and Attraction force between nuclei and free electron increases.