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Simple Explanation:
Electronegativity
The electronegativity of an atom determines how much force its nucleus pulls on the electrons. Higher values of electronegativity in an element means that the pull is stronger for those atoms of that particular element.
When two atoms of different electronegativity values bond, the more electronegative atom exerts more pull on the bonding electrons, causing unequal charge distributions across the molecule.
If two atoms with a significant difference in electronegativity interact, the more electronegative atom likely has sufficient force to rip the valence electron off of the other atom, resulting in ionization and the formation of ions.
Electronegativity Chart of Elements
Courtesy of: https://www.chem.ucla.edu/~harding/IGOC/E/electronegativity.html
What is Polarity?
A bond is polar when two atoms have a difference in electronegativity, causing unequal charge distribution.
A compound is polar when it has a net dipole, where its sides have differences in charge.
Molecules: Because they are covalently-bonded, they have partially positive (δ⁺) and negative (δ⁻) ends. These ends are known as dipoles.
Ionic Compounds: Because they are composed of ions, they have discretely positive and negative ends. The strength of their polarity is determined by the charge number of the ions.
Properties of Polar Substances
Polar bonds are generally stronger than nonpolar bonds.
Polar compounds have stronger intermolecular forces than nonpolar molecules due to electrostatic attraction. Therefore, they usually exhibit properties such as:
Higher melting/boiling points, requires more energy input to weaken these interactions.
Crystal lattice structures in their solid phases, oppositely-charged ends link in a way that maximizes attraction while minimizing repulsion.
Higher solubility in polar solvents due to their similar charge distribution.
More viscosity and surface tension, particles are more strongly attracted to each other than nonpolar compounds.
Drawing Dipole Moments on Molecules
Dipole arrows point towards the more electronegative atom.
The H-Cl bond in HCl is polar. The molecule itself is also polar.
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