BONDING

Bonding refers to the association (formation of a chemical bond) between two or more atoms. The outermost electrons of atoms, called the valence electrons, are involved in this process. There are three ways in which atoms bond with one another (intramolecular forces of attraction): ionic bonding, covalent bonding, and metallic bonding.The purpose of chemical bonding is to achieve stability, which, at this level, is equivalent to an atom possessing the electron configuration of the nearest noble gas.

Ionic Bonding

Ionic bonds are strong electrostatic forces of attraction between oppositely charged ions.This type of bonding occurs between a metal and nonmetal ion. The metal atom, with less than 4 valence electrons,loses these valence electrons to form an ion with a positive charge, called a cation. Alternatively, the nonmetal atom, with greater than 4 valence electrons, gains these valence electrons provided by the metal ions, forming negatively charged ions called anions. These non-metal atoms gain electrons until the nearest noble gas electron configuration is reached. These compounds which are formed by ionic bonding are called ionic compounds.

Note: Atoms with 4 valence electrons (such as carbon and silicon) will not form ions under normal conditions, as it is not energetically feasible for these atoms to lose/gain four electrons. As  a result, these atoms undergo covalent bonding (discussed below).

Example:

Na → Na+ + e- Zn → Zn2+ + 2e-

Cl + e- → Cl- Cl + e- → Cl-

Na+ + Cl- → NaCl Zn2+ + 2Cl- → ZnCl2

Covalent Bonding

Unlike ionic bonds, covalent bonds involve the sharing of valence electrons between nonmetal or semimetal atoms.These compounds are called covalent compounds. When the valence shells of two nonmetal (or semimetal) atoms overlap, unpaired valence electrons are shared between the two atoms. This sharing of electrons forms the covalent bond. One atom will share one-three electrons, and the other atom will share an equal amount. This results in one to three electron pairs being shared. If one electron pair is shared between the atoms (one electron from atom A and one from atom B), a single covalent bond is formed (Cl2). Similarly, if two electron pairs are shared between atoms, a double covalent bond is formed (O2), and if three electron pairs are shared between atoms, a triple covalent bond is formed (N2).

These atoms, held together by covalent bonds, are called molecules. This covalent bonding is also an electrostatic force of attraction, but unlike ionic bonding, this force of attraction is between each nucleus and the shared electron pair.

Note: Covalent bonds are able to be formed since all participating atoms need to obtain electrons to become stable, ie, all molecules have greater than or equal to 4 valence electrons. See future note on polar and nonpolar molecules.

Examples:

CH4 has four single covalent bonds

C2H4 has one double C=C bond and four single C-H bonds

N2 has one triple covalent bond

O2 has one double covalent bond

NH3 has three single covalent bonds

H2O has two single covalent bonds

Metallic Bonding

Metallic bonding is very different from both ionic and covalent bonding. Metallic bonds form only between metal atoms, and can be defined as the strong electrostatic force of attraction between metal cations (positive) and a ‘sea’ of delocalized electrons (negative). The metal ions are packed very closely together, and form a metal lattice (a repeating, regular structure).These metal ions are formed since valence electrons of the metal atoms are ‘released’, and are no longer associated with any particular atom. As a result, the electrons become mobile (free to move throughout the lattice) and are said to be delocalized. These positive metal cations are attracted to the negative electrons. However, metals are still considered to be made out of atoms, because although the valence electrons are not attached to the atom, they are not lost and are still present in the structure eg (Na(s)).

Note: Pure metals consist solely of metallic bonding between metal atoms of the same element. However, in alloys, these metals are mixed with other metal atoms and/or nonmetal atoms (e.g. of alloys: carbon steel, brass, magnalium, duralumin, stainless steel).