Periodic Classification of Elements

Position of Elements in Modern Periodic Table

By now you already know about the periodic table. You have seen the colour coded grid and can perhaps recite the first few elements by memory. But have you ever wondered why the elements are so arranged? Now let us learn a few facts about the periodic table of elements.

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Modern Periodic Table of Elements


The arrangement of elements in the periodic table follows a certain rule known as the periodic law, which is:

“Properties of elements are a periodic function of their atomic number”

So the periodic table of elements has been arranged in increasing order of their atomic numbers. The arrangement of these elements is in a grid or matrix format. There are 118 elements in total, spread out on the periodic table. Now, this matrix divides itself into rows and columns. Let us learn about these in detail.

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  • Horizontal rows of the periodic table of elements are the periods.
  • There are seven such periods in the table, each numbered from 1 to 7.
  • All the elements in the period have the same number of shells. The number of electrons in this last shell increase by one across any given period
  • The first period is the shortest having only two elements, namely Hydrogen and Helium. The next two rows or periods, that is the second and the third, are known as short periods. They both contain eight elements each. The next two periods have 18 elements each. And finally, the last (sixth) period is the very long period and has 32 elements.


  • The vertical columns of the periodic chart are what we call Groups.
  • There are 18 groups in the periodic table.
  • Elements belonging to a particular group make a family and are generally named after the first element in that particular group
  • If you notice the electronic configuration of all elements in one group you will see they all have the same number of valence electrons. Take Chlorine and Fluorine for example. Both belong to group 17 and both have seven valence electrons.
  • As you go down the group the valence electrons remain same but the number of shell increase.
  • On the extreme left of the table is Group 1 which consists of alkali metals (Li, Na, K, Rb, Cs, Fr). They are highly reactive metals that form strong alkaline hydroxides.
  • The last group is occupied by noble gases (He, Ne, Ar, Kr, Xe, Rn). These are highly nonreactive. The eighteenth group they occupy is also called Group 0.

Inner Transition Elements

  • These are Lanthanides and Actinides.
  • Lanthanides are rare earth metals. They are fifteen in number and found deep in the earth’s crust.
  • These elements are placed below the periodic table of elements and are called the 4f series.
  • Actinides are all radioactive elements. Some of them are not found in nature but are man-made.
  • These actinides are also placed below the table in a row after lanthanides.

Learn more about D Block and F Block elements here in detail here.

Solved Example for You

Q: Electronic configuration of element T is 2, 8, 7. What is the period number of T?

  1. 2
  2. 3
  3. 5
  4. 4

Sol: The correct answer is option “b”. Given configuration is 2, 8, 7. The first shell has 2 electrons and the second will have 8. Hence the remaining 7 will be in the third shell as the third shell has a capacity of 18 electrons. Since the valence shell is three, the period will also be 3.

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What about the Al Razi classification??

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