唉 問幾條化學

do you mean "down the group"?
1.
density of group I metal generally increases down the group. as atomic amss of elements increases with little changes in volume of unit cells, density of metal increases. it's easy to understand.

the melting point depends on strength of attraction between metals' nuclei and valence electrons (sea of electrons in this case). no. of valency of all group I metals is ONE. however, as no. of electron shells increases, the distance between nuclei and sea of electron increases. the attraction follows inverse cubic rule, that attraction falls more rapidly than the strengthening due to increased no. of protons.


2.
halogens are simple molecular diatomic molecules. they have same packing pattern. density increases down the group as atomic mass increases.

melting point of halogens depends on van der waals' force, the intermolecular attraction. it increases as molecular size increases.

difference between alkali metals and halogens is the difference in bonding. notice their types -- metals VS discrete molecules.

3&4.
salt is commonly referred to "the salty white substances obtained by evaporating sea water". from these substances halogens are found. therefore halogens are referred as "salt producers".

salt is defined as "product of neutralization reaction between acid and base apart from water". all halogens can form (hydrohalic) acids, like hydrochloric acid, hydrobromic acid and hydroiodic acid. conversely, all alkali metals can form metal hydroxides, soluble bases.

when acids and bases react, salts are produced.
even halogens are not converted into acids, and alkali metals not converted into bases, the reaction products between them are still the same (water absent).

hydrochloric acid + sodium hydroxide ------> sodium chloride + water
chlorine + sodium ------> sodium chloride

hydrobromic acid + potassium hydroxide ------> potassium bromide + water
bromine + potassium ------> potassium bromide

as simple as that.