CBSE Class 10 Chemistry Revision Notes for Periodic Classification of Elements

Find out how to create a balanced chemical equation and learn about chemical reactions with CBSE Class 10 Chemistry learning resources. On TopperLearning, our experts support you to understand chemistry with CBSE Class 10 Chemistry notes such as CBSE Class 10 Chemistry notes, MCQs and NCERT solutions as per the latest syllabus.

Discover the nomenclature of organic compounds, properties of hydrocarbons and a lot more through our CBSE Class 10 Chemistry Carbon and its compounds worksheets. Learn to identify elements based on their properties using our CBSE Class 10 Chemistry Periodic Classification of Elements notes.

For practicing questions and answers, you can check out our CBSE Class 10 Chemistry Chapter-1 solutions for NCERT textbook questions. Similarly, you can move ahead by practicing CBSE Class 10 Chemistry Chapter-2 questions or reviewing CBSE Class 10 Chemistry Chapter-2 notes.

In the CBSE Class 10 Science Syllabus 2019-2020, you will find 25 marks allocated to Chemical Substances – Nature and Behaviour in Unit-I. You can easily score those marks by practicing questions from our CBSE Class 10 Chemistry sample papers with answers.

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Periodic Classification of Elements

Early Attempts of Classification of Elements

Dobereiner’s Triads

Law of Triads: When elements are arranged in the order of their increasing atomic masses, the atomic mass of the middle element was approximately the mean of the atomic masses of the other two elements.

For example:
Consider the triad of lithium, sodium and potassium. The atomic mass of sodium is the mean of the atomic masses of lithium and potassium.

Element  Atomic Mass
Lithium 6.9
Sodium Atomic mass of Na begin mathsize 11px style equals fraction numerator 6.9 plus 39 over denominator 2 end fraction equals 23 end style 
Potassium 39


Newlands’ Law of Octaves

  • Law of Octaves: When elements are arranged in the increasing order of their atomic masses, the properties of every eighth element is similar to the first.
  • Limitations
  • Newland could arrange elements only up to calcium, out of the total 56 elements known.
  • After calcium, every eighth element did not possess properties similar to that of the first.
  • Only 56 elements were known at the time of Newland, but later several new elements were discovered.
  • In order to fit the existing element arrangement, Newland placed two elements in the same position which differed in their properties.
  • For example: Iron, an element which resembles cobalt and nickel in its properties is placed far away from these elements.
  • The periodic table did not include inert gases because they were not discovered then.

Mendeleev’s Periodic Table

  • Mendeleev’s Periodic Law: The physical and chemical properties of elements are a periodic function of their atomic masses.

Features of Mendeleev’s Periodic Table

  • There are seven horizontal rows in the periodic table, numbered from 1 to 7. These seven rows are called periods.
  • There are eight vertical columns numbered from I to VIII. These eight columns are called groups. Groups I to VII are further divided into sub groups A and B.
  • The properties of elements in a particular period show regular gradation from left to right.

Merits of Mendeleev’s Periodic Table

  • Mendeleev kept some blank spaces in the periodic table for the elements which were yet to be discovered.

Predicted element  Actual element discovered later
Eka-boron Scandium
Eka-aluminium Gallium
Eka-silicon Germanium


  • He also predicted properties of some elements even before their discovery which were later found to be correct.
Property Eka-aluminium Gallium
Atomic mass  68 69.7
Formula of oxide E2O3 Ga2O3
Formula of chloride ECl3 GaCl3


  • Mendeleev’s periodic table could accommodate noble gases when they were discovered.

Demerits of Mendeleev’s Periodic Table

  • Hydrogen resembles alkali metals as well as halogens. So, a correct position could not be assigned to hydrogen in the periodic table.
  • The position of isotopes could not be explained. Isotopes are atoms of the same element having similar chemical properties but different atomic masses. If the elements are arranged according to atomic masses, the isotopes should be placed in different groups of the periodic table.
  • At certain places, an element of higher atomic mass was placed before an element of lower atomic mass.
    For example: Cobalt (Co = 58.93) was placed before nickel (Ni = 58.71).
  • Some elements placed in the same sub group had different properties.
    For example: Manganese is placed with the halogens which are totally different in their properties.

Modern Periodic Table

  • In 1913, Henry Moseley proved that the atomic number is the fundamental property rather than its atomic mass.
  • Modern Periodic Law: Properties of elements are a periodic function of their atomic numbers.
  • The periodic table, based on the Modern Periodic Law is called the Modern Periodic Table.

Position of Elements in the Periodic Table

  • The horizontal rows in the Modern Periodic Table are called periods.
  • The Modern Periodic Table consists of seven periods which are numbered from 1 to 7.
  • In each period, a new shell starts filling up. The period number is also the number of shell which starts filling up


  • The vertical columns are called groups and consist of eighteen groups numbered from 1 to 18.
  • Elements having the same number of valence electrons are present in the same group.
  • Elements present in the same group show the same chemical properties.

Trends in the Modern Periodic Table

  • The valency of an element is determined by the number of valence electrons present in its outermost shell.
  • In a group, all the elements have the same number of valence electrons.
  • On moving from left to right in each short period, the valency increases from 1 to 4 and then decreases to zero.

Atomic Size

  • Atomic size refers to the radius of the atom.
  • It is the distance between the centre of the nucleus and the outermost shell of an isolated atom.
  • In a period, the atomic radius decreases from left to right. This is because electrons are added to the same shell and so they experience a greater pull from the nucleus.
  • Moving in a group from top to bottom, the atomic radius increases as new shells are added, resulting in the outermost electrons being farther away from the nucleus.

Metallic & Non-metallic Properties

  • Metals show a tendency to lose electrons and are said to be electropositive.
  • Non-metals show a tendency to accept or share electrons and are said to be electronegative.
  • Moving from left to right in a period, the metallic character decreases and the non-metallic character increases. The atomic size decreases and so electrons are not released easily.
  • In a group, the metallic character increases from top to bottom and the non-metallic character decreases. This is because, as the atomic size increases the valence electrons can be easily removed.
  • Elements on the left of the periodic table are all metals and on the right of the periodic table are all non-metals.
  • A zigzag line in the periodic table separates the metals from non-metals. The borderline elements show intermediate properties and are called metalloids.

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