Covalent compounds are formed between nonmetals.

Examples of nonmetals
H , O , N , C , S , P , F , Cl , Br , I.
Examples of covalent compounds: - CH2O, H2O etc.,

Ionic compound is formed between a metal and nonmetal.

Examples of ionic compound: - CuSO4, NaCl.

NH4^+ = ammonium ion it is a polyatomic ion it form ionic salt which are ionic in nature
Example:- NH4Cl, (NH4)3PO4 etc.,

Ionic compounds are formed by transfer of electron between a metal atom and nonmetal atom.

Covalent compounds are formed by sharing of electrons between two nonmetals atoms.

Number of electron transferred can be determined by electronic configuration or by knowing the oxidation states.

(NH4)2SO4 is the chemical formula of ammonium sulfate. This compound is formed due to the presence of ionic bonds between ammonium ions and sulfate ions. Ammonium ions have a positive charge of positive charge of 1+ and sulfate ions have a negative charge of 2-. When ammonia and hydrogen sulfate are reacted with each other two positively charged ammonium ions and one negatively charged sulfate ion are attracted to each other due to their opposite charges. This results in the formation of an ionic compound that is ammonium sulfate or (NH4)2SO4.