# Covalent Bond/Hydrogen Bond

During the formation of a molecule of water ($\ce{H2O}$), what kind of bond occurs between the elements? I'm studying this subject on my own and from some sources I saw that there is a covalent bond and others showed an hydrogen bond, responsible for the non-evaporation of water at $-90\ ^{\circ} \rm{C}$. Can you guys explain to me exactly how this bonds behave? Both of them occurs to the molecules of water?

• Welcome to Chemistry.SE! This looks like an interesting question. We have some tools available to help make chemistry and maths look pretty in your posts. I edited your question to put them in. Check out ourfaq for more info. – Ben Norris Mar 7 '13 at 2:57

Covalent bonds are the bonds between the atoms within the same water molecule. Hydrogen bonds are the bonds between two water molecules. All molecules have covalent bonds, but only some molecules have hydrogen bonds. As an example, water has hydrogen bonds, but carbon dioxide does not.

One of the requirements for hydrogen bonding is that the molecule must be polar. Water molecules are polar because of two effects. The first is that oxygen is more electronegative than hydrogen, so each of the OH bonds will be polar. The electrons shared in each of those bonds will spend more time near the oxygen than near the hydrogen. The other effect is the geometry of the molecule: the oxygen atom has two free electron pairs in addition to the two bonds, so it assumes a tetrahedral arrangement. The $\ce{O-H}$ bonds are two corners of that tetrahedron, and the free electron pairs are the other two corners. This arrangement means that, if you only look at the atoms, the molecule has a bent geometry. Because of that, the polarity of the two bonds partially adds together. By contrast, $\ce{CO2}$ has a linear geometry, and the two polar bonds cancel each other out, leaving the molecule non-polar.

So the water molecule is polar, with a lot of excess positive charge around the hydrogen atoms, and a lot of negative charge on the side of the oxygen atom away from the hydrogen atoms. Hydrogen bond is the name given to the electrostatic interaction between the positive charge on a hydrogen atom and the negative charge on the oxygen atom of a neighboring molecule. The covalent bond is the electrostatic interaction between two atoms in the same molecule. Covalent bonds are much stronger than hydrogen bonds: the $\ce{O-H}$ has a strength of 467 kJ/mol, while the hydrogen bond is usually between 4 to 40 kJ/mol.

Just want to point out that there is a subtlety to the word "water":

Water can mean the molecule, $\ce{H2O}$, and those bonds (between central O and H's) may be covalent.

Water also means the bulk water, many molecules of water. The interactions between two or more molecules is what dictates the bulk properties of a liquid, such as evaporation and freezing. Here we would talk about hydrogen bonding.

Below is a picture of bonding of one configuration of "water" in "water." This is computed from electronic structure theory, and the Quantum Theory of Atoms in Molecules:

• Does this answer the question? – Jan Sep 25 '15 at 13:59

In H2O molecule, two water molecules are bonded by a Hydrogen bond but the bond between two H - O bonds within a water molecule are covalent.

Refer this diagram : http://goo.gl/84w9T

The dotted lines represent a hydrogen bond and the solid lines represent a covalent bond.

Water is a polar covalent molecule. Take a look at their electronegativity values, Hydrogen is 2.20 and Oxygen is 3.44. This is a difference of 1.24 meaning that it is Polar Covalent. A Covalent bond is present when there is a difference 1.7 +/- 0.5. Also there is Hydrogen Bonding in water also, this is because the hyrdogens will have a slightly psotove charge and the oxygen will have a slightly negative charge, which causes hydrogens in other water molecules to become attracted to the oxygens. I hope this helps