Periodic Table


·        The physical and chemical properties of elements are the periodic function of their atomic numbers.

Period number-

  • Periods are the horizontal rows of elements in a periodic table. 
  • There are seven periods in the periodic table. 
  • The period number is related to the number of electron occupied shells in the element.
  • For example, If an element having electronic configuration 2,5.This shows that the number of electron occupied shells in the element is 2. Hence this element belongs to 2nd period.

Number of Elements in a Period

Period Number

Number of Elements
















Group number-

  • There are 18 vertical columns in the periodic table. Each column is called a group.
  • All elements in a group have similar chemical and physical properties because they have the same number of outer electrons.
  • If the element is in s block, then the group number is equal to the number of valence electrons. 
  • 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).

Anomalies of Modern Periodic Table

·      Isotopes are placed at one place in the same group.

·      There is no element between hydrogen and helium as atomic masses always comes in whole numbers.

·      Atomic number is represented by Z, and it is equal to the number of protons in the nucleus of the atom.

·      It also consists of 18 vertical columns known as Groups and 7 horizontal rows known as Periods.

·      Elements having same number of valence electrons are placed in the same group.

·      As we go down in a group, number of shell increases.

·      Elements having same number of occupied shells are placed in same period.

·      Each period has a new electronic shell getting filled.

·      Number of elements placed in a particular period depends on the point how electrons are filled into various shell.

·      To find out the number of electrons in a shell, 2n2 formula can be used, where is n is shell number.

K Shell n = 1 or 2n2 = 2(1)2 = 2

L shell n = 2 or 2n2 = 2(2)2 = 8

·      Valence electron also determines the number of bonds which is formed by an element.