CBSE Class 10 Answered

The properties which show gradual variation in a group and in a period, and repeat themselves after a certain interval of atomic number are called periodic properties.

     Valency

• The valency of an element is determined by the number of valence electrons present in the 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 the 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, bringing the outermost electrons farther 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 the 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, the atomic size increases and so the valence electrons can be easily removed.
• Elements on the left of periodic table are all metals and on the right of 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.

Limitations of Modern periodic table:

1. Hydrogen resembles both the alkali metals and halogens. But it has been placed with the alkali metals.

2. The lanthanides and actinides have not been placed in the main body of the table.