**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 top to bottom in
group, number of shells increases but mass is more and more increases with
respect to volume, so finally the atomic density in a 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 '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.

_{11}Na= 1s^{2} 2s^{2} 2p^{6} 3s^{1}
(Completely filled Inner shell s)

_{19}K= 1s^{2} 2s^{2} 2p^{6} 3s^{1}
3p^{6} 3d^{0}, 4s^{1} (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.

_{12}Mg= 1s^{2} 2s^{2} 2p^{6} 3s^{2}(Completely
filled Inner shell s)

_{20}Ca= 1s^{2} 2s^{2} 2p^{6} 3s^{1}
3p^{6} 3d^{0}, 4s^{2}** **(incomplete inner
shells)

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