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,
Cl
2, HCl
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.
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