|Why is it that we have two hydrogen bonding with
one oxygen and not four?|
|Question Date: 2012-12-12|
The fact that water has two hydrogens
attached to a single oxygen is determined by the
number of bonds that an oxygen atom can form. As
you may know, all atoms are comprised of a small
nucleus surrounded by 'cloud' of electrons.
While these electrons don't orbit the nucleus
like the planets orbit the sun, they do tend to
spend more time at certain distances from that
central spot. These distances depend on the
energy of the electrons (how 'fast' they're
travelling around the nucleus). The most
energetic, so furthest away, electrons are
called "valence" electrons and these are the
electrons that interact with other atoms. An
oxygen atom has 6 valence electrons. As it turns
out, it doesn't cost much energy to add up to 8
electrons to an atom like oxygen's valence
shell, but adding a 9th electron will cost a
great of energy (you'll learn why if you study
chemistry/physics in college) so the oxygen atom
has space for two more electrons before they get
very hard to add. The hydrogen atoms provide
these electrons when water forms. Adding more
hydrogen on top of H2O would require
those extra electrons from hydrogen to occupy
much higher energy levels than is typically
available. If H4O were ever to form,
it would spontaneously decompose into
H2 + H2O because the
energy of an H2 molecule is much
lower than those extra two O-H bonds.
Hope this helps!
The proton has a +1 charge, oxygen has -2.
Hence to make neutral molecule H2O
you need 2x(+2) +1x(-2)=0
Atoms are made up a nucleus, which contains
protons and neutrons, as well as electrons,
which form shells around this nucleus. In each
electron shell, there are different numbers of
electrons. The first shell can only fit two
electrons, and the second shell fits eight.
Oxygen, number 8 in the periodic table, has
eight electrons, 2 in the first shell and 6 in
the second. Hydrogen only has one electron which
is in the first shell.
Oxygen and hydrogen share one electron when
they bond. By bonding with two hydrogen atoms,
oxygen has its 6 outer shell electrons + 2
shared electrons from the hydrogen making 8 (and
a filled shell makes a happy atom. Each of the
hydrogen atoms has their 1 electron + 1 shared
electron to fill their shell with 2
If oxygen bonds with 4 hydrogen atoms, it
would have too many electrons to fit into its
Hope that helps.
Oxygen needs two electrons to fill its
outermost electron shell. Hydrogen has one
electron to play with. Therefore, an oxygen atom
needs two hydrogens to bond to in order to
complete its shell.
Oxygen atoms (O) try to find 2 electrons on
other atoms that the other atoms will share with
them. So 2 hydrogen atoms (H) each share their
1 electron with 1 oxygen atom. That makes 1
molecule of water, H2O.
Different kinds of atoms try to share different
numbers of electrons with other atoms. Some
kinds of atoms have electrons to share, and
other kinds of atoms are trying to find more
electrons. So when 1 carbon atom (C) makes 1
molecule with hydrogen atoms, it needs 4
hydrogen atoms, and the molecule is
CH4, which is methane.
In the same way that you have two hands and
generally can hold two things, oxygen tends to
favor bonding with two things because of the
energy of its electrons. However, depending on
the conditions, oxygen can bond to three
hydrogens, which happens where there are many
hydrogens (that is, something is strongly
Click Here to return to the search form.
Copyright © 2017 The Regents of the University of California,
All Rights Reserved.