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Name: Megan
Status: student
Age: 13
Location: N/A
Country: N/A
Date: 2000


Question:
How does hydrogen and oxygen bond together to form water? Two non-metals combining to form a liquid.


Replies:
Both elements, hydrogen [H2] and oxygen [O2], exist as diatomic molecules. Envision that we break the bond in each of these molecules somehow to give 2 atoms of hydrogen [H], and 2 atoms of oxygen [O]. Each [H] atom has a single electron, and each [O] atom has 8 electrons.

The hydrogen and oxygen atoms can "share" electrons, one each, to form new bonds between hydrogen and oxygen atoms. However, this sharing obeys certain rules that determine how many [H] atoms and how many [O] atoms can share electrons to form new bonds, and dictate what the H--O--H angle will be. In this case the two rules are:

1. Hydrogen can share only its single electron, and oxygen can share two of its 8 electrons.

2. The water molecule [H2O] is not linear. Rather the H--O--H angle is ~104 degrees.

When atoms share electrons, those electrons "spend more time" between the atoms and "spend less time" on "the other side" of the respective nucleii. This means that the oxygen atom still has a high concentration of electrons (negative charge) surrounding its more positively charged nucleus, but the hydrogen atom, having no other electrons remaining after it shares its sole electron with the oxygen atom, leaves its proton nucleus with its positive charge, "exposed". Molecules that have an asymmetrical distribution of charge are called "polar". This "exposed" positively charged ends can form weak, temporary linkages with the negative ends of surrounding water molecules. These weaker bonds are called "hydrogen bonds" because they only form when [H] atoms form asymmetrical polar bonds.

These hydrogen bonds form a 3 dimensional network. It is this network that gives water its unique properties, such as the fact that it boils at 100 C, and freezes at 0 C, while non-polar compounds of similar mass, for example methane [CH4], boil and freeze at a much lower temperatures -- for methane B.P.= -161C and freezing point = - 183 C.

Vince Calder



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