Dative covalent bond
When one of the atoms forming a covalent bond provides both the electrons needed for the sharing, the bond formed is called a
dative covalent bond or a coordination bond (e.g., H3N→BF3).
Polar covalent bonds
Pure covalent bonds are rare, and most of the pure covalent bonds only occur in elements (e.g, Br-Br). When the
electronegativity of the two atoms forming the bond are different, the shared electrons will be located nearer to the more
electronegative atom in the bond. A positive pole will develop on the more electropositive atom, and a negative pole will
develop on the more electronegative atom. This causes the bond to become a polar covalent bond (e.g., H-Cl).
Ionic compounds with covalent character
Small cations with high charge (e.g., Al3+) are polarizing, whereas large anions with high charge (e.g., SO4
2-) are polarizable.
Polarization means distortion of electron cloud in the anion. As the electron cloud is distorted and partially returned to the
cation, the electron transfer is incomplete, and the ionic bond formed is having certain covalent character (i.e, there is a certain
degreed of electron sharing instead of having a total transfer of electrons).
Structure, Properties and Bonding
Simple molecular substances
These have weak van der Waals’ forces between molecules, so they have low melting points and low boiling points, and the
molecular crystals they form are usually soft solids (e.g., CO2, S8, etc.)
Ionic compounds
Ionic compounds form giant ionic crystals, and they usually have high melting points and they are usually hard solids at room
temperatures.
Covalent giant structures
A number of covalent substances form giant structures (e.g., diamond, quartz). These are hard solids with high melting points.
Graphite is an example of a giant covalent structure with a high melting point but a low tensile strength, as it only forms
2-dimensional sheets in the giant structure.
Metallic giant structures
Mobile electrons account for the electrical and thermal conductivity of metals in both solid and liquid states. The giant
structure accounts for the facts that most metals are solids at room temperature. The metallic bond is not broken when the
metal ions rearrange their positions in the crystal, so metals can change shape without breaking (malleable and ductile).
Hybridization in covalent compounds
Before covalent bonds are formed, the s, p and even d orbitals can hybridize to form a number of equivalent orbitals to hold
bonding electron pairs or lone electron pairs. This can be used for understanding the shapes of molecules and explaining
delocalization of electrons in covalent compounds.
Introduction:
CFCs, or chlorofluorocarbons, are a class of organic compounds containing nly the elements carbon, chlorine and fluorine.
These are a class of very stable and volatile organic liquids with a wide range of uses. Examples of CFCs include: CFCl3,
CF2Cl2 and CClF2CClF2.
Properties of CFCs
Chemical inertness
CFCs are stable, because of the strong C-X and C-C bonds (X stands for Cl or F).
These bonds are difficult to break and so reaction of the CFCs with most other reactants would require a high activation energy.
As a result CFCs are chemically unreactive. This makes the CFCs ideal for use as solvents.
Physical properties of the CFCs
The CFCs molecules are non-polar and are relatively small in size. Intermolecular forces are weak and as a result these have
low boiling points. Most of the CFCs are therefore volatile liquids, or gases that can be liquefied easily by applying pressure.
This, together with their chemical inertness, make the CFCs ideal for making aerosol propellants, refrigerants and electronic
contact cleaners.
Problems with using CFCs
Though CFCs are such useful chemicals, their use has been banned and discontinued in most part of the Earth, as they can
cause depletion of the ozone layer, and cause global warming by the greenhouse effect.
Importance of the ozone layer
Ozone in the stratosphere undergoes photodissociation by absorbing ultraviolet radiation:
O3 → O2 + O (1)
This absorbs the harmful UV rays from space and protects humans and other forms of life on Earth from the harmful high
energy radiation.
Ozone layer depletion
During the past few decades, there has been a drastic change in the concentration of ozone in the stratosphere. Ozone
concentration has fallen over the Antarctica and Arctic. Evidence shows that ozone depletion is related to the release of
chlorofluorocarbons (CFCs) to the atmosphere. The destruction of ozone by CFCs is a complex chemical process. Very much
simplified, an outline of this process is illustrated below by CCl3F and CC12F2 :
CCl3F → CCl2F• + Cl • (2)
CCl2F2 → CClF2
• + Cl • (3)
The Cl • from the above initiation steps readily depletes ozone via a sequence of reactions (4) and (5).
Cl • + O3 → ClO • + O2 (4)
ClO • + O → Cl • + O2 (5)
The depletion of ozone by reaction (4) is fast, and has the effect of disturbing the balance in the production and destruction of
ozone. What worries environmentalists more is the fact that since the reactive species Cl • consumed in (4) is actually
regenerated in (5), the presence of one Cl • can effectively destroy many ozone molecules.
Ozone depletion leads to reduced crop yield, and higher incidence of skin cancer and eye cataract. There is therefore an urgent
need to minimize the use of CFCs and to develop suitable CFC substitutes which do not adversely affect the environment.
Possible alternatives for chlorofluorocarbons
Attempts have been made to develop compounds which have low ozone depletion potential (ODP) to replace CFCs as
refrigerants, aerosol propellants and solvents. Possible alternatives for CFCs include:
• Hydrochlorofluorocarbons (HCFCs) such as CF3CHC12
They break down more quickly in the atmosphere.
• Hydrofluorocarbons (HCFs) such as CF3CH2F
They have no chlorine and thus are “ozone safe”. However, safety question on toxicity is still unsolved.
• Hydrocarbons such as butane and propane
They are cheap and readily available. As they contain no chlorine, they are “ozone safe”. Water and steam
They are effective for some cleaning applications and thus can replace some CFCs as solvents in cleaning.
UV light
UV light
UV light