Classify Each Compound As Ionic Or Covalent

The Short Answer

Most people learn this in chemistry 101 and then immediately forget the exceptions. It works like this: when you have a metal combined with a nonmetal, treat it as ionic. Two nonmetals bonded together? That is covalent. But the actual practice is messier than the textbook version. I am going to walk through the classification process the way you would actually use it on a worksheet or in a lab notebook, including the edge cases that show up every semester and cause students to lose points they did not need to lose.

How To Classify Each Compound As Ionic Or Covalent

Start by looking at the elements involved. Write down which ones are metals and which are nonmetals. The periodic table makes this straightforward if you keep the stair-step line in mind. Everything to the left and below the line from boron to astatine is a metal. Everything to the right is a nonmetal, with hydrogen being the annoying exception that sits on its own. Here is the working rule set I use when grading papers or checking my own work: NaCl — sodium is a metal, chlorine is a nonmetal. Ionic.

CO — carbon is a nonmetal, oxygen is a nonmetal. Covalent. NHNO — this one trips people up. Ammonium nitrate contains the ammonium ion (NH) and the nitrate ion (NO). Even though nitrogen and hydrogen are both nonmetals, the compound is ionic because it exists as discrete charged polyatomic ions held together by electrostatic attraction. The bonds within each ion are covalent, but the overall compound is ionic. This distinction matters on exams. PbCl — lead is a metal, chlorine is a nonmetal. Most students will mark this ionic and be marked correct on a basic quiz. But lead(IV) chloride has significant covalent character. The high charge density on Pb polarizes the chloride electron clouds enough to create substantial covalent bonding. This is where electronegativity difference becomes more useful than the metal-nonmetal shortcut.

That leads me to the second method, and the one I rely on when the first method gets ambiguous. Calculate or look up the electronegativity difference between the bonded atoms. This is the Pauling scale. The thresholds are approximate but reliable for classroom purposes: Below 0.4 — nonpolar covalent

0.4 to 1.7 — polar covalent Above 1.7 — ionic Take aluminum chloride, AlCl. Aluminum is a metal, chlorine is a nonmetal. The metal-nonmetal rule says ionic. The electronegativity difference is about 1.55, which falls in the polar covalent range. AlCl is actually covalent. It sublimes at 180°C, forms dimeric AlCl molecules in the gas phase, and dissolves in organic solvents. Calling it ionic gets you the wrong answer every time you look beyond the introductory level.

I ran into this exact problem when I was TA-ing for general chemistry. A student argued that SnCl should be ionic because tin is a metal. I had them check the electronegativity values. The difference came out to 1.26. Tin(IV) chloride is a liquid at room temperature with a boiling point of 114°C. No ionic compound behaves like that. The student understood the concept faster after seeing the data contradict the shortcut. There are other compounds where the shortcut fails in the opposite direction. ConsiderBeF. Beryllium is a metal. Fluorine is the most electronegative nonmetal. The electronegativity difference is 1.94, squarely in the ionic range. And BeF is ionic, unlike BeCl which has significant covalent character. The beryllium ion is so small and has such high charge density that it polarizes larger halide ions dramatically, but fluoride is small enough to resist that polarization. This is an exception worth memorizing because it shows up regularly on upper-level exams. Another thing to watch for: transition metal compounds. They live in a gray zone. Take FeCl. Iron is a metal, chlorine is a nonmetal. The electronegativity difference suggests ionic character, but FeCl sublimes and has considerable covalent bonding. The higher the oxidation state of the metal, the more covalent the compound tends to be. Fe³ compounds are more covalent than Fe² compounds for the same anion. This trend is consistent enough that you can use it as a secondary check.

Here is a practical breakdown you can reference without overthinking it: BaO — ionic. Barium is a group 2 metal, oxygen is a nonmetal. Electronegativity difference is 2.6. Clear-cut. PCl — covalent. Phosphorus and chlorine are both nonmetals. Phosphorus pentachloride exists as PCl and PCl in the solid state, but it is classified as a covalent compound in standard coursework.

KCrO — ionic. Potassium is a metal. The dichromate is a polyatomic ion. Ionic compound containing covalent bonds within the anion. This pattern repeats with every polyatomic ion you will encounter: NO, SO², PO³, MnO, CrO², CHO, OH, NH. The compound is ionic. The bonds inside the ion are covalent. HSO — covalent. All nonmetals. Even though it ionizes completely in water as a strong acid, the pure compound consists of covalently bonded molecules. Do not confuse behavior in solution with bonding in the pure substance. This mistake costs points on every exam I have ever proctored. The limitations of this system are real. The electronegativity method gives you a continuum, not a binary. Bonds exist somewhere along a spectrum from purely covalent to purely ionic, and the cutoff points are somewhat arbitrary. Different textbooks use slightly different thresholds. Some say 2.0 instead of 1.7 for the ionic boundary. The variation is small but it can flip borderline compounds.

The metal-nonmetal method is faster and covers about 85 percent of the compounds you will see in an introductory course. The electronegativity method covers the remaining 15 percent where the shortcut breaks down. Use both when you are uncertain, and if they disagree, trust the electronegativity data. One more edge case that deserves mention: compounds containing hydrogen. Hydrogen is a nonmetal, but it behaves differently depending on what it is bonded to. In NaH, hydrogen forms the hydride ion H and the compound is ionic. In HCl, hydrogen shares electrons with chlorine and the compound is covalent. You have to look at the other element, not just note that hydrogen is present. So when you are working through your assignments, start with the element types, flag the polyatomic ions, run the electronegativity calculation on anything that feels borderline, and remember that the periodic table position of each element is your primary tool. The exceptions are few once you know where they live.