Question

How do you identify the group and period number of an element with example

Answer 1

The period is equal to the number of shells present in an atom.

The group number is equal to the number of valence electrons.

What is the period in the periodic table?

A period in the periodic table is a row of chemical elements.

The horizontal rows (which Mendeleev called series) are called periods and the vertical columns, are groups.

Elements having similar outer electronic configurations in their atoms are arranged in vertical columns.

The period indicates the value of n for the outermost or valence shell. The period is equal to the number of shells present in an atom. To find a period, the no. of shells =no. of periods.

Example: Na- 2,8,1 - Three shells are there, so period no. is 3.

Calcium - 2,8,8,2 - Four shells are there, so period no. is 4.

To determine the group, we need to understand some rules:

1. If the element is in s block, then the group number is equal to the number of valence electrons.

Example: Mg(12) - 2,8,2

Group = no. of valence electrons = 2

2. If the element is in the p block, then the number of the group can be determined by the formula: (number of valence electrons + 10).for groups.

Example: S(16) - 2,8,6

Group = no. of valence electrons+ 10 = 6+ 10 = 16

3. If the element is in the d block, then the number of the group can be determined by the formula: number of electrons in(n−1)d subshell + (number of electrons in (n)s subshell).

Example: Fe(26):

$[Ar]3d^64s^2$

Group = number of electrons in(n−1)d subshell + (number of electrons in (n)s subshell) = 6+2 = 8

4. If the element is in the f block, then the number of the group is always 3.  Example: Cerium (58) belongs to the 3rd group.

Learn more about the period in the periodic table:

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