Chapter 5 Section 3 Notes The chemical bond formed when two atoms share electrons is called a covalent bond. Except for noble gases, nonmetals can bond to other nonmetals by sharing electrons in covalent bonds. The force that holds atoms together in a covalent bond is the attraction of each atom’s nucleus for the shared pair of electrons. A neutral group of atoms joined by covalent bonds is called a molecule.

The number of covalent bonds a nonmetal atom can form equals the number of valence electrons needed to make a total of eight. Some atoms share two pairs of electrons, forming a double bond. Some atoms even form triple bonds in which their atoms share three pairs of electrons.

A molecular compound is a compound that is composed of molecules. Molecular compounds have very different properties than ionic compounds. Compared to ionic compounds, molecular compounds have lower melting points and boiling points, and they do not conduct electricity when dissolved in water.

Atoms of some elements pull more strongly on shared electrons than do atoms of other elements. As a result, the electrons are pulled more toward one atom, causing the bonded atoms to have slight electrical charges.

These charges are not as strong as the charges on ions. A covalent bond in which electrons are shared unequally is a polar bond. When valence electrons are shared equally, the bond is nonpolar.

A molecule is nonpolar if it contains only nonpolar bonds, or if it has polar bonds that cancel each other out. For example, in carbon dioxide (CO 2 ), the oxygen atoms attract electrons more strongly than does the carbon atom. However, the two oxygen atoms pull with equal strength in opposite directions, so the polar bonds cancel each other out. Some molecules with polar bonds are themselves polar.

In a water molecule, the oxygen atom attracts electrons more strongly than do the hydrogen atoms. As a result, the molecule has a slightly negative charge at the oxygen end and a slightly positive charge at the hydrogen end. Polar molecules are more strongly attracted to one another than are nonpolar molecules. This difference leads to different properties in polar and nonpolar compounds. Polar and nonpolar molecules also do not mix.

For example, water, which is polar, does not mix with oil, which is nonpolar. The nonpolar oil molecules have little attraction for the polar water molecules, while the water molecules are more strongly attracted to one another.