Bond, in chemistry, is the attraction that holds atoms together in groups of two or more. Bonds hold together every substance in the universe that is made of atoms. Bonds arise from the activity of electrons, negatively charged particles that whirl about the positively charged nucleus of an atom. A bond forms when an electron that belongs to one atom forms a pair with an electron that belongs to another atom.
There are two main types of bonds: (1) covalent and (2) ionic. A group of atoms held together by covalent bonding is called a molecule. Ionic bonds hold together ionic compounds.
A covalent bond
forms when each of two atoms contributes one electron to a pair, which the atoms then share. The bond is the electrical attraction of the positively charged nuclei of the two atoms for the negatively charged electron pair. A hydrogen molecule consists of two hydrogen atoms linked by a covalent bond. The atoms share the electron pair equally. A polar covalent bond forms when one atom has a stronger attraction for the electron pair. This atom gains a slight negative charge. The other atom becomes slightly positive.
An ionic bond
results when one atom loses an electron to form a positive ion, and another atom accepts the electron to form a negative ion. The attraction between the two atoms holds them together. Most ionic compounds, such as table salt—sodium chloride (NaCl)—are solids in which the positive and negative ions pack together to form a crystal.
Other linkages.
A linkage called an intermolecular force results when the negative charge associated with a polar bond in one molecule interacts with the positive charge of another molecule. A hydrogen bond is a weak intermolecular force between a hydrogen atom in one molecule and an electron pair associated with an oxygen, nitrogen, or fluorine atom in another molecule. Hydrogen bonds are largely responsible for the attraction between water molecules. This attraction gives water its ability to form drops.
See also Ion; Materials (The role of bonds); Molecule.
