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What is atomic density ? give the periodicity of atomic density in periods and groups.

ATOMIC DENSITY:
Density is the ratio of mass and volume. Higher the atomic volume leads to lower density as vice versa lower the atomic volume leads to higher density .
(A) Trends in group:
When we move top to bottom in a group the atomic density generally increases , it is due to increasing in atomic volume and atomic mass because too to bottom in group , number of shells increases but mass is more and more increases with respect to volume, so finally the atomic density ina group increases.
(B) Trends in period: 
When we move from left to right along with a period , the atomic density increases and become maximum at mid period , and after that decrease.
(C) Atomic density depends upon two factors these are given as.
(i) the electronic configuration of inner most shells
(ii) packing capacity 
EXCEPTION:
(1) Density of Na > Density of K
Reason: 
The density of 'Na' is greater than the density of of 'K' .It is due to the inner shells of 'Na' atom is fully filled but the shells the inner most shells of 'K' not full filled.
11Na= 1s2 2s2 2p6 3s1 (Completely filled Inner shell s)
19K= 1s2 2s2 2p6 3s1 3p6 3d0, 4s1 (incomplete inner shells)
(2) Density of Mg > Density of Ca 
Reason:
The density of 'Mg' is greater than the density of of 'Ca' .It is due to the inner shells of 'Mg' atom is fully filled but the shells the inner most shells of 'Ca' not full filled.
12Mg= 1s2 2s2 2p6 3s2(Completely filled Inner shell s)
20Ca= 1s2 2s2 2p6 3s1 3p6 3d0, 4s2 (incomplete inner shells)

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