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Periodic Classification of Elements

  • In periodic table Elements are classified on the basis of similarities in their properties.
  • Döbereiner grouped the elements into triads and Newlands gave the Law of Octaves.
  • Mendeléev arranged the elements in increasing order of their atomic masses and according to their chemical properties.
  • Mendeléev even predicted the existence of some yet to be discovered elements on the basis of gaps in his Periodic Table.
  • Anomalies in arrangement of elements based on increasing atomic mass could be removed when the elements were arranged in order of increasing atomic number, a fundamental property of the element discovered by Moseley.
  • Elements in the Modern Periodic Table are arranged in 18 vertical columns called groups and 7 horizontal rows called periods.
  • Elements thus arranged show periodicity of properties including atomic size, valency or combining capacity and metallic and non-metallic character.
Periodic Law: ‘Properties of elements are a periodic function of their atomic number.’

Atomic number of an element gives us the number of protons in the nucleus of an atom and this number increases by one in going from one element to the next.
Elements, when arranged in order of increasing atomic number Z, lead us to the classification known as the Modern Periodic Table.

Position of Elements in the Modern Periodic Table:
  • The Modern Periodic Table has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’.
  • All elements of a group contain same number of valence electrons, and hence similar chemical characteristics.
  • On moving left to right in periodic table atomic radius decreases, this owes to increase in nuclear charge which tends to pull the electrons closer to the nucleus and reduces the size of the atom.
Metallic & Non-metallic Properties:

Metals like Na and Mg are place towards the left-hand side of the Periodic Table while the non-metals like sulphur and chlorine are found on the right-hand side.
In the middle is silicon, which is classified as a semi-metal or metalloid as it shows characteristics of both metals and non-metals.

In the Modern Periodic Table, a zig-zag line separates metals from non-metals.
The borderline elements – boron, silicon, germanium, arsenic, antimony, tellurium and polonium – are intermediate in properties and are called metalloids or semi-metals. Metals tend to lose electrons while forming bonds, that is; metals are electropositive in nature.


Atomic Radius - The atomic radius of an element is half of the distance between the centers of two atoms of that element that are just touching each other. Generally, the atomic radius decreases across a period from left to right and increases down a given group. The atoms with the largest atomic radii are located in Group I and at the bottom of groups.

Ionization Energy - The ionization energy, or ionization potential, is the energy required to completely remove an electron from a gaseous atom or ion. The closer and more tightly bound an electron is to the nucleus, the more difficult it will be to remove, and the higher its ionization energy will be.

Group I elements in the periodic table have low ionization energies because the loss of an electron forms a stable octet.


Electron Affinity- Electron affinity is the ability of an atom to accept an electron. It is the energy change that occurs when an electron is added to a gaseous atom. Atoms with stronger effective nuclear charge have greater electron affinity.

Group VIII elements of periodic table also called as noble gases, have electron affinities near zero, since each atom possesses a stable octet and will not accept an electron readily. Elements of other groups have low electron affinities.

Electronegativity- Electronegativity is a measure of the attraction of an atom for the electrons in a chemical bond. The higher the electronegativity of an atom, the greater its attraction for bonding electrons.

Electronegativity is related to ionization energy. Electrons with low ionization energies have low electronegativities because their nuclei do not exert a strong attractive force on electrons.

Elements with high ionization energies have high electronegativities due to the strong pull exerted on electrons by the nucleus.

In a group in periodic table, the electronegativity decreases as atomic number increases, as a result of increased distance between the valence electron and nucleus (greater atomic radius).

Important Periodic Table Trends

Moving Left → Right
  • Atomic Radius Decreases
  • Ionization Energy Increases
  • Electronegativity Increases

Moving Top → Bottom
  • Atomic Radius Increases
  • Ionization Energy Decreases
  • Electronegativity Decreases