what is chemical bonding

The atoms of a compound are held together by chemical bonds formed by the interaction of electrons from each atom. According to the octet rule Section 5.7C1, atoms bond together to form molecules in such a way that each atom participating in a chemical bond acquires an electron configuration resembling that of the noble gas nearest it in the periodic table. Thus the outer shell of each bonded atom will contain eight electrons (or two electrons for hydrogen and lithium).

The simplest chemical bond is that formed between two hydrogen atoms. Each hydrogen atom has one electron. As the two atoms approach each other, the nucleus of one atom attracts the electron of the other. Eventually the two orbitals overlap, becoming a single orbital containing two electrons (see Figure 7.1).

A. Covalent, Polar covalent, and Ionic Bonds
Because the hydrogen molecule contains two identical atoms, it can be assumed that the bonding electrons in this covalent bond are shared equally by these atoms.

Most chemical bonds are not between like atoms but form between atoms of different elements. These bonds are slightly different from that in a hydrogen molecule. Consider the bond between hydrogen and chlorine: Again both atoms require one more electron to satisfy the octet rule. As the atoms come together, their orbitals overlap and the two atoms share a pair of electrons. However, the hydrogen-chlorine bond differs from the hydrogen-hydrogen bond because the electrons are not shared equally between hydrogen and chlorine but are more strongly attracted to the chlorine. They are more apt to be found close to the chlorine than close to the hydrogen. Because of this unequal sharing, the chlorine atom assumes a slightly negative character and the hydrogen atom a slightly positive character. We say that the bond is polar covalent, meaning that the bond consists of electrons shared between two atoms (therefore covalent) but shared unequally, thus giving the bond a positive and a negative end, a condition described by the term polar. We can also say that the bond is a dipole or has a dipole moment, meaning that the bond has a positive end (the hydrogen) and a negative end (the chlorine). The more negative atom in a bond is often shown with the symbol and the more positive atom is shown with the symbol. The bond between hydrogen atoms is nonpolar (has no positive and negative ends) covalent (electrons are shared).

An ionic bond is the extreme case of a polar covalent bond. In an ionic bond, the bonding atoms differ so markedly in their attraction for electrons that one or more electrons are essentially transferred from one atom to the other. The sodium-chlorine bond is an example of an ionic bond. The attraction of the chlorine atom for electrons is so much greater than that of a sodium atom that the 3s electron of sodium is said to be completely transferred from sodium to chlorine.

In summary, then, the three types of bonds are: (1) a covalent bond, in which the electrons are shared equally; (2) a polar covalent bond, in which the electrons are shared unequally; and (3) an ionic bond, in which electrons are transferred from one atom to the other. These bonds are illustrated in Figure 7.2.