A.Group whose element has zero valency.
B. A covalent compound formed by an element in period $2$ and a halogen.
C. Non metallic element present in period $3$ of groups $15$ and $16$ .
D. An electrovalent compound formed by an alkaline earth metal and a halogen.
E. Bridge elements of period $3$ of group $1,2$ and $3$ .
F. Alkali metal in period $3$ that dissolves in water giving a strong alkali.
G. Typical elements of group $14$ and $15$ .
H. Alkaline earth metal in period $3$.

Hint:Periodic table is defined as an arrangement of elements in the increasing order of the atomic number. They are arranged left to right and top to bottom. It consists of horizontal rows known as periods and vertical columns known as groups.

Complete step-by-step answer:Let us discuss the answers of the given questions one by one:
A. Group $18$ elements have zero valency because the outermost shell of these electrons are complete and form a stable electronic configuration. They are non reactive elements. These elements are also known as noble gases. The elements are helium, neon, argon, krypton, xenon and radon.
B. $NC{{l}_{3}}$ or nitrogen trichloride is a covalent compound formed by Nitrogen in period $2$ and a halogen that is chlorine. It is a yellow coloured explosive substance which is prepared when ammonia reacts with excess of chlorine.
Let us see the preparation-
$N{{H}_{3}}+3C{{l}_{2}}\to NC{{l}_{3}}+3HCl$
C. Non metallic elements that are present in period $3$ of groups $15$ and $16$ are phosphorus and sulphur. Non metals are the elements that are formed by accepting electrons and acquiring negative charge.
D. An electrovalent compound that is formed by an alkaline earth metal and a halogen is magnesium chloride. The chemical formula of magnesium chloride is $MgC{{l}_{2}}$ . Here, magnesium is an alkaline earth metal and chlorine is a halogen. When a highly electropositive element reacts with electronegative elements, it forms an electrovalent compound.
E. Lithium and magnesium, beryllium and aluminium, boron and silicon are the bridge elements of period $3$ of group $1,2$ and $3$ .
F. Alkali metal in period $3$ that dissolves in water giving a strong alkali is Sodium element. Alkali metals are the elements that are present in group one of the periodic tables.
G. The third period of the elements is known as typical elements. Silicon and Phosphorus are the typical elements of period $3$ that belong to groups $14$ and $15$ .
H. Alkaline earth metal in period $3$is magnesium. Alkaline earth metal is defined as the elements that are present in group two of the periodic table.

Note:The electronic configuration of an atom is defined as a representation of arrangement of electrons that are distributed in the orbital of an atom. The electronic configuration helps in determining the position of an element and the chemical behaviour.