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why does covalent bonds determine the polarity of water?

Why does hydrogen bonds determine the water's special propeties?

Comments

Arthur,

Yeah, I should have looked up the angle instead of trusting my memory. But you do not understand that my description of the geometry of the water molecule is exactly equivalent to yours (except, of course, for the angle). I see a water molecule as 3 atoms in space, which are not collinear and, therefore, determine a plane and a triangle in that plane. If your picture of that triangle contains the two covalent bonds represented as lines, then it looks like a bent line as you describe it. However, I do not see the covalent bonds as lines connecting atoms but rather as sharing of electrons. The Tinker Toy model of a water molecule does not work for me.

By the way, the exact angle, as long as it is obtuse, plays no role at the level the questions that were presented. That's why I didn't bother to look it up.

My real concern about my answer is whether it is intelligible to Brenda. Without feedback I have no way to tell.

George

George,

I understand however the angle actually plays a role in explaining why there are excess charges at the oxygen and hydrogen ends of this molecule.  The angle allows the right amount for repulsion between the hydrogen atoms while minimizing the attraction of the hydrogen atoms by the oxygen atoms. The bond length itself is defined by this angle and thus the polarity. To put it simply, any angle larger than this or smaller (but neither is possible) would change the characteristics of the water molecule.  While I know this is far more than is required to make it intelligible it is key to understanding the water molecule.

Art

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4 Answers

Any chemical reaction involves an energy change or transfer. Atoms combine to form molecules, because the sharing or transferring of electrons result in a lower, and more stable energy state. That is, energy input from endothermic reactions are stored in the bonds; exothemic reactions release energy as heat and radiation; oxidation-reduciton reactions involves the interconversion of chemical and electrical energy to do work, etc.

                In a covalent bond, two nonmetals share electrons equally to fullfill their valence electron configurations. Hydrogen's 1s1 can hold one more electron, and oxygen's (1s22s22p4) 2p orbitals can hold two more electrons. Oxygen, however, is more electronegative than hydrogen; it pulls the shared electrons closer to itself, and away from the hydrogen. The result of the unequal sharing of electrons is a polar covalent bond, and a molecule that is dipolar, or a molecule that has a dipole momment. Due to the greater electron distribution around oxygen, it is the center of a partial negative charge. Hydrogen is the center of the partial positive charge.

In ionic bonding, a metal and nonmetal combine by a transfer of electrons to complete their outer shell. Generally, the more electrognegative nonmetals tend to gain electrons to form anions; whereas, metals tend to loose electrons easily, and become cations. There is no totally ionic character however, in any bond types. A transfer of electron is considered when the difference in electrogenativities between the bonded atoms are significantly large.

Furthermore, the polarity of the molecule is determined by its geometry, and the vector sum of the diploes. For example, CH3Cl (methyl chloride) is polar, but CCl4 is nonpolar. Thus, linear, trigonal planar, tetrahedral, etc., molecules with identical groups have dipoles which cancel, and are nonpolar. The geometry is determined by either the VSEPR model, or Molecular Orbital Model.

Brenda:
     You seem to have two questions:

1. Why is the water molecule polar?
2. How does hydrogen bonding give water special properties?

Answer to question 1: A water molecule contains 1 oxygen atom and 2 hydrogen atoms. The 3 atoms are not arranged in a straight line but rather in a triangle with the angle between the hydrogen atoms as seen from the oxygen atom being about 135°. Since the 6 outer electrons of the oxygen atom and the 2 electrons from the hydrogen atoms tend to hang around the oxygen nucleus, the oxygen corner of the triangle becomes more negatively charged and the hydrogen corners more positive. This gives the water molecule its polarity, negative toward the oxygen atom and positive opposite the oxygen atom.


Answer to question 2: A hydrogen bond is the electromagnetic attraction of a hydrogen atom with a negative pole of a neighboring atom. In water, the oxygen end of each water molecule is such a negative pole so it can form a hydrogen bond with the hydrogen atom of a neighboring water molecule. In fact, each water molecule can form a hydrogen bond with as many as 4 other water molecules. One consequence, among many, of this high number of hydrogen bonds is the high boiling point of water; it is difficult to break the hydrogen bonds so that a high temperature (lots of energy) is required for a water molecule to break away from the liquid water to become steam.

Comments

George,

Some misinformation has been supplied by you. First the orientation of water is a "bent" not triangle configuration as you described it. It is this bent formation that allows for the ease of rotation of the covalent bonds to form hydrogen bonds between water molecules or other species. Second, the angle between the oxygen and hydrogen is approximately 105°. No where near the 135° you metion.

Best Regards,

Art 

Brenda,

Water is said to be polar because there is a net negative charge at the Oxygen end of the molecule and a net positve charge at the hydrogens end of the molecule.  Water molecules therrefore behave like magnets in that they arrange themselves such that hydrogen bonds form between the hydrogen atoms of one molecule and the oxygen atoms of other molecules. While these are weak bonds they can hold a specific orientation and allow water to form hydrogen bonds with other species as welll.


Art

Comments

it is van der Waals forces that hold water molecules together not hydrogen bonds

Hydrogen covalently bonded to a very electronegative atom (like Oxygen) is also weakly bonded/attracted to an unshared electron pair of another electronegative atom. In water, this other electronegative atom is other oxygen parts of surrounding molecules. So, not only is the hydrogen bonded directly to its oxygen but it is also attracted to the surrounding oxygen of other water molecules. Try to go with me here for the diagram, it is just a basic input for typing/symbols so the spacing is not perfect. A and B are oxygen, the little x’s are hydrogen. Each water has one oxygen with two hydrogen to make the molecule. The x’s from A are directly bonded to A, but they are also attracted to B; the x’s from B are directly boned to B but are also attracted to A. x x A B x x Polar Molecule- Molecule that has a positive and a negative end (or pole) The more electronegative atom has a slightly negative pole and the less electronegative has a slightly positive pole. In water, oxygen is the negative pole, hydrogen the positive If you needed to draw a symbol showing the polarity you would have an arrow that pointed to the hydrogen away from the oxygen and place a (+) sign on the oxygen side of the arrow O +--------> H

Comments

pretty good description of intramolecular attraction due to polarity

hans