III.

Bonds Within the Molecule

The bonds that hold a molecule together form because of the structure of the atoms in the molecule. Atoms are made of a nucleus surrounded by a cloud of electrons. The nucleus contains positively charged particles called protons and, in almost all atoms, neutral particles called neutrons. The electrons in an atom arrange themselves in shells, like the layers of an onion, around the atom’s nucleus. Each shell can contain a certain number of electrons, and electrons normally fill the shells closest to the nucleus first. Atoms bond with each other to form molecules by sharing their valence, or outermost, electrons.

Each chemical element has a characteristic number of electrons. For example, a carbon atom has six electrons and a neon atom has ten electrons. The first, or innermost, shell of each of these atoms can contain two electrons, and it is full for both of them. The second shell—which is the outermost shell for both of these elements—can contain eight electrons. Carbon has only four electrons in its outer shell, so it needs four more electrons to fill this layer. Neon has eight electrons in its outer shell, so its outer shell is full. Atoms are very stable when their outermost electron shell is full. Neon and the other so-called noble gases all have full outer electron shells. They are extremely stable and rarely react with other elements. Atoms of other elements bond with each other to fill their outermost shell of electrons and thus attain the stable configuration of the noble gases.

When two atoms bond by sharing some of their outer electrons, the atoms create a covalent bond, forming a molecule. To create a covalent bond, two atoms share a pair of electrons; in most cases, each atom contributes one of the shared electrons. Each atom becomes more stable, because the covalent bond has effectively provided each atom with one more electron in its outer shell. This type of bond, in which one pair of electrons is shared, is called a single bond. Sometimes, two atoms share two or three pairs of electrons with each other. These bonds are called double or triple bonds, respectively.

Two hydrogen atoms, each of which contains one electron, form the simplest covalent bond and the simplest molecule. In the resulting hydrogen molecule, the electrons are much more likely to be located between the hydrogen nuclei than on the far side of either one. The bond is strong because the positively charged nuclei are attracted to the negatively charged electrons between them. The electrons belong to the molecule as a whole. However, each hydrogen atom now has a complete outer shell of two electrons. The formula H₂ describes a hydrogen molecule, a discrete unit. When a molecule contains just two atoms, such as the hydrogen molecule does, it is called a diatomic molecule. Some atoms can form covalent bonds with more than one other atom and thus create a larger molecule.

Atoms form molecules with covalent bonds when they have similar electronegativity values. Electronegativity is a measure of how strongly an atom attracts electrons. If atoms A and B form a molecule with a covalent bond and atom B is slightly more electronegative than atom A is, the molecule’s electrons will shift slightly toward atom B. The side of the molecule near atom A will have a slight positive charge, while the side closer to atom B will have a slight negative charge. This arrangement results in a polar molecule, which is similar to a tiny magnet.

If the electronegativity difference is very large between atoms A and B, the atoms will not bond covalently. Instead, atom B will effectively steal an electron from atom A. As a result, atoms A and B become electrically charged atoms, or ions. Atom B is now a negative ion, while atom A becomes a positive ion. Although the two atoms do not share electrons to form a covalent bond, they are strongly attracted to each other because of their opposite charges. Based on this electrical attraction, they form an ionic bond, and together with other ions, they form an ionic compound. Atoms do not form individual molecules in an ionic compound. Instead, all the ions are mutually attracted. They build up a lattice structure to form a crystal.

When the atoms that join together are all metallic elements, they form a metal. Any number of metal atoms can bond together in a metallic crystal. To form metallic bonds, each atom releases its outer electrons to the metal. The remainder of the atom becomes part of a crystal structure, surrounded by a sea of electrons shared by the entire metal. Metals conduct electricity because these outer electrons can move easily throughout the structure.