L15. Covalent Bonds

Chemical Bonding

R-report

L15. Covalent Bonds

How can two neutral atoms share electrons so they act like a single connected unit?

Phenomenon: a glowing lamp and a wire of identical atoms

Imagine a thin strand of a nonmetal material that glows when you pass a small current through it. The material stays neutral overall, but its atoms stay together as the strand conducts lightly. Unlike metals or ionic salts, some substances are made of molecules — groups of atoms stuck together by shared electrons. In this lesson we focus on the particle-level idea that makes molecules hold their shape: covalent bonds. You already learned about ions and ionic bonds earlier in the unit; covalent bonds produce stable particles without charged ions by sharing electrons instead of transferring them.

What happens in a covalent bond (particle model)

At the particle level, a covalent bond forms when two atoms each contribute one or more electrons to a shared pair. The shared electrons spend time around both nuclei and lower the total energy of the system, so the atoms stay close together. This keeps each atom’s outer electron shell more like the nearby noble gas configuration, which is often more stable. Covalent bonds can be single (one shared pair), double (two shared pairs), or triple (three shared pairs). Covalent bonds do not create charged ions; the bonded atoms together form a neutral molecule if the atoms themselves were neutral. Simple examples: H + H → H2 (two single electrons become one shared pair), O + O → O2 (each O brings two electrons to make a double bond). When writing these particle-level sketches, draw the nuclei with their protons and neutrons, and show valence electrons as dots or small marks; indicate the shared pair with a short bond line or a pair of dots between atoms.

Key contrasts: covalent vs. ionic bonding

A quick comparison helps when deciding which model to use to explain an observation. Use the covalent model when atoms remain neutral and form identifiable molecules; use the ionic model when atoms become charged ions and form lattices.

• Electron movement: covalent = electrons shared in pairs; ionic = electrons transferred (one atom becomes +, the other -). Product form: covalent = discrete molecules (H2, O2, H2O); ionic = extended crystal lattices (NaCl). Electrical behavior: covalent molecular substances often do not conduct as solids and may be poor conductors in solution; ionic compounds conduct when molten or dissolved because ions move. Polarity: covalent bonds can be nonpolar (shared equally) or polar (unequal sharing), leading to different physical properties.

Everyday application: water and oxygen molecules

Water (H2O) and oxygen gas (O2) show how covalent bonds shape properties. In O2, two oxygen atoms share two pairs of electrons (a double bond); the molecule is nonpolar and a gas at room temperature. In H2O, each hydrogen shares one electron with oxygen; oxygen also keeps two lone pairs. The geometry (bent shape) and unequal sharing make water polar. Polarity lets water dissolve many substances and form hydrogen bonds between molecules, explaining water’s higher boiling point than other small covalent molecules. When you draw or balance reactions that involve covalent molecules, always conserve atoms and charge: for example, to form two water molecules from hydrogen and oxygen gas, write and balance the reaction as 2 H2 (g) + O2 (g) → 2 H2O (l). Count atoms on both sides to show conservation (H: 4 → 4; O: 2 → 2), and note that all species are neutral so overall charge is conserved.

Big idea and where this leads

Covalent bonds hold neutral atoms together by sharing electron pairs, producing molecules with shapes and polarities that explain many everyday properties (gases, liquids, solubility, melting and boiling points). At the particle level, shared electrons reduce energy and keep nuclei close without creating charged ions.

Next, you will learn how chemists name molecular compounds and use formulas to represent covalent molecules. For now, practice drawing simple covalent bonds, counting valence electrons, and checking that atoms and overall charge are conserved in any reaction sketches you write.