CHEMICAL BONDS

Basic concept on chemical bonds

1. Ionic bond: The bond formed when one atom looses electrons and the other accepts  

      them. It is formed when a metal bonds to a non-metal, example: NaCl

Original electronic configuration : Na  2:8:1                      Cl    2:8:7

Sodium loses one electron from its outermost shell (valence shell)  to become   Na+ 2:8                   and  chlorine gains this electron to become Cl^-   2:8:8   

 Hence  Na⁺Cl^{-    ----------->} NaCl

2. Covalent bond: This is the bond formed through the sharing of electrons. It is formed

    when non-metal bond to each other. Examples: H₂, Cl₂, HCl

Illustration of covalent bonding

Water is neither purely ionic nor purely covalent because the electrons are not shared equally. The more electronegative element, oxygen, tends to pull the shared electrons from the less electronegative (electropositive), hydrogen atom. One end, or pole, of the molecule has a partial positive charge (+), and the other end has a partial negative charge (-).Water is best described as a polar compound.

As a rule, when the difference between the electronegativities of two elements is less than 1.2, we assume that the bond between atoms of these elements is covalent. When the difference is larger than 1.8, the bond is assumed to be ionic. Compounds for which the electronegativity difference is between about 1.2 and 1.8 are best described as polar, or polar covalent.

Hydrogen bonding: When hydrogen atoms are joined in a polar covalent bond with a small atom of high electronegativity such as O, F or N, the partial positive charge on the hydrogen is highly concentrated because of its small size. If the hydrogen is close to another oxygen, fluorine or nitrogen in another molecule, then there is a force of attraction termed a dipole-dipole interaction. This attraction is known as hydrogen bonding.

Hydrogen bonding has a very important effect on the properties of water and ice. Hydrogen bonding is also very important in proteins and nucleic acids and therefore in life processes.