Page 83 - Theory and Problems of BEGINNING CHEMISTRY

72 CHEMICAL BONDING [CHAP. 5

Electron dot structures are not usually written for transition metal or inner transition metal atoms even though
they do lose electrons, forming ions. If you are asked to draw an electron dot diagram for a compound containing
a monatomic transition metal ion, show the ion with no outermost electrons.

5.6. COVALENT BONDING
The element hydrogen exists in the form of diatomic molecules, H 2 . Since both hydrogen atoms are identical,
they are not likely to have opposite charges. (Neither has more electron-attracting power than the other.) Each
free hydrogen atom contains a single electron, and if the atoms are to achieve the same electronic configuration
as atoms of helium, they must each acquire a second electron. If two hydrogen atoms are allowed to come
sufficiently close to each other, their two electrons will effectively belong to both atoms. The positively charged
hydrogen nuclei are attracted to the pair of electrons shared between them, and a bond is formed. The bond
formed from the sharing of a pair of electrons (or more than one pair) between two atoms is called a covalent
bond. The hydrogen molecule is more stable than two separate hydrogen atoms. By sharing a pair of electrons,
each hydrogen atom acquires a configuration analogous to that of a helium atom. Other pairs of nonmetallic
atoms share electrons in the same way.
The formation of covalent bonds between atoms can be conventionally depicted by means of the electron
dot notation. The formation of some covalent bonds is shown in this manner below:

H + H HH
F + F F F

H + F H F

F
C + 4 F F C F
F

In these examples, it can be seen that the carbon and fluorine atoms can achieve octets of electrons by sharing
pairs of electrons with other atoms. Hydrogen atoms attain “duets” of electrons because the first shell is complete
when it contains two electrons. We note from Sec. 5.4 that many main group atoms lose all their valence electrons
and their cations have none left in their valence shell.
Sometimes it is necessary for two atoms to share more than one pair of electrons to attain octets. For example,
the nitrogen molecule N 2 can be represented as follows:
N N

Three pairs of electrons must be shared in order that each nitrogen atom, originally with five valence electrons,
attains an octet of electrons. The formation of strong covalent bonds between nitrogen atoms in N 2 is responsible
for the relative inertness of nitrogen gas.
Every group of electrons shared between two atoms constitutes a covalent bond. When one pair of electrons
is involved, the bond is called a single bond. When two pairs of electrons unite two atoms, the bond is called
a double bond. Three pairs of electrons shared between two atoms constitute a triple bond. Examples of these
types of bonds are given below:

H H O C O N N Cl S Cl
Single bond Two double bonds Triple bond Two single bonds

Two small points might be noted now that double bonds have been introduced. Hydrogen atoms rarely
bond to oxygen atoms that are double-bonded to some other atom. Halogen atoms do not form (simple) double
bonds.