Elements of II^nd period are known as _________.

The correct answer is Bridge Element.

Explanation:

• Mosley gave the modern Long form of Periodic table where the base of classification was the atomic number.
• According to Mosley's periodic law, the physical and chemical properties of elements are periodic functions of their Atomic number.

• In the long-form periodic table, horizontal rows are called Periods and vertical rows are called groups.
• A diagonal relationship is said to exist between certain pairs of diagonally adjacent elements in the second and third periods (first 20 elements) of the periodic table. 
• These pairs are (lithium (Li) and magnesium (Mg), beryllium (Be), and aluminum (Al), boron (B), and silicon (Si).

• For example, Lithium and magnesium are showing similar properties, so second-period elements are known as Bridge elements.
• Bridge elements are the gap between the alkali and alkali earth metals (groups 1 & 2) on one side and the metalloids/nonmetals (groups 13 - 18) on the other side.
• The third period elements( Na, Mg, Al,Si,P,S,Cl) are called the TYPICAL elements.They are called so because they represent properties of their own group.
• Transition elements refer to elements of d -block in the periodic table because their valencies show transitions.
• Elements belonging to the s- and p-blocks in the periodic table are called the normal elements or main group elements.

 So, Elements of the II^nd period are known as Bridge elements.