Difference between covalent and ionic compound

In general, the most common covalent compounds do not contain a metal as one of its constituent elements. So, with few exceptions to be mentioned momentarily, if you do not see a metal atom in a formula for a compound, it is probably the formula for a covalent compound. (An exception to this is butyl lithium, BuLi of the class of organometallic compounds, which have very little ionic character, even though they contain an ion.)

Most Ionic compounds contain at least one positively charged metal atom, such as NaCl. The metal is sodium as its positively charged ion, Na+. (There are exceptions to this rule. For example, NH4Cl, ammonium chloride, is an ionic compound, containing the ions, NH4+ and Cl-.)

Suggestion: Memorize the common elements and what ions they can form. Study atomic structure and quantum mechanics theory, the aufbau theory, the major trends of the periodic theory, which will give you a systematic way to estimate or predict the ions and the charges of many of the elements. Knowing the trends and patterns in the periodic table will allow you to easily recognize which elements are metals and which are nonmetals. This knowledge, combined with the rules given above in paragraphs one and two, will empower you to distinguish between covalent and ionic compounds, because you will be able to recognize when a compound contains metals or only nonmetals.