Polarity can be described as a separation of charge leading to a molecule or its chemical groups having an electric dipole moment with a negatively charged end and a positively charged end. In other words, bond or the molecular polarities depend upon the electro-negativities of the atoms or the molecules. A molecule is basically said to be either a polar molecule, nonpolar molecule or ionic molecule.
What Are Polar Molecules?
A polar molecule is a molecule in which one end of the molecule is slightly positive while the other end is slightly negative. A polar molecule has an asymmetric shape, lone electron pair or central atom bonded to other atoms with different electronegativity values. Usually, a polar molecule contains ionic or polar covalent bonds.
In a polar molecule, the more electronegative atom will have a higher density of electrons near it. Since electrons carry a negative charge, this atom will also have a partial negative charge on it. In turn, the other atom will have a lower electron density around it, so it will have a partial positive charge. The partial charges create a dipole, which is a separation of charges between the two atoms. The greater the difference in electro-negativities, the greater the dipole and the more polar the molecule is.
Polar molecules orient themselves in the presence of an electric field with the positive ends of the molecules being attracted to the negative plate while the negative ends of the molecules are attracted to the positive plate.
Polar molecules are often hydrophilic and soluble in polar solvents. They also have higher melting and boiling points than nonpolar molecules with similar molar masses generally due to strong intermolecular forces between the molecules. Examples of polar molecules include:
- Water (H2O)
- Ammonia (NH3)
- Sulfur dioxide (H2S)
- Carbon monoxide (CO)
- Ozone (O3)
- Hydrofluoric acid (HF)
- Ethanol (C2H6O)
- Sucrose (C12H22O11)
What You Need To Know About Polar Molecules
- Polar molecules will have an excess charge due to the imbalance of the electronegativity of the atoms forming the bond that creates a difference of charge in the poles of molecules.
- A polar molecule has a dipole where part of the molecule has a partial positive charge and part has a partial negative charge.
- A polar molecule has an asymmetric shape, Ione electron pair or central atom bonded to other atoms with different electronegativity values.
- Polar molecules contain ionic or polar covalent bonds.
- Polar molecules are often hydrophilic and soluble in polar solvents.
- A polar molecule creates a dipole-dipole intermolecular force. A dipole is the separation of positive and negative electric charge.
- The polarization of polar molecules is highly temperature dependent.
- Polar molecules often have higher melting points than non-polar molecules with similar molar masses. This is due to intermolecular forces between polar molecules such as hydrogen bonding.
- Electronegativity difference between atoms is <0.4.
- Examples include water, HF and CHF3.
- Polar molecules interact through hydrogen bonds and dipole-dipole intermolecular forces. The charges attract the other molecules and also will attract other polar molecules of different substances.
What Are Non-polar Molecules?
Nonpolar molecules are those that do not possess regions of positive and negative charge. In other words, the electrical charges of nonpolar molecules are evenly distributed across the molecule. Nonpolar molecules form either when electrons are equally shared between atoms in a molecule or when the arrangement of electrons in a molecule is asymmetrical so that dipole charges cancel each other out.
Nonpolar molecules are water insoluble at room temperature; they tend to dissolve well in nonpolar solvents which are frequently organic solvents. Nonpolar molecules have a low boiling point, melting point, high vapor pressure and low surface tension.
Typically, the only completely nonpolar molecules consist of a single type of atom or of different types of atoms that display a certain spatial arrangement. Many molecules are intermediate, neither completely Nonpolar nor polar.
Examples of Nonpolar molecules include:
- Any of the noble gases (He, Ne, Ar, Kr, Xe, though technically these are referred to as atoms and not molecules)
- Any of the homonuclear diatomic elements: H2, N2, O2, Cl2 (These are truly nonpolar molecules).
- Carbon dioxide –CO2
- Boron trifluoride CC4
- Hydrocarbon liquids such as gasoline and toluene
- Most organic molecules with exceptions (like alcohols and sugars)
What You Need To Know About Non-polar Molecules
- Non-polar molecules will be neutral, due to a balance of electronegativity of the atoms.
- Nonpolar molecules form either when electrons are equally shared between atoms in a molecule or when the arrangement of electrons in a molecule is asymmetrical so that dipole charges cancel each other out.
- Nonpolar molecules tend to be water insoluble at room temperature, hydrophobic and able to dissolve other nonpolar compounds.
- Nonpolar molecules have a low boiling point, melting point, high vapor pressure and low surface tension.
- The polarization in this type of molecules is independent of temperature.
- Electronegativity difference between atoms is >0.4.
- Examples include: Pentane, Hexane, helium, oxygen, gasoline, neon, krypton, hydrogen and carbon dioxide.
Difference Between Polar And Non-polar Molecules In Tabular Form
|BASIS OF COMPARISON||POLAR MOLECULES||NON-POLAR MOLECULES|
|Shape||Polar molecules are asymmetrical in shape.||The Nonpolar molecules are symmetrical in shape.|
|Electric poles||The polar molecules have electrical poles.||The Nonpolar molecules do not have electrical poles.|
|Poles||In polar molecules, one end of the molecule is positive while there is a negative charge on the other end.||There is no profusion of charges on opposite ends of non-polar molecules.|
|Bonds||Hydrogen bonds occur in polar molecules.||The Van Waal interactions amongst the Nonpolar bonds.|
|Nonpolar Covalent||Minimum one polar covalent in polar bonds is present in all polar molecules.||There is no Nonpolar covalent in all Nonpolar molecules.|
|Charge Separation||The polar bond has charge separation.||There is no charge separation in the non-polar molecules.|
|Dipole moment||Has dipole moment.||Has no dipole moment.|
|Surface Tension, Boiling And Melting Point||It has high surface tension, melting point & boiling point.||It has low surface tension, melting and boiling point.|
|Interaction With Other Molecules||It interacts with other polar molecules.||It does not interact with other Nonpolar molecules.|
|Dielectrics In External Electric Field|
|Examples||Water, ammonia and ethanol||Oil, benzene, methane.|