Ask A Scientist© Chemistry Archive

name         Megan
status       student
age          13

Question -   How does hydrogen and oxygen bond together to form
water?  Two non-metals combining to form a liquid.
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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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Hi Megan!
The concept of metals and non-metals is not considered
nowadays a really correct one. The elements can be classified following
the trends of their eletronic structure.
Atoms can combine in different ways. Some that are quite different in
eletronic
structure ( are in opposite ends of the periodic table)  react by the
complete
transfer of an electron from one atom to another (forming an ionic bond) .
Others that have "similar" eletronic structure can combine by sharing one
or more pairs of electrons ( forming a covalent bond). That is a simple
way to state things. even in nature things are not always simple as that.
The water molecule has a molecular formula of H2O.The oxigen tend to
share 2 pairs of electrons; but an hydrogen atom only tends to share one
pair
of electrons, therefore an oxigen atom must bond with 2 hydrogen atoms,
both completing the 8 electrons required by the octet rule.
I hope all this is not too complicated for you. If my answer (or other
one done by a fellow scientist) doesn't satisfies you, ask again and
we can try to give a more clearer answer.
Ad thanks to ask NEWTON!
Mabel
(Dr. Mabel Rodrigues)
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Your question: "What is the biggest obstacle in creating water out of the
abundance of Hydrogen and Oxygen?" needs some clarification.
     First, let us put aside an irrelevant issue:
In the string of communications with NEWTON BBS you make reference to
"hydrogen bonding", specifically your quote is: "I have been reading various
papers on the web about hydrogen bonding." Hydrogen bonding refers to the
special association of hydrogen atoms in polar molecules with electrons in
neighboring molecules. While it is important as a chemical concept, it has
no bearing on the issues you seem to be articulating about the utility and
economics of the direct combustion of molecular hydrogen (H2) and oxygen
(O2) as an energy source. Specifically, if I interpret your inquiry
properly, you want a prioritized list of why this is challenging. Assuming
my interpretation is correct here goes:
     1. While oxygen (O2) forms about 20% of the atmosphere, hydrogen is
present only in trace amounts. So the use of hydrogen (H2) as a fuel
requires somehow manufacturing it. In assessing the utility of  H2 one needs
to do a complete manufacturing cost analysis. It makes little sense to use
H2 if the cost of producing it greatly exceeds the benefits of using it as a
fuel. Experimental work studying the use of H2 as a fuel is certainly worthy
research, but at present there is no economical way to produce it on a large
scale.
So at present the economics are not favorable.
     2. Hydrogen gas under high pressure that would be required for its use
as a fuel is not easy to handle. It is explosive even when present in very
small amounts (above about 1% or so in air). Static electricity will cause
it to detonate, so fail-safe storage and transfer systems need to be
engineered. The engineering is known, but the necessary procedures and
protocols not cost effective for consumer or vehicular use.
     3. The protagonists of hydrogen fuel, and the media, propose that since
water is the only reaction product (that is not quite true, see #4) H2 is a
pollution-free fuel. What is ignored is the fact that water vapor IS A
GREENHOUSE GAS. It absorbs and emits infrared radiation (heat) and of course
forms clouds in the atmosphere if present in large amounts. The addition of
large amounts of water vapor into the atmosphere from the wide use of H2 as
a fuel needs to be assessed. It is not free of potentially bad consequences.
     4. Assuming that the source of oxygen (O2) for the combustion of
hydrogen (H2) is air, nitrogen gas (N2) will be present in the combustion
chamber however it is designed. The flame temperature of hydrogen and oxygen
is very high, and under the useful combustion conditions some nitrogen
oxides (called NOx) will be formed. If hydrogen is to be a useful clean fuel
using air as the oxidant these nitrogen oxides will have to be destroyed
just as they are now in automotive exhaust pipes. So the nitrogen oxide
problem remains.
     This list of reasons why hydrogen in not a panacea for air pollution is
not complete, but it calls attention to enough issues that one needs to be
circumspect about promoting hydrogen as a pollution free, cheap fuel for
general commercial and consumer use.

Vince Calder
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