Summary

Atoms are joined together in compounds by chemical bonds. A covalent bond is formed by two atoms sharing electrons, either equall to yield a nonpolar bond or unequally to form a polar bond. An ionic bond is formed by an almost complete transfer of electrons from one atom or group to another atom or group. Electronegativity measures the attraction an atom has for the electrons in a chemical bond between that atom and another. If the difference in electronegativity is slight, the bond will be nonpolar covalent. If the difference is greater, the bond will be polar covalent. A large difference in electronegativity results in an ionic bond. Atoms may share two, four, or six electrons to form single, double, or triple bonds. One atom may donate both electrons of a single bond.

The Lewis structure of a molecule shows the arrangement of its atoms and the distribution of their electrons. The arrangement of atoms is usually the most symmetrical. If a molecule (or ion) has several equally likely arrangements of its electrons, that molecule (or ion) is said to exhibit resonance.

The number of electron clouds, or regions of high electron density around the central atom of a molecule, determines the geometry aroud that atom. Two clouds result in a linear geometry, three result in trigonal planar or bent geometry, and four may yield either bent, or pyramindal, or tetrahedral geometry.

A molecule is polar if its atoms are so arranged that the polar bonds it contains are not balanced. Polar molecules and ionic compounds are most apt to dissolve in water to yield ions; such compounds are called electrolytes. Compounds that are partially ioized in water solution are weak electrolytes. Compounds that do not dissociate into ions in water are called nonelectrolytes.