(a) 1.Low melting point
2.Low electric conductivity
3.Low density
(b) 1.According to periodicity,within a same group (IA in the case of alkali metals),atomic radius increases from top to bottom because more valance levels are filled.
2.According to periodicity,within a same group (IA in the case of alkali metals),ionization energy decreases from top to bottom because the single valence electron is less tightly bond to the nucleus.
3.In alkali metal group,melting point decreases from top to bottom becasue the stabilty of metal bond,which is formed by s1 valence electrons,decreases when valence level increases.
4.Because lithium ion has the smallest radius,and thereby the strongest ability to polarize water molecule to give the largest hydration energy.
(C)atomic radius:alkali metals>alkaline earth metals
ioniztion energy:alkali metals