how we can know that a particular element is from which particular group or period?
A group is determined by the number of electrons in the outermost shell.
Sodium with atomic number 11 has electronic configuration 2, 8, 1.
It has one electron in the outermost shell hence it is placed in Group 1.
Similarly, magnesium with atomic number 12, electronic configuration 2, 8, 2 is placed in group2.
The number of shells present in an atom determines its period.
Elements of period one like hydrogen and helium have one shell.
Elements of period 3 like sodium and magnesium have three shell.
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