The bonds between the electrons
There is no "generally". For alkali metals (group 1) and alkali earths (group 2) they decrease. For groups 4 to 10 they rise, for group 11 it goes down from copper to silver and then up to gold.
Generally melting point and boiling point increase up to group 6 and then decrease.
The melting point decreases down the group for alkali metals because the metallic bonding weakens as the atomic size increases, resulting in weaker intermolecular forces. For halogens, the melting point increases down the group due to stronger van der Waals forces between larger molecules with increased electron cloud size, resulting in higher melting points.
The melting point of magnesium is lower than calcium because although melting points generally decrease down a group, there can be exceptions due to variations in atomic size, packing efficiency, and bonding forces. In this case, the smaller size and stronger metallic bonding in calcium result in a higher melting point compared to magnesium, despite being further down the group.
The melting point of magnesium is 650 degrees celcius. It has the lowest melting point among all the group II metals (alkaline earth metals), though the melting points generally decrease down the group - magnesium is an exception and has the lowest melting point among them.
In Group 1 (alkali metals), the melting and boiling points decrease as you move down the group due to the increase in atomic size and metallic bonding. In Group 7 (halogens), the melting and boiling points increase as you move down the group due to the increase in atomic size and London dispersion forces.
The metallic bonds of the alkali metals are are weak.
Group Two elements form oxide consisting of ionic bonds. With that in mind, as you go down group two, the ionic radius increases due to an addtion of a electron shell, so therefore, melting point of group2 oxide decreases down group because the distance between the O2- and Group two ion are further apart. The increase of bond length results in less energy needed to break the ionic bonds, resulting in decrease of energy.
Alkali metals have relatively low melting points compared to the other metals. Their exact melting points depend on the element itself. But it would be useful to note that their melting points decreases down group I.
From BCIT.ca: The alkali metals show a decrease in melting points and boiling points due to the weaker metallic bonds between atoms as their size increase down the group.From AUS-e-TUTE: melting point decreases down the Group as the elements become less metallic in natureFrom okstate.edu: As one goes down the Periodic Table, the metallic radius of alkali metals increases. The increase in radius results in much decreased attractive forces between atoms within the liquid metals, resulting in a decrease in heat of vaporization and boiling points [and melting points] because less heat is needed to separate atoms from the liquid [or solid] state within the larger alkali metals. See Web Links to the left for the original sources.
Melting points generally decrease as you go down a group for group I and group II metals. This does not apply to the transition metals. Reactivity of metals increases down a group due to a larger size and less effective charge between the nucleus and valence electrons. Atomic radius increases due to a higher principle number of electrons.
Alkali metals get softer down Group 1 due to an increase in atomic size and weaker metallic bonding. As you move down the group, the atomic radius increases, leading to a decrease in the strength of metallic bonding and making the metals softer.