- Water is a polar molecule because it has an unequal sharing of electrons.
- A water molecule has one oxygen (negative charge) and two hydrogen atoms (positive charge).
- The difference between the atoms’ electronegativity and their spatial arrangement makes water polar.
The term ‘polarity’ is often used in electricity and magnetism. In chemistry, it refers to the distribution of electric charge(s) around molecules, atoms, or chemical groups.
To know whether water is polar or nonpolar, we need to first understand the two important groups of molecules: polar molecules and nonpolar molecules. While most of the molecules in the world have some polarity, a few are completely polar or nonpolar.
When two atoms don’t share electrons evenly in a covalent bond, they give rise to polar molecules. One portion of the molecule carries a tiny negative charge while the other portion carries a tiny positive charge.
Polar molecules form when their atoms have different electronegativity. Each atom has a specific electronegativity, which defines its ability to gain electrons and form negative ions in chemical reactions.
A lesser difference in atoms’ electronegativity gives rise to a polar covalent bond, while a huge difference forms an ionic bond. In other words, polar covalent bond forms when one atom attracts more electrons, whereas ionic bond forms when the atom with greater electronegativity strips the electrons from another atom and not shares them at all.
So polarity describes how different the electrical poles of molecules are. Ammonia (NH3), Hydrogen sulfide (H2S), Sulfur dioxide (SO2) are some examples of polar molecules.
Nonpolar molecules form when their atoms share electrons equally in a covalent bond, and there is a zero electrical charge across the molecule. All electrons in a nonpolar covalent bond are equally spread.
This happens when two atoms with the same or similar electronegativity form a bond. Nonpolar molecules can also form when their atoms share a polar bond that is arranged in such a manner that their electric charges cancel each other out.
Carbon dioxide (CO2), Methane(CH4), Benzene (C6H6) are some of the most common examples of nonpolar molecules.
How To Know Polarity Of A Molecule?
There is an easy method to predict whether a molecule is polar or nonpolar; you need to look up electronegativity on the periodic table.
If the difference between the two atoms’ electronegativity is more than 2.0, the atoms form an ionic bond, and ionic molecules are highly polar. If the difference lies between 0.5 and 2.0, the molecule is fairly polar.
If the difference between atoms’ electronegativity is less than 0.5, the atoms form a nonpolar bond. Only identical atoms form completely nonpolar molecules.
Now back to the original question: Is water polar or nonpolar? Water (H2O) is a polar molecule since it has an unequal sharing of electrons.
Why Is Water A Polar Molecule?
The molecule of water consists of one oxygen and two hydrogen atoms. Since oxygen is a highly electronegative atom, it pulls pair of electrons (from two hydrogen atoms) towards itself, thus acquiring a minute negative charge. The hydrogen, on the other hand, acquires a partial positive charge.
The electronegative value of oxygen and hydrogen is 3.44 and 2.20, respectively.
However, this is not the only factor that determines the polarity of the molecule. The other major factor is the spatial arrangement of atoms — how atoms within molecules are arranged around the central atom.
To better understand this concept, let’s look at the structural formula of carbon dioxide.
It contains two equally electronegative oxygen atoms, both standing at 180-degree angles from carbon. Since they pull electrons from carbon with equal force in opposite directions, the net density on the carbon remains unaffected.
Both ends of the molecule carry a slight negative charge while the carbon atom at the middle carries a slight positive charge, making carbon dioxide molecule nonpolar.
Now Let’s take a look at a water molecule.
In a water molecule, oxygen shares an electron with each hydrogen atom. Since oxygen atom contains 6 electrons in its outermost shell, the four remains unbonded. These bonded and unbonded electron pairs rearrange themselves in a tetrahedral structure.
Since electronegativity of oxygen is more than that of hydrogen, it pulls electrons towards it. Thus, the area around the oxygen becomes more negative compared to the regions around the two hydrogen atoms. This makes water a polar molecule.
Reference: Northern Arizona University | London South Bank University
Note: Although both covalent bonds between oxygen and hydrogen in water are polar, the overall molecule is electrically neutral. Every water molecule contains 10 electrons and 10 protons, for a net charge of zero.
Polarity Of Water Makes It A Special Chemical Compound
Since water is a polar molecule, it can form electrostatic interactions (charge-based attraction) with other polar molecules and ions. And since water is attracted to either the negative or positive electrical charge on a solute, it acts as a polar solvent. This means it has an incredible ability to dissolve substances.
The temporary hydrogen bonds between different water molecules give it interesting properties. It allows water to store substantial heat energy, or have high heat capacity.
Although it takes a lot of energy to heat water, it remains warm for a longer time than most liquid compounds. This makes life possible on Earth, as water carries heat to regions that receive little sunlight.
In addition to possessing strong bonds and impressive dissolving capability, water also has unique density features. When frozen, water molecules are farther apart yet strongly connected to each other via hydrogen bonds.
The density of water increases as temperature goes down but only up to 4°C. Below this point, the density decreases, and when the temperature reaches 0°C, it gets lighter than liquid water. This is why ice floats in water (which supports marine life).