Chemical Bonds, Molecules, and Compounds
Students learn how valence electrons influence ionic and covalent bonding and how chemical formulas represent the composition of substances.
Illustrations are auto-generated and may be placeholders. They can be refreshed to match the narration.
Why Atoms Form Bonds
Atoms can join through chemical bonds, which are attractions that hold particles together. Bonding involves valence electrons, the electrons in an atom's outermost occupied energy level. Many atoms become more stable when their outer level has a complete arrangement of electrons. They may gain, lose, or share electrons to reach that condition. For example, a sodium atom has one valence electron that it can lose relatively easily, while a chlorine atom needs one additional electron. Their complementary tendencies help explain why sodium and chlorine readily combine. Models that show valence electrons as dots can help students track which electrons move or are shared, although real electron behavior is more complex than the model.
Ionic Bonds
An ionic bond forms after electrons are transferred from one atom to another. Losing electrons produces a positively charged ion, while gaining electrons produces a negatively charged ion. Opposite charges attract, holding the ions together in an ionic compound. In sodium chloride, sodium transfers one electron to chlorine. The resulting sodium ion has a charge of positive one, and the chloride ion has a charge of negative one. They combine in a one-to-one ratio, represented by the formula NaCl, so the compound is electrically neutral overall. Ionic compounds often form rigid crystals, have high melting points, and conduct electricity when melted or dissolved because their charged particles can move.
Covalent Bonds and Molecules
A covalent bond forms when atoms share pairs of electrons. Covalent bonding commonly occurs between nonmetal atoms and creates particles called molecules. In a water molecule, one oxygen atom shares electrons with two hydrogen atoms, giving the formula H2O. The subscript 2 means there are two hydrogen atoms for every oxygen atom. In carbon dioxide, CO2, one carbon atom is bonded to two oxygen atoms. Different arrangements of atoms produce substances with different structures and properties. Many covalent substances have lower melting and boiling points than ionic compounds, although exceptions exist. Molecular models use colored spheres and sticks to make atom ratios and bonding patterns easier to visualize.
Reading and Building Chemical Formulas
A chemical formula identifies the elements in a substance and shows their relative numbers. Element symbols tell which atoms are present, and subscripts show how many atoms of each element occur in one molecule or formula unit. If no subscript appears, the number is understood to be one. For example, NH3 contains one nitrogen atom and three hydrogen atoms. A coefficient placed before a formula counts whole molecules or formula units. Thus, 2H2O represents two water molecules containing four hydrogen atoms and two oxygen atoms altogether. When building a model from a formula, students should first list each element, read each subscript, and then count the correct number of pieces before connecting them according to the bonding information.