The Table that defines chemistry turns 150

Headline : The Table that defines chemistry turns 150

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The News

  • 2019 is celebrated as ‘The International Year of the Periodic Table of Chemical Elements’ as the Period Table of Elements has turned 150 years in 2019.

Periodic Table of Elements: A backgrounder

  • The ‘Periodic Table of Elements’ was written by Dmitri Ivanovich Mendeleev on in 1869.
  • While there were other efforts to classify elements, Mendeleev was unique in that he was the first one to propound a ‘law of periodicity of elements’.
  • According to Mendeleev’s law of periodicity, ‘elements, if arranged according to their atomic weights, exhibit an evident periodicity of properties’.
  • Currently, the modern periodic table has 118 elements, compared to 63 in Mendeleev’s version.
  • Out of 118 elements, 100 are naturally occurring and 18 and artificially produced.
  • While Mendeleev’s law put emphasis on ‘atomic weight’ to classify elements, modern periodic law classifies elements based on periodicity of their properties in accordance with their ‘atomic number’.
  • In recognition of his achievement, Mendeleev has found a place in the periodic table in the form of an element 101 named as Mendelevium (Md).

Basics of Periodic Table

  • Each chemical element is made of a specific type of atom.
  • Each specific atom has a characteristic number of protons in its nucleus which defines the atomic number for that particular element.
  • Elements in the periodic table are arranged in the increasing order of their atomic number.

Position of Elements in the Periodic Table (See figure below)


  • The elements in the periodic table are classified into ‘groups’.
  • ‘Groups’ are the vertical columns in the periodic table.
  • There 18 such ‘groups’ in the periodic table.
  • Elements in the same group have similar chemical properties.


Alkali Metals (Group 1)

  • Alkali Metals are elements found in ‘group 1’ except hydrogen.
  • Hydrogen is an exception in group 1 and is a non-metal.
  • Alkali Metals are soft, silvery metals that are very reactive.
  • Therefore, they not found in pure form in nature and often found in combination with other elements.
  • All alkali metals react with water.

Alkaline Earth Metals (Group 2)

  • Alkaline Earth Metals are found in group 2.
  • Alkaline earth metals are also reactive and thus not found in pure form in nature.
  • However they are not as reactive as alkali metals in group 1.

Metals (Groups 3 to 12)

  • Elements in groups 3 to 12 are classified as metals in general.
  • Metals are solids at room temperature except for mercury (Hg) which is a liquid at room temperature.
  • Metals are very malleable which means they are not brittle and thus can be worked into different shapes.
  • Metals are also ductile which means they can be formed into wires.
  • Further metals are good conductors of heat and electricity.


  • Non-metals in general are found in different states of matter in nature.
  • Solid non-metals are brittle unlike metals.
  • Non-metals are poor conductors of heat and electricity.

Halogens (Group 17)

  • One type of non-metals are halogens found in group 17 in the periodic table.
  • Halogens are very reactive non-metals and ‘salt formers’ and hence the name.
  • They are usually very corrosive.

Nobel Gases (Group 18)

  • Nobel gases are found in group 18 of the periodic table.
  • They are colorless gases and generally very unreactive and thus very stable in nature.


  • The zig-zag line shown in the figure divides the metals from non-metals in the periodic table.
  • However some of the elements on the zig-zag line exhibit properties in between metals and non-metals and are called metalloids.
  • Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium are some of the famous metalloids.
  • Silicon is the most famous metalloid because of its semiconducting properties.


  • A ‘period’ is a horizontal row in the periodic table.
  • There are 7 such ‘periods’ in the periodic table.
  • The elements in the periods are arranged based on their electron configuration.
  • Elements of the same period have the same number of electron shells.
  • Elements in Period 1 has its electrons arranged in one shell, those in Period 2 in 2 electron shells, 3 shells in period 3 and so on and so forth.

Note to students: This comes under basic science part of the syllabus. a detailed note is provided considering the significance of the year.Section : Science & Tech