
Amarjeet K. answered 11/28/14
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Metallic properties increases down the group as the size of an atom increases which leads to lesser attraction between the nuclei and the electrons thus the outer most electrons of a metallic atom is loosely bound which makes metals a good conductor of heat and electricity. As we move across the period we notice that the size of the atom decreases which means the attraction between the nuclei and the electrons increases so the metallic character decreases.
As we move down the group the number of shells increases. This causes the effective nuclear charge to decrease due to the outer shells being further away: in effect the atomic size increases. The electrons of the outermost shell experience less nuclear attraction and so can lose electrons easily thus showing increased metallic character.