Why does hydrogen bonds determine the water's special propeties?
why does covalent bonds determine the polarity of water?
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.
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.
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.