Periodic Table and Periodicity

Periodic Table and Periodicity

There are 7 periods (Horizontal rows) and 18 groups (vertical columns)


Periodic Table and Periodicity History

  1. Doebereiner discovered group of three elements having similar properties and called them triads. The atomic mass of the central element was the arithmetic mean of the atomic mass of other two elements e.g., Li, Na, K

  2. Newlands law of Octaves: He arranged elements in their increasing atomic weights of the elements and properties of first element was found to be similar to that of eighth element.

  3. Lother Meyer plotted graph between atomic volumes and atomic weight of the elements. Elements with similar properties had the similar position on the curve.

  4. Mendeleev stated that "Physical and chemical properties of elements are periodic function of their atomic weight". Only 57 elements were known at that time.

  5. Modern periodic Law: Moseley stated that physical and chemical properties of elements are periodic function of their atomic number.

  6. Bohr given long form of periodic table.

    Long Form of Periodic Table
    There are 7 periods (Horizontal rows) and 18 groups (vertical columns)

    1st period : Very short period (only two elements H1and He42)
    2nd period : Short period (8 elements 3Li - 10Ne)
    3rd period : Short period (8 elements 11Na - 18Ar)
    4th period : Long period (18 elements 19K - 36Kr)
    5th period : Long period (18 elements 37Rb - 54Xe)
    6th period : Longest period (32 elements 55Cs - 86Rn)
    7th period : Incomplete period (26 elements 87Fr - 112Uub)

In the long form of periodic table elements can also be classified as:

  1. s-Block Elements: Elements of group 1 and 2 having general electronic configuration ns1-2
    ns- Alkali metal (Li to Cs)
    Fr is radioactive
    ns2 - Alkaline earth metal (Be to Ba)

  2. p-Block Elements: Elements of group 13, 14, 15, 16, 17 and 18 having electronic configuration ns2  np1-6. Elements of group 18 are also called as inert gases.

  3. d-Block Elements: Also called as transition element having electronic configuration (n-1)d1-10  ns1-2 (except 46Pd)

  4. f-Block Elements: Also called as inner transition element, having electronic configuration (n-2) f1-14  (n-1) d0-1  ns2. It includes 4f or lanthanoid (Ce to Lu) 5f or actinoid (Th to Lr)

The elements are broadly divided into three types

  1. Metals comprise more than 78% of the known elements.
  2. Non-metals are less than twenty (C, N, P, O, S, Se, H, F, Cl, Br, l, He, Ne, Ar, Kr, Ar, Kr, Xe and Rn)
  3. Elements which lie in the border line between metals and non metals are called metalloids. (B, Si, Ge, As, Sb, Te, Po, At)

Periodicity is observed in a number of properties which are directly or indirectly or linked with electronic configuration.

Effective nuclear charge increase across each period and remain almost constant down the group.

Zeff = Z - 

where 'Z' is nuclear charge
 is shielding effect



Atomic radii (is generally taken as covalent radii)

n = Principal quantum number of outer orbit

It generally decreasing across a period and increases down the group.

It is of three types

  1. Covalent radius
  2. Metallic radius
  3. van der Waal radius

Order: van der waal's radius > Metallic radius > Covalent radius

Ionic Radii



Cationic radii is smaller than anionic radii for some atom.

Cationic radii < Atomic < anionic radii

Elements of 2nd and 3rd transition series belonging to same group are similar in size and properties due to lanthanide contraction.

Ionisation Enthalpy is the energy required to remove the most loosely held electron from the gaseous isolated atom





Ionisation enthalpy generally decreases down the group.

Successive I.E of an atom have higher values.

IE<  IE<  IE.......

Electron gain enthalpy is the enthalpy change taking place when an electron is added to an isolated gaseous atom of the element.

The Ist electron gain enthalpy of most of the elements is negative as the process is exothermic.

Electron gain enthalpy becomes more negative from left to right in a period and less negative from the top to bottom in a group.

Successive electron gain enthalpies are always positive.

Electro negativity is the tendency of an atom to attract the shared pair of electron towards itself in a molecule.

  1. Electro negativity increases across the period and decreases down the groups.

  2. Metals have low electro negativity and non-metals have high electro negativities.

Application of Electro negativity

  1. Percentage ionicity = 16 (XA ~ XB) + 3.5 (XA ~ XB)2

  2. For oxides Xo ~ X element higher the difference more basic the oxide


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