Water — Molecular Structure & Bonding

Physical Properties of Water

Water has a few features that make it one of the most versatile and interesting molecules on Earth. Water's unique physical and chemical properties are the result of its molecular structure and bonding.

Water Structure

A water molecule is made of one oxygen atom and two hydrogen atoms. The oxygen atom has six electrons, but it really wants to have eight. Both hydrogen atoms have one electron, but each of them wants to have two.

The oxygen shares one pair of electrons with the hydrogen atoms so that the oxygen has eight electrons and both hydrogens have two. By sharing electrons with each other, the oxygen and hydrogen atoms bond. This specific type of interaction is known as a covalent bond.

The oxygen atom still has two extra sets of electrons that nothing is bonded to (these are called lone pairs. Those poor electrons). These lone pairs push away the bonded electrons, which cause the water molecule's structure to look like a boomerang.

The Covalent and Hydrogen Bonding of Water

When atoms are joined together in a molecule and one of the atoms is really greedy for electrons, it will draw the electrons closer to itself. This is known as a polar covalent bond. Polarity is when one or more bonded atoms are bad at sharing their electrons. This causes an uneven distribution of negatively charged electrons across the molecule.

Water is a polar molecule, which means that it has a partial positive charge ($\delta$+) on the hydrogen atoms and a partial negative charge on the oxygen ($\delta$-). The partial charges on a water molecule are similar to a bar magnet, where oppositely charged poles repel each other but opposite poles attract.

Because of the uneven charge distribution, the partially positive hydrogen atoms can form a weak bond with other oxygen atoms. This is called a hydrogen bond.

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$\color{#8925AE}{\text{Polarity}}$ : Partial positive and negative charges caused by some atoms wanting electrons more than other atoms in a molecule

$\color{#8925AE}{\text{Hydrogen Bond}}$ : Positively charged hydrogens of one molecule are attracted to the negative charge on another molecule

Polarity and Hydrogen Bonding

Water is so unique and so versatile because it is a polar molecule (it has polarity). Polarity is when there is an unequal sharing of electrons between atoms that causes a difference in molecular charge across the water molecule.

This difference in charge is known as a dipole. In water, the larger oxygen is greedy and wants electrons more than hydrogen does, so it holds them tighter. Because electrons are negative, it results in a partial negative charge on the oxygen atom and a partial positive charge on hydrogen.

Water's most important characteristic that arises due to its polarity is its ability to form hydrogen bonds.

Hydrogen Bonding: This occurs when the partial positive hydrogens of the water molecule are attracted to and form a weak bond with the partial negative end of the oxygen from a different water molecule. This weak bonding has many important effects necessary for life to exist.

(Video) How Polarity Makes Water Behave Strangely

by Christina Kleinberg on TED-Ed

Many of the interesting and life-supporting properties of water are due to the fact that water is partially polar. Water is drawn to other water molecules by hydrogen bonds. Hydrogen bonds create phenomena like surface tension, water's high boiling point, and ice (the solid phase of water) being less dense than liquid water.