Atoms in a molecule or compound are held together by electrostatic forces. These electrostatic forces are called chemical bonds.
- 1 Bonding Forces
- 2 Chemical Bonds
- 3 Polar and Nonpolar Bonds
- 4 Electronegativity
- 5 Dipole Moment and Polarity
- 6 Molecular Geometry and Polarity
- 7 Examples of Polar Molecules
- 8 Examples of Non-polar Molecules
The forces that bond the atoms can be of two types:
- Intermolecular forces
- Intramolecular forces
Let me explain the difference between intermolecular force and intramolecular force.
The weak forces that held the molecules together are called intermolecular forces. The example of these forces are London dispersion forces, dipole-dipole interactions and most commonly known hydrogen bonding (present in water molecules).
The strong forces that held the atoms together in a molecule are called intramolecular forces. The examples are ionic bonds (NaCl) and covalent bonds (CH4).
There are two major types of intramolecular forces:
- Ionic forces
- Covalent Forces
The bonds which are formed by complete transfer of electrons between metals and non-metals are called ionic bonds. The positive ions are called cations (formed by losing an electron by a metal atom) while negative ions are called anions (formed by gaining an electron by a non-metal atom). The most common ionic compound is NaCl formed by the combination of Na+ and Cl- ions.
The bonds which are formed by sharing of electrons between two non-metals are called covalent bonds. The atoms can share one, two, three or four electrons depending upon the number of their valence shell electrons.
Most common examples of covalent bonds are the bond between Cl2, SOCl2, CH4 etc.
Figure 3: Single covalent bond between two chlorine atoms.
Covalent bonds can be divided into two parts depending upon the polarity of bonding atoms:
- Polar covalent bonds
- Non-polar covalent bonds
Polar and Nonpolar Bonds
What is a Polar Bond?
A polar bond is a bond in which the shared electrons are unequally distributed between the bonded atoms depending upon the electronegativity of the bonded atoms. These bonds are unsymmetrical in nature.
The unequal distribution of electrons is donated by delta “δ”. The atom with greater electronegativity will have partial negative charge while the atom with the laser electronegativity will have a partial positive charge.
The direction of increase in the electronegativity is donated by a crossed arrow.
Figure 4: Partial positive and partial negative charges on covalently bonded polar atoms.
What is a non-polar bond?
The covalent bonds formed between the two same atoms or the electrons are equally distributed between the two atoms are called non-polar covalent bonds. The bonds in most of halogens and diatomic gases are non-polar in nature.
Figure 5: Non-polar bond between two chlorine atoms.
Electronegativity is the tendency of an atom to attract the electrons towards itself while bonded with other atoms. Fluorine atom have the highest electronegativity value which is 4. The electronegativity values increase from left to right in a periodic table.
The electronegativity difference between two bonded atoms can also predict the nature of the bond.
How electronegativity difference predicts the nature of the bond?
The electronegativity difference can predict whether a bond is ionic, polar or non-polar. The maximum electronegativity an atom can have is 4.
- If the electronegativity difference is less than 0.4 the bond is non-polar.
- If the electronegativity difference is between 0.4-1.7 the bond is polar in nature.
- If the electronegativity difference is more than 1.7 than the bond is ionic in nature.
Figure 6: Effect of electronegativity difference on type of bonding (polar, non-polar, ionic)
In case of F2 molecule the electronegativity difference is zero as both of the fluorine atoms have electronegativity value of 4.
Dipole Moment and Polarity
The charge separation between the partially negative charged atom and partially positive charged atom of the covalently bonded polar molecule is called dipole moment. It is a vector quantity. The arrow is pointing from lower electronegative atom to the higher electronegative atom.
Q=Magnitude of the partially charged atoms
r= Distance between the atoms
Figure 7: Direction of increasing electron density in polar bond (H-Cl).
The diatomic molecules like halogens (Cl2, F2, Br2) have zero dipole moment as there is no charge separation. In case molecule have more than one bond, there will be a net dipole moment that depends upon the shape of the molecule (geometry).
Molecular Geometry and Polarity
The geometry of the molecule explains if the molecule is polar and non-polar. If the molecules are arranged in a manner that their dipole moment cancel each, then the molecule will be non-polar.
Why CO2 is non-polar? In case of CO2, individual C=O bonds have dipole moment and they are polar but in the molecule of CO2, two C=O bonds are arranged linearly, canceling individual bond polarities and giving zero dipole moment for the whole molecule and making it non-polar.
Figure 8: Liner geometry of CO2 with zero dipole moment (non-polar bond).
Why H2O is polar? In case of H2O the central atom (oxygen) have two lone pairs which bends the O-H bonds and gives rise to bent or angular geometry. In the bent geometry the individual O-H bonds are arranged in a manner that they don’t cancel the individual bond polarities making it a polar molecule.
Figure 9: Bent geometry of H2O (polar bond).
Examples of Polar Molecules
Given below are the Lewis structure of some polar molecules.
Figure 9: Lewis structures of some of the polar compounds.
Examples of Non-polar Molecules
Given below are Lewis structures of some of the non-polar molecules.
Figure 10: Lewis Structure of some of the non-polar compounds