Structure of a Water Molecule

Water is the basis of life on Earth. It is everywhere around us and covers almost three-quarters of the Earth’s surface. In famous poems like The Rime of the Ancient Mariner, it has been talked about its importance in ensuring that we are alive. Water makes up over half of the human body.

At its core, this life-giving compound that we easily find anywhere we want is a very special molecule with a unique structure. The structure of the water molecule is one of the first structures that we learn about. It is simple yet fascinating. Water has both usual and some unusual properties, but all these properties can be explained when we look at the structure of water molecules.

How is a water molecule constituted?

Water is one of the two oxides that are formed when hydrogen reacts with oxygen, the other being hydrogen peroxide. H2O is a compound that is formed when hydrogen reacts with oxygen. The most common way in which water can be formed is by combustion. 

The structure of the water molecule consists of three atoms, two hydrogens, and one oxygen. The bonds that are formed between the hydrogen atom and the oxygen atom are polar in nature due to the high electronegativity of oxygen. The high electronegativity means that the oxygen molecule has an innate tendency to attract the shared pair of electrons strongly when in a covalent bond. 

We first need to see the type of bond that hydrogen and oxygen form to understand this. Hydrogen has a valency of 1 and therefore needs another 1 electron to complete a filled shell and become stable. On the other hand, Oxygen has 6 valence electrons in its outermost shell and therefore has a valency of 2 since it requires 8 electrons to reach a filled shell and achieve stability. 

Therefore, oxygen bonds with two hydrogen atoms. Hydrogen atoms share their electrons with oxygen to create filled shells for all three atoms. This gives rise to a covalent bond.  A covalent bond is one in which the electrons are shared by the two atoms bonded to each other. 

Nature of the bonds in a Water molecule

The structure of the water molecule consists of two hydrogen atoms that are bonded to a single oxygen atom. The hydrogen and oxygen atoms are bonded by a covalent bond created by sharing a single pair of electrons. The oxygen atom has a higher electronegativity compared to the hydrogen atom, which means that the electrons in the covalent bond are attracted more towards the oxygen atom than the hydrogen atom.

This creates a disbalance in the charge on these molecules. Due to the electrons being farther away from the hydrogen atom, both the hydrogen atoms in the structure of the water molecule acquire a partial positive charge denoted by +. This means that the hydrogen atoms behave like positively charged atoms now.

Similarly, since the electron pairs are closer to the oxygen atom, the oxygen atom gains a partial negative charge – on it. The tendency of the oxygen atom goes from being neutral to behaving like a negatively charged atom. These partial charges that develop on the atoms give a shape to the water molecule.

The shape of the water molecule

The ideal shape of the water molecule was supposed to be a linear molecule with oxygen in the center flanked by a hydrogen atom on both sides, but slight variations occur to this shape. It is common knowledge that like and like repel, while opposites attract. Similarly, the partial positive charges on the hydrogen atoms push them away from each other. And the partial negative charge on the oxygen atom attracts the hydrogen atoms towards the oxygen atom.

An added influence is the two lone pairs of electrons that the oxygen atom has. After forming two covalent bonds with the hydrogen atoms, four electrons are left in the valence shell of oxygen that forms two lone pairs of electrons. These lone pairs of electrons repel each other. 

The repulsion of these lone pairs is greater than the repulsion between the hydrogen atoms due to their partial charges. The result is that the H–O–H bond gets pushed inwards to create a bent shape. The angle between the H–O–H bond is 105 degrees, which is a deviation from the ideal 180 degrees. 

The bent shape ensures that the O–H bonds do not cancel each other out since they both are polar. This makes the entire structure of water molecules polar in nature. When the water molecule is viewed as a whole, the negative polarity of the molecule is the oxygen atom, whereas the positive polarity of the molecule is the common area that is flanked by the two hydrogen atoms. 

The surface tension of water

One of the most fascinating properties of the water molecule structure is the high surface tension that it results in. It is a fact that water molecules have a strong attraction between them. And the reason for this is the special structure of the water molecules. The oxygen atom has two lone pairs of electrons that allow the formation of hydrogen bonds with the hydrogen of other water molecules. 

Hydrogen bonds are special bonds formed between hydrogen and oxygen, fluorine, chlorine, and other highly electronegative atoms. A hydrogen bond is not a true chemical bond but rather a dipole-dipole interaction that causes the water molecule to have high affinity towards each other, resulting in high surface tension. 

Conclusion

The structure of a water molecule is a combination of three atoms, out of which two are hydrogen atoms, and one is an oxygen atom. There are partial negative and positive charges on the oxygen and hydrogen atoms, respectively. The two lone pairs of electrons on the oxygen atom cause the water molecule to have a bent shape. The special bent shape of the water molecule causes it to have a high affinity towards other water molecules, resulting in high surface tension of the water.