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The two main types of chemical bonds are ionic bonds and covalent bonds. Ionic bonds form when one atom transfers electrons to another, creating oppositely charged particles (ions) that attract each other. This typically happens between metals and nonmetals. Covalent bonds form when two atoms share electrons, which usually happens between two nonmetals. The type of bond affects the properties of the resulting substance — ionic compounds tend to be hard, brittle solids with high melting points, while covalent compounds tend to have lower melting points and may be liquids or gases.
Compare two familiar substances: table salt (ionic — NaCl) and water (covalent — H2O). Discuss how salt forms hard crystals that dissolve in water and conduct electricity when dissolved, while water is a liquid at room temperature. Then connect these observable differences to the underlying bond types.
You have learned that atoms bond to achieve more stable electron arrangements. Now it is time to look at the two main types of chemical bonds: ionic bonds and covalent bonds. Understanding the difference between them explains a great deal about why substances look and behave the way they do.
Ionic bonds form when one atom transfers electrons to another. This typically happens when a metal meets a nonmetal. Metals have a few outer electrons that they lose easily. Nonmetals need a few electrons to complete their outer arrangement. So the metal gives its electrons to the nonmetal. When sodium (a metal with 1 outer electron) meets chlorine (a nonmetal that needs 1 electron), sodium transfers its electron to chlorine. Sodium becomes a positively charged ion (Na+) and chlorine becomes a negatively charged ion (Cl-). The opposite charges attract each other powerfully, holding the ions together.
What makes ionic compounds distinctive is that they do not form individual molecules. Instead, billions of positive and negative ions pack together in a repeating three-dimensional pattern called a crystal lattice. Table salt is a perfect example: each sodium ion is surrounded by chloride ions, and each chloride ion is surrounded by sodium ions, extending in every direction. This extensive network of attractions is why ionic compounds tend to be hard, brittle solids with high melting points. Breaking a salt crystal apart requires overcoming many strong attractions at once.
Covalent bonds form when two atoms share electrons rather than transferring them. This usually happens between two nonmetals, because both need to gain electrons and neither is willing to give them up. In a water molecule, oxygen shares electrons with two hydrogen atoms. In an oxygen gas molecule, two oxygen atoms share electrons with each other. The shared electrons hold the atoms together, creating distinct molecules. Covalent compounds tend to have lower melting points than ionic compounds because the forces between separate molecules are weaker than the forces inside an ionic crystal lattice. Many covalent compounds are liquids or gases at room temperature — water, carbon dioxide, and methane are all examples.
It is worth noting that the ionic/covalent distinction is not always black and white. Some bonds fall in between, with electrons being shared unequally rather than transferred completely. But for most introductory purposes, asking "is this a metal bonding with a nonmetal (ionic) or nonmetals bonding with each other (covalent)?" gives you a reliable prediction of the bond type and the properties of the resulting substance.