CHEMICAL BONDS

       A chemical bond is the physical phenomenon of chemical substances being held together by attraction of atoms to each other through sharing, as well as exchanging, of electrons -or electrostatic forces.

          Since opposite charges attract via a simple electromagnetic force, the negatively charged electrons that are orbiting the nucleus and the positively charged protons in the nucleus attract each other. An electron positioned between two nuclei will be attracted to both of them, and the nuclei will be attracted toward electrons in this position. This attraction constitutes the chemical bond. Due to the matter wave nature of electrons and their smaller mass, they must occupy a much larger amount of volume compared with the nuclei, and this volume occupied by the electrons keeps the atomic nuclei relatively far apart, as compared with the size of the nuclei themselves. This phenomenon limits the distance between nuclei and atoms in a bond.

            In general, strong chemical bonding is associated with the sharing or transfer of electrons between the participating atoms. The atoms in molecules, crystals,metals and diatomic gases— indeed most of the physical environment around us— are held together by chemical bonds, which dictate the structure and the bulk properties of matter.

IONIC BONDS

         Ionic bonding is a type of electrostatic interaction between atoms which have a large electronegativity difference. There is no precise value that distinguishes ionic from covalent bonding, but a difference of electronegativity of over 1.7 is likely to be ionic, and a difference of less than 1.7 is likely to be covalent.

• Ionic compounds form crystals- Ionic compounds form crystal lattices rather than amorphous solids. Although molecular compounds form crystals, they frequently take other forms plus molecular crystals typically are softer than ionic crystals.

• Ionic compounds have high melting points and high boiling points- High temperatures are required to overcome the attraction between the positive and negative ions in ionic compounds. Therefore, a lot of energy is required to melt ionic compounds or cause them to boil.

• Ionic compounds have higher enthalpies of fusion and vaporization than molecular compounds- Just as ionic compounds have high melting and boiling points, they usually have enthalpies of fusion and vaporization that may be 10 to 100 times higher than those of most molecular compounds. The enthalpy of fusion is the heat required melt a single mole of a solid under constant pressure. The enthalpy of vaporization is the heat required for vaporize one mole of a liquid compound under constant pressure.

• Ionic compounds are hard and brittle- Ionic crystals are hard because the positive and negative ions are strongly attracted to each other and difficult to separate, however, when pressure is applied to an ionic crystal then ions of like charge may be forced closer to each other. The electrostatic repulsion can be enough to split the crystal, which is why ionic solids also are brittle.

• Ionic compounds conduct electricity when they are dissolved in water- When ionic compounds are dissolved in water the dissociated ions are free to conduct electric charge through the solution. Molten ionic compounds (molten salts) also conduct electricity.

• Ionic solids are good insulators- Although they conduct in molten form or in aqueous solution, ionic solids do not conduct electricity very well because the ions are bound so tightly to each other.

An analogy of  ionic bonding

COVALENT BONDS

        Covalent bonding is a common type of bonding, in which the electronegativity difference between the bonded atoms is small or nonexistent. Bonds within most organic compounds are described as covalent. 

• Most covalent compounds have relatively low melting points and boiling points- While the ions in an ionic compound are strongly attracted to each other, covalent bonds create molecules that can separate from each other when a lower amount of energy is added to them. Therefore, molecular compounds usually have low melting and boiling points.

• Covalent compounds usually have lower enthalpies of fusion and vaporization than ionic compounds- The enthalpy of fusion is the amount of energy needed, at constant pressure, to melt one mole of a solid substance. The enthalpy of vaporization is the amount of energy, at constant pressure, required to vaporize one mole of a liquid. On average, it takes only 1% to 10% as much heat to change the phase of a molecular compound as it does for an ionic compound.

• Covalent compounds tend to be soft and relatively flexible- This is largely because covalent bonds are relatively flexible and easy to break. The covalent bonds in molecular compounds cause these compounds to take form as gases, liquids and soft solids. As with many properties, there are exceptions, primarily when molecular compounds assume crystalline forms.

• Covalent compounds tend to be more flammable than ionic compounds- Many flammable substances contain hydrogen and carbon atoms which can undergo combustion, a reaction that releases energy when the compound reacts with oxygen to produce carbon dioxide and water. Carbon and hydrogen have comparable electronegativies so they are found together in many molecular compounds.

• When dissolved in water, covalent compounds don't conduct electricity- Ions are needed to conduct electricity in an aqueous solution. Molecular compounds dissolve into molecules rather than dissociate into ions, so they typically do not conduct electricity very well when dissolved in water.

• Many covalent compounds don't dissolve well in water- There are many exceptions to this rule, just as there are many salts (ionic compounds) that don't dissolve well in water. However, many covalent compounds are polar molecules that do dissolve well in a polar solvent, such as water. Examples of molecular compounds that dissolve well in water are sugar and ethanol. Examples of molecular compounds that don't dissolve well in water are oil and polymerized plastic.

Note that network solids are compounds containing covalent bonds that violate some of these "rules". Diamond, for example, consists of carbon atoms held together by covalent bonds in a crystalline structure. Network solids typically are transparent, hard, good insulators and have high melting points.

An analogy of covalent bonding

SOURCES:
http://en.wikipedia.org/wiki/Chemical_bond
http://www.sciencedaily.com/articles/c/chemical_bond.htm
http://chemistry.about.com/od/moleculescompounds/a/Covalent-Compound-Or-Molecular-Compound-Properties.htm
http://chemistry.about.com/od/moleculescompounds/a/Ionic-Compound-Properties.htm