Question

How can we find in which period and group in periodic table a element exist. Explain whole method thoroughly?

Answer 1

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)
  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: $\mathrm{Fe}\left(26\right):\left[\mathrm{Ar}\right]3{\mathrm{d}}^{6}4{\mathrm{s}}^2$
  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 3rd group