Notes and WKST
Covalent Bonds
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The formation of a covalent bond between two atoms occurs when the atoms in question are both
nonmetals, or when the absolute difference in electronegativity for the two atoms is less than 0.8.
In such atoms, there is a tendency to gain enough valence electrons to complete the p sublevel in
the valence energy level.
A complete p sublevel has 6 electrons. Remember that the elements in the p block also have 2
electrons in the s sublevel of the valence energy level. The total number of valence electrons in
the completed s and p sublevels is 8. This is the basis for the octet rule for covalent bonding:
the sharing of electrons occurs when the atoms involved can acquire particularly stable
valence electron configurations. Since the most stable valence electron configuration is that of
the noble gases, s2p6, covalent bonding allows certain atoms to have such stable configurations.
There are four types of covalent bonds: single, double, triple, and coordinate.
Single Covalent Bond: formed when one pair of valence electrons is shared between two atoms
(total of 2 electrons).
Double Covalent Bond: involves the sharing of two pairs of valence electrons (total of 4
electrons).
Triple Covalent Bond: involves the sharing of three pairs of valence electrons (total of 6
electrons).
Coordinate Covalent Bond: forms when one atom contributes (or donates) both bonding
electrons (a bonding pair) in a covalent bond.
Single Covalent Bonds
Single covalent bonds form between two atoms of the same element when the result is a
completed valence energy level.
For example, 2 H atoms will form a single covalent bond and form a hydrogen molecule, H2.
The skeleton structure for H2 is H — H. The single line between the two hydrogen atoms
represents a single pair of shared valence electrons.
Hydrogen can form only one single covalent bond: the hydrogen atom has only one electron in
one sublevel of the 1st principle energy level. Since two electrons would fill and complete that
sublevel and energy level, the single covalent bond between hydrogen and another atom results
in the formation of a stable valence configuration.
Double and Triple Covalent Bonds
Atoms can sometimes share more than one pair of electrons to attain the stable noble gas valence
configuration.
Oxygen is an example of an atom that can form double covalent bonds. Each oxygen atom has 2
unpaired p valence electrons. Carbon is an atom that has 4 unpaired electrons (1 in the s
sublevel, and 3 in the p sublevel).
The compound carbon dioxide, CO2, involves one carbon atom forming 2 double covalent bonds.
The skeleton structure for CO2 is:
O═C═O
The two parallel lines between each oxygen atom and the carbon atom represent 2 pairs of shared
valence electrons.
Two nitrogen atoms can form a triple covalent bond between them to form the diatomic
molecule N2:
N≡N
Coordinate Covalent Bond
The compound carbon monoxide, CO, is very different from the molecule of carbon dioxide,
CO2, and not just in having one less oxygen atom.
In this molecule, the oxygen forms a normal double bond with the oxygen but also donates a pair
of electrons to form a coordinate covalent bond.
..
: C :: O :
The carbon and oxygen form a normal double covalent bond between them with each atom
donating one pair of electrons to the bond. But then the oxygen donates a pair of electrons to
form a coordinate covalent bond.
The pair of electrons at the top of what looks like a triple bond are the two electrons donated by
the oxygen atom. This arrangement gives both atoms a stable s2p6 valence configuration.
Because one of the three bonds between the two atoms is a coordinate covalent bond, this “triple
bond” is quite different from the triple bond between the two nitrogen atoms in which each
nitrogen atom donates 3 electrons to the bond.
Draw skeleton structures for the following molecular compounds:
CH4
NH3
H2S
HCN
SO2
H3C2NO
C2N2
H2CO
OF2
C2H2
NI3
CH3OH