Polarity is the distribution of electric charge between atoms connected by a chemical bond. This property of molecules is commonly discussed in terms of dipole moments, geometry, polar and non-polar. Polarity is also a periodic trend that shows specific patterns on the periodic table.
Polarity is a property that determines how a molecule will interact with other molecules. It is the explanation behind “polar solutes dissolve in polar solvents and non-polar solutes will dissolve in non-polar solvents”.
Just like other periodic trends, polarity also influences other properties of molecules and elements. This article focuses on the periodic trend of polarity on the periodic table, its relationship with other trends, and its influence on other properties of molecules.
What is the trend for polarity on the periodic table?
Polarity increases from left to right across the periods and decreases from top to bottom down the groups on the periodic table. Polarity depends on the position of the element on the periodic table and its electronegativity.
Elements on the right side of the periodic table tend to be more electronegative and form more polar covalent bonds than elements on the left side. It is the opposite from top to bottom in groups.
This is the explanation for fluorine being the most electronegative halogen. Also, it is why compounds that contain chlorine and iodine have less polar bonds.
Polar and non-polar bonds
Polar molecules and bonds
A polar molecule is a molecule with positive and negative charges on opposite ends, creating an electrical pole. A polar bond exists in two atoms of different electronegativities.
In a polar molecule, the two atoms share electrons, and the more electronegative atom attracts the shared electrons more strongly than the electropositive atom. This results in an unequal sharing of electrons and a distribution of partial negative and partial positive charges.
The partial positive and partial negative charges create a separation of electric charge within the bond and form a dipole.
Non-polar molecules and bonds
Non-polar molecules and bonds are the exact opposite of polar molecules and bonds. Instead of partial charges at opposite ends, the electrons are distributed equally.
Also, there is a symmetrical arrangement of bonds (which may be polar), which cancel out each other and make the overall molecule non-polar.
Is polarity the same as ionic character?
Polarity and ionic character are usually seen in the same context to describe the distribution of electrons in a chemical bond. However, they are not the same.
Polarity is peculiar to covalent molecules. A covalent bond is said to be polar when there is an unequal sharing of electrons between the atoms that share the bonds. This results in a polar molecule with partial positive and partial negative charges on the atoms.
A good example of a polar molecule with partial positive and negative charges is water (H2O). The oxygen atoms have partial negative charges while the hydrogen atom has a partial positive charge.
On the other hand, ionic character describes the degree of ionic bonding in a compound. When electrons are transferred from one atom to another, they form ions with positive and negative charges.
For example, in the formation of sodium chloride (NaCl), sodium donates an electron to chlorine, and this results in the formation of Na+ and Cl− ions.
In essence, polarity describes the sharing of electrons between atoms, while ionic character refers to electron sharing between atoms.
How does electronegativity affect polarity?
Electronegativity describes the ability of an atom to attract electrons. The polarity of a chemical bond is determined by the electronegativity of the atoms in the bond.
The difference in electronegativity of atoms in a chemical bond is a major determiner of the polarity of the bond.
When two atoms with significant electronegativity values form covalent chemical bonds, the atom with the higher value will attract more strongly, resulting in an unequal sharing of electrons in the bond.
Furthermore, when the difference in electronegativity is low, electron distribution is almost equal, and the covalent bonds are non-polar.
On the other hand, when the electronegativity difference is high, there is a stronger electron pull towards the more electronegative atom. As a result, the electron distribution is more unequal, creating a polar covalent bond.
In addition, the Pauling electronegativity scale makes quantitative measurements of polarity easier. The greater the electronegativity difference, the more polar the bond.
What other factors explain bond polarity?
Molecular geometry
Molecular geometry describes the overall shape of a molecule. The shape of a molecule can affect its polarity. Molecular geometry influences the distribution of charges, which in turn determines polarity.
For instance, the individual bonds in tetrahedral structures are polar. But these structures have a symmetrical distribution of charge which means that the polar bonds cancel out each other and make the molecule non-polar.
Dipole moment
Dipole moment uses bond polarity and molecular geometry to measure the magnitude and direction of charge separation in a molecule.
The individual bond dipole moments and their arrangement within the molecule also determine the overall dipole moment. In non-polar molecules, bond dipoles cancel out each other. On the other hand, they do not cancel out each other in polar molecules.
Resonance structures
This factor does not determine polarity for all molecules, only in molecules that exhibit resonance. Resonance occurs when a molecule can be drawn in multiple valid Lewis structures. The actual structure, the most valid, is usually a hybrid of all other structures.
Resonance produces polarity by the delocalization of π-bonds. This effect is a result of the interaction between two π-bonds or one π-bond and the lone pair of electrons on an adjacent atom.
Bond length
Bond length also influences polarity. The shorter the bonds, the less polar they are. In chemical compounds with longer bonds, the electrons are farther from the nuclei, which makes them more polar.
Presence of multiple bonds
Multiple bonds – double and triple – are more polar than single bonds. The formation of these bonds requires the sharing of more electrons between the atoms.
The degree of electron sharing determines polarity. Triple bonds are more polar than double bonds and double bonds are more polar than single bonds.
Does polarity affect the physical properties of molecules?
Yes, polarity affects the physical properties of molecules. The effects of polarity on some of these properties are discussed below:
Melting and boiling point
In a comparison of polar and non-polar molecules of similar sizes and molecular weights, the polar molecules have higher melting and boiling points than the non-polar molecules.
Polar molecules have dipoles which makes them have stronger attractive forces and higher melting and boiling points.
Density
The more polar a molecule is, the denser it is. Also, polar molecules are denser than non-polar molecules of similar size.
Viscosity
Polar molecules have strong intermolecular attractions and are highly resistant to flow. Therefore, the more polar a molecule is, the more viscous it is.
Solubility
The polarity of molecules is crucial to solubility. The degree of polarity determines the degree of solubility of the molecule. This is why polar solutes will dissolve well in polar solvents and non-polar solutes will dissolve well in non-polar solvents.
Intermolecular forces
The type and strength of the intermolecular forces that hold molecules together are determined by polarity. Polar molecules have strong forces like hydrogen bonding while non-polar molecules form weak induced dipole forces.
Surface tension
Polar molecules have strong intermolecular forces which also means higher surface tension than non-polar molecules.
Dielectric constant
Polar molecules have a higher dielectric constant. When they enter an electric field, they can align their dipoles to store electrical energy.
Electrical conductivity
Polarity also determines electrical conductivity. Ionic molecules are highly polar and are good conductors of electricity when dissolved in water. Non-polar molecules, on the other hand, are poor conductors of electricity.
Does polarity affect the chemical properties of molecules?
Polarity can also determine some chemical properties, such as:
Reactivity
Polar molecules usually have ions of opposite charges or partial positive and partial negative charges (like water). These molecules readily participate in reactions that involve the breaking of their bonds and the formation of new bonds. This is the opposite in non-polar molecules.
Acid-base reactions
Non-polar molecules cannot participate in acid-base reactions because they do not have regions of partial positive and partial negative charge. Polar molecules have both acid and base properties because they have these regions of partial positive and partial negative charge.
Electrophilic and nucleophilic reactions
Electrophiles and nucleophiles are chemical species that donate or accept electrons to form new chemical bonds. Electrophiles are electron-deficient species and nucleophiles are electron-rich species. They are naturally attracted to each other and form bonds.
Polarity influences this behavior of molecules. Polar molecules have both electrophilic and nucleophilic sites and can participate in these reactions, unlike non-polar molecules.
FAQs
Is bond polarity a physical or chemical property?
Bond polarity is a physical property. It is determined by the interaction of the bonds between molecules and atoms with different electronegativities.
Polarity, in turn, affects the physical properties of the molecules and atoms. For instance, molecules with higher polarity have stronger intermolecular forces between them because of the partial distribution of charges.
These molecules have higher boiling points and lower vapor pressure. They are also more soluble in polar solvents.
Can polar bonds form non-polar molecules?
Polar bonds can form a non-polar molecule. This happens when the polar bonds in the molecule cancel out each other. Cancellation of dipole moments of polar bonds occurs in molecules with symmetrical geometry.
For example, carbon tetrachloride, CCl4. This molecule is non-polar, but all its four bonds are polar.
Can polar bonds be ionic?
Ionic bonds are polar bonds. These bonds form during electron transfer from one atom to another, resulting in the formation of ions.
All ionic bonds are polar, but not all polar bonds are ionic. Polar bonds may also be covalent if there is an unequal sharing of electrons.
Conclusion
In summary, polarity increases with increasing electronegativity. That is, polarity will also decrease as you go down groups and increase from left to right across periods on the periodic table.
Polarity is not one of the known periodic trends, but it largely influences the physical and chemical properties of molecules. Some of these properties are melting and boiling points, solubility, intermolecular attraction, and reactivity.
The uneven distribution of charges makes the molecule more likely to interact with other polar molecules. This is the explanation for why polar solvents will dissolve polar molecules.
Additionally, on periodic trends, learn why ionization energy increases across periods and the factors that affect this trend.
Thanks for reading.