The correct statement is that ionic bonds form when electrons transfer from one atom to another, while covalent bonds form when atoms share electrons. This is the fundamental distinction between the two bond types, and it drives every other difference in their physical properties, structure, and behavior. Many comparison questions on this topic try to trip you up with statements that sound plausible but reverse or blur this core difference, so understanding exactly how each bond works will help you spot the right answer every time.
How Electrons Behave in Each Bond
Both ionic and covalent bonds involve valence electrons, the outermost electrons of an atom. The difference is what happens to those electrons. In a covalent bond, two atoms share one or more pairs of electrons roughly equally. In an ionic bond, one atom pulls so strongly on the electrons that they effectively transfer completely to the other atom, creating a positively charged ion and a negatively charged ion. The electrostatic attraction between those opposite charges is what holds an ionic compound together.
A useful way to think about it: ionic and covalent bonds sit on a spectrum defined by how evenly the electron pair is shared. When two atoms have similar pulling power (electronegativity), they share electrons and form a covalent bond. When one atom is far better at attracting electrons, the pair shifts almost entirely to that atom, and the bond is ionic. The conventional cutoff is an electronegativity difference of about 1.5 on the Pauling scale. Below that threshold, a bond is considered polar covalent or nonpolar covalent. Above it, the bond is classified as ionic.
Which Atoms Form Which Bond
Ionic bonds typically form between a metal and a nonmetal. Sodium chloride (NaCl), magnesium chloride (MgCl₂), and calcium hydroxide (Ca(OH)₂) are classic examples. The metal donates one or more electrons, and the nonmetal accepts them. Covalent bonds form between two nonmetals. Water (H₂O), carbon tetrachloride (CCl₄), hydrogen sulfide (H₂S), and ammonia (NH₃) are common covalent compounds. If you see a compound made of two nonmetals on an exam, it is almost certainly covalent. If it pairs a metal with a nonmetal, it is almost certainly ionic.
Structure: Lattices vs. Molecules
Ionic compounds do not exist as individual molecules. Instead, they arrange themselves into a repeating three-dimensional crystal lattice, with alternating positive and negative ions packed tightly together. Table salt is the textbook example: every sodium ion is surrounded by chloride ions, and vice versa, in a rigid, orderly grid. This lattice structure is why ionic crystals are hard but brittle. A strong enough force can shift one layer of ions, suddenly aligning like charges next to each other, and the crystal fractures.
Covalent compounds, by contrast, typically form discrete molecules. Each molecule is a self-contained unit of atoms bonded together. The forces between individual molecules are much weaker than the covalent bonds within them. This distinction between strong bonds inside a molecule and weak attractions between molecules is key to understanding why covalent substances behave so differently from ionic ones.
Melting Points and Physical State
Ionic compounds generally have high melting and boiling points because breaking apart a crystal lattice held together by strong electrostatic forces requires a lot of energy. Sodium chloride melts at 801°C. Most ionic compounds are solids at room temperature.
Covalent compounds generally have much lower melting and boiling points. When you melt or boil a covalent substance, you are not breaking the covalent bonds themselves. You are overcoming the weak attractions between molecules, which takes far less energy. That is why many covalent compounds are liquids or gases at room temperature. Water is a liquid, carbon dioxide is a gas, and rubbing alcohol evaporates easily.
One important exception: covalent network solids like diamond and quartz have extremely high melting points because their atoms are locked into a continuous three-dimensional covalent framework. There are no separate molecules to pull apart, so the entire structure must be disrupted at once.
Electrical Conductivity
A common comparison question asks about conductivity, and the correct distinction has a nuance that matters. Solid ionic compounds do not conduct electricity. Their ions are locked in place in the crystal lattice and cannot move. However, when you dissolve an ionic compound in water or melt it, the ions become free to move, and the solution or liquid conducts electricity well.
Covalent compounds generally do not conduct electricity in any state, because they have no charged particles to carry a current. There are no free ions or mobile electrons in a sugar solution or a puddle of oil. This difference in conductivity when dissolved in water is one of the most reliable ways to distinguish ionic from covalent compounds in a lab setting.
Solubility Patterns
Ionic compounds tend to dissolve in water and other polar solvents. Water molecules, which are polar, pull the ions apart from the crystal lattice and surround them individually. This is why salt dissolves readily in water but not in cooking oil.
Covalent compounds follow a “like dissolves like” rule. Polar covalent molecules (like alcohol) dissolve well in water. Nonpolar covalent molecules (like fats and oils) do not dissolve in water but dissolve easily in other nonpolar substances. You cannot determine covalent solubility from a simple table the way you can for many ionic compounds. Instead, you need to look at the molecule’s shape and polarity.
Common Incorrect Statements to Watch For
Comparison questions often include statements that are close to correct but wrong in a specific way. Here are patterns to recognize:
- “Ionic bonds share electrons; covalent bonds transfer them.” This reverses the definitions. Ionic bonds transfer, covalent bonds share.
- “Covalent compounds have higher melting points than ionic compounds.” The opposite is true for the vast majority of cases. Covalent network solids are the exception, not the rule.
- “Ionic compounds do not conduct electricity.” This is only true in the solid state. When dissolved or melted, they conduct well. A correct statement would specify the condition.
- “Ionic bonds form between nonmetals.” Ionic bonds form between metals and nonmetals. Bonds between two nonmetals are covalent.
- “Covalent bonds are always weaker than ionic bonds.” Individual covalent bonds can be very strong. The low melting points of many covalent compounds reflect weak intermolecular forces, not weak bonds.
The single most reliable correct statement in any comparison is this: ionic bonds involve the transfer of electrons from one atom to another, while covalent bonds involve the sharing of electrons between atoms. Every other property difference, from melting point to conductivity to solubility, flows from that foundational distinction.