Tuesday, May 5, 2009

Chemical Bonds.

A chemical bond is an attraction between atoms brought about by a sharing of electrons between to atoms or a complete transfer of electrons. There are three types of chemical bonds: Ionic, Covalent and Polar covalent. In addition chemists often recognise another type of bond called a hydrogen bond.



Ionic Bonds:
  Ionic  bonds arise from elements with low electronegativity(almost empty outer shells) reacting with elements with high electronegativity (mostly full outer shells). In this case there is a complete transfer of electrons

A well known example is table salt, sodium chloride. Sodium gives up its one outer shell electron completely to chlorine which needs only one electron to fill its shell. Thus, the attraction between these atoms is much like static electricity since opposite charges attract.


Covalent bond:

Ionic, Covalent, Polar covalent, Hydrogen bond .


   Covalent bonds involve a complete sharing of electrons and occurrs most commonly between atoms that have partially filled outer shells or energy levels. Thus if the atoms are similar in negativity then the electrons will be shared. Carbon forms covalent bonds. The electrons are in hybrid orbitals formed by the atoms involved as in this example: ethane. Diamond is strong because it involves a vast network of covalent bonds between the carbon atoms in the diamond.


Polar Covalent Bond:
 These bonds are in between covalent and ionic bonds in that the atoms share electrons but the electrons spend more of their time around on atom versus the others in the compound. This type of bond occurs when the atoms involved differ greatly in electronegativity. The most familiar example is water. Oxygen is much more electronegative than hydrogen, and so the electrons involved in bonding the water molecule spend more time there.

The fact that water is a polar covalently bonded moleccule has a number of implications for molecules that are disolved in water. In particular, molecules with polar covalent bods can break apart when they encounter water molecules. They are broken apart because of the electrical attraction between the dissimilar charges of the molecules. Also, since ionically bonded molecules involve ions with opposite charges, water with its polar covalent bonds can separate ions from each other and then surround the ions which prevents them from recombining. The properties of water all relate to this polar covalent bonding. Indeed the sorts of so called hydrophilic and hydrophobic interactions water has with varios organic compounds depend on the nature of the polar covalent bond in water.

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