Lesson 1: Overview of Chemical Bonding

There are three types of chemical bonds: ionic, metallic, and covalent.

Ionic bonds are formed when one atom transfers valence electrons to another atom. These atoms are now opposite charges and will bond together through electrostatic attraction. Ionic compounds are brittle and have high melting points. They are also electrolytes, or substances that conduct electricity when they are in a liquid or aqueous state. This occurs because they ionize—or separate into the cations and anions—when they are in these states. The anions go to the positive end of a circuit and donate electrons into the circuit, becoming neutral atoms. The electrons flow through the circuit toward the positive end of the circuit, where cations take the electrons out of the circuit and also become neutral atoms. Ionic bonds are relatively weak. Ionic compounds exist as formula units, which are crystal lattice structures that have set ratios of the cations to the anions. This ratio is expressed in the chemical formula of the compound. Formula units can be any size. The difference in electronegativity between the cations and anions in ionic compounds is 2.0 or greater. These compounds are often what rocks and minerals are comprised of, and are often solid at room temperature.

Metallic compounds form when many metal cations crowd together and share their valence electrons equally among themselves. Metallic compounds are ductile, which means they are able to be pulled into a thin wire. They are also malleable and conduct in solid or liquid form. Metallic bonds are relatively stronger than ionic bonds and weaker than covalent bonds. They exist as alloys, or mixtures of two or more metals. The arrangement of atoms in alloys are determined by size, charge, and other factors. Differences in electronegativity don’t apply to metallic bonds because all the cations share the electrons equal. Metallic bonds and metals in general are incredibly complex and we don’t need to worry about them much for this unit.

Covalent bonds are formed when electrons are shared or pulled on (not always equally) by two different atoms. Covalent (aka molecular) compounds are usually not brittle, have low melting points, and cannot conduct electricity. This is because molecular compounds do not dissociate or separate into atoms, so they cannot donate and take electrons into and out of circuits like ionic compounds. Covalent bonds are relatively stronger than both ionic and metallic bonds. They exist as discrete molecules, where each one has the exact amount of atoms of each element as they are listed in the chemical formula. The differences in electronegativity between two covalently bonded atoms depends on the molecules’ polarity, a topic that will be covered later. Polar compounds have electronegativity differences between 0.1 and 1.9, while non-polar compounds’ electronegativity differences are 0.