Question
How can a period, group and block of the element be determined?
✓
Answer
The group, period and the block of the element can be determined on the basis of its electronic configuration.
$i.$ Period: The principal quantum number of the valence shell corresponds to the period of the element.
e. g. The principal quantum number $(n)$ of the valence shell $(3s^1$) of $Na (1s^2 2s^2 2p^6 3s^1)$ is $3.$ This corresponds to third period.
$ii.$ Block: The subshell in which the last electron enters, corresponds to the block of the elements $($with exception being He$)$.
e. g. The subshell $3d ($in which the last electron enters) for $Sc (1s^2 2s^2 2p^6 3s^2 3p^6 3d^1 4s^2)$ corresponds to d block.
$iii.$ Group: The group of the element is determined on the basis of number of electrons present in the outermost or penultimate $[$next to outermost, i.e. $(n-1)]$ shell:
For $s-$block elements, group number $=$ number of valence electrons.
For $p-$block elements, group number $= 18 –$ number of electrons required to complete octet.
For $d-$block elements, group number $= 2 +$ number of $(n-1)d$ electrons.
$i.$ Period: The principal quantum number of the valence shell corresponds to the period of the element.
e. g. The principal quantum number $(n)$ of the valence shell $(3s^1$) of $Na (1s^2 2s^2 2p^6 3s^1)$ is $3.$ This corresponds to third period.
$ii.$ Block: The subshell in which the last electron enters, corresponds to the block of the elements $($with exception being He$)$.
e. g. The subshell $3d ($in which the last electron enters) for $Sc (1s^2 2s^2 2p^6 3s^2 3p^6 3d^1 4s^2)$ corresponds to d block.
$iii.$ Group: The group of the element is determined on the basis of number of electrons present in the outermost or penultimate $[$next to outermost, i.e. $(n-1)]$ shell:
For $s-$block elements, group number $=$ number of valence electrons.
For $p-$block elements, group number $= 18 –$ number of electrons required to complete octet.
For $d-$block elements, group number $= 2 +$ number of $(n-1)d$ electrons.
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