CBSE Class 10 Answered

Brisk effervescence is defined as the rapid or vigorous evolution of a gas in a chemical reaction. For example, when hydrochloric acid is added to sodium bicarbonate, bubbles of carbon dioxide gas are formed vigorously.

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.