Group I and Group II?

Question

Why the elements in Group I and II are always soft and less denser while others are hard and high denser? What structure cost this? They all are both held by metallic bond.Is that because of less electrons in electron sea, so the attractive between the sea and the cation become smaller?

Don't tell me Group I is less dense because they got less outermost shell electrons only.... I want to know WHY!

Answer 1

Two things involved here.

Other things being equal, atomic size decrease is from left to right across any row because of increased nuclear charge.

The electrons are, as you say, shared in a sea. This is where the bonding comes from, and for Group II you have twice as many electrons involved.

[Advanced aside: for simplicity, I have neglected mutual screening by electrons. I have assumed good overlap between s- and p-derived bands]

Answer 2

In other words, as you go down the Group, the elements become less ... and that the first three in the Group are less dense than water (less than 1 g cm-3). ...