Classification of elements

Evolution of periodic table

  • After the discovery of large numbers of elements and their compounds, the study of these elements and compounds individually became difficult so it was felt that these elements should be classified into a few groups to make their study more systematic and easier
  • The first attempt in this direction was given by Dmitri Ivanovich Mendeleev who arranged the elements in the increasing order of their atomic masses

Mendeleev’s periodic table-1869

  • The periodic table is based upon Mendeleev’s periodic law which states that ”the physical and chemical properties of the elements are the periodic function of their atomic masses”
  • It contains 7 periods and 8 groups but 0groups(inert gases) is absent in this table
  • It contains gaps for the elements not known at that period of time e.g: eka-boron, Eka-aluminum, and eka-silicon the properties which had been found were found to be similar to the scandium, gallium, and germanium respectively
  • It has no fixed position for hydrogen, no space for isotopes, and not a regular arrangement of atomic masses

Modern periodic law:

  • Mosely(1913) modified Mendeleev’s periodic law proposed modern periodic law as “the physical and chemical properties of the elements are the periodic function of their atomic numbers”
  • He discovered that atomic number has more fundamental property than atomic mass

Long form of periodic table:

  • The long form of the periodic table or Bohr’s table is based on Bohr-burry concept of the electronic configuration
  • It contains 7 periods(horizontal rows) and 18 groups(vertical rows)
  • The number of periods denotes the number of outermost shells of the elements
  • The first element of each period(Except period I ) is an alkali earth metal and the last is an inert earth gas


I PERIODVery short period2 elements
II AND III PERIODshorts periods8 elements
IV AND V PERIODlong periods18 elements
VI PERIODVery long periods32 elements
VII PERIODincompleteVery long period
  • Lanthanoids and actinoids belonging to VI and VII periods are placed, in two separate rows at the bottom of the periodic table. all the members of a particular group have similar outer shell electronic configuration
  • The elements of groups 1,2,13,14,15,16,17 and 18 are known as normal or representative elements
  • The elements of group 3,4,5,6,7,8,9,10 and 11 are known as transition elements
  • It is just a graphical representation of the Aufbau is based on the electronic configuration of the elements and contains 118 elements
  • At a difference of 2,8,8,18,18,32 electrons, the elements have the same properties, thus elements with atomic number 3,11,19,37,55 and 87 belong to the same group

Charcterisstcs of periods:

  • The number of the valence electron in elements increases from 1 to 8 on moving from left to right in a period
  • The elements in a period have a consecutive atomic number
  • The valency of the elements increases from 1 to 4 and then it decreases to 0 on moving from left to right in a period concerning hydrogen

Characteristics of groups:

  1. All the elements of a group of the periodic table have the same number of valence electrons and hence, have almost similar chemical properties

Block elements:

The elements can be classified into 4 blocks

  1. s-block elements
  2. p-block elemets
  3. d-block elements
  4. f-block elements
  • s-block elements
  • it contains groups 1 and 2 i.e hydrogen and alkali metal(Li,Na,K,Rb,Cs,Fr) and alkali earth metals (Be,Mg,Ca,Sr,Ba,Ra) with general configuration ns^1-2.
  • These elements are soft metals, electropositive, and form basics oxides
  • p-block elements
  • it comprises the last six groups i.e 13 to 18 groups with general configuration (n-1) d^1-10 ns^1-6
  • it is the only block that contains metals,non-metals, and metalloids
  • heavier elements of this block show inert pair i.e, their lower valency becomes more stable
  • d-block elements
  • it comprises groups 3to12 with general configuration (n-1)d^1-10 ns1-2
  • these elements are called transition elements
  • elements of this block contain unpaired electrons and are paramagnetic
  • these elements show variable valency due to less difference in the energy of the outer and penultimate shell
  • these are generally colored and used as a catalyst
  • f-block elements
  • it usually offset below the rest of the periodic table two rows of 14 elements called the lanthanides and actinides respectively with general configuration(n-2)f^1-14(n-1)d^0-1 ns^2
  • elements of this block are called inner transition elements  and are present in IIIB(3) group only


ionisation enthalpy

it is the minimum energy required to remove an electron from an isolated gaseous atom of an element to form a positive ion

it increases along a period but boron and oxygen have smaller ionization energy than beryllium and nitrogen respectively due to stable electronic configuration

ionisation enthalpy decreases down the group

Electron gain enthalpy

It is the energy released by an element when an extra electron is added to its neutral gaseous atom

It increases along with period and decreases down the group with some exception


It is the ability of an atom to attract the shared pair of electron towards it

It increases along a period and decreases down the group

Metallic character

It is the tendancy of an element to form cation by the loss of electrons

It decreases along a period and increases down the group

Oxidising and reducing character

Reducing character decreases along the period and increases down the group

Oxidising character increases along the period and decreases down the group


Valency with respect to hydrogen increases from 1-7 but with respect to oxygen first increases from 1-4 and then decreases to one

Valency remains same in a group

Basic nature of oxides

It decreases along a period while acidic nature decreases down the group

Basic nature of oxides increases while acidic nature decreases

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