Why H20?
Ever wonder why it’s HTWOO? Why not HO or H3O? Why not HO2?
Read for information:
Atoms react so that they become stable. For an atom, being stable means that the outer
energy level is full. When the atom is stable, it does not react anymore. Maybe it’s like the feeling
you get after a big Thanksgiving meal.
The elements in the last group to the right on the periodic table are stable and unreactive.
These atoms all have full outer energy levels (see your Bohr-Rutherford models). These elements
are called the “noble gases” or “inert gases”.
When is an atom stable?
What happens when an atom is stable?
 When atoms react to form a molecule, they share electrons until their outer energy levels are
full. Some of the electrons are shared between the two atoms. The shared electrons are the
chemical bond, the glue that holds the atoms together in a molecule.
How many electrons does hydrogen need to
become stable?
What group is hydrogen in in the periodic
table?
Check out the Bohr-Rutherford molecule for oxygen and for hydrogen. Reproduce and analyze
them in the table below.
Table 1. The Bohr-Rutherford models of hydrogen and oxygen.
Oxygen
Bohr-Rutherford
model
Number of electrons
in outer energy level
Number of jumps to
the nearest atom
with full outer energy
level
Number of electrons
needed to fill outer
energy level
Number of atoms
needed to react with
to form water, H20
Hydrogen
For the H20 molecule, an oxygen atom reacts with two atoms of hydrogen. The hydrogen and
oxygen atoms become stable in this ratio. No more oxygen or hydrogen can be added to the
molecule because the atoms are stable.
Why does oxygen become stable when it
reacts with two hydrogen atoms?
Why NaCl?
Draw the Bohr-Rutherford model for sodium and chlorine.
Sodium
Bohr-Rutherford
model
Number of electrons
in outer energy level
Number of jumps to
the nearest atom
with full outer energy
level
Number of electrons
needed to fill outer
energy level
Number of atoms
needed to react with
to form sodium
chloride, NaCl
Chlorine
Check out the models of molecules on p.43 and on p. 45 in your text. Use a ball-and-hook model
or a structural diagram to draw models of the molecules below.
Draw a model of the molecule that results from the reaction of nitrogen and hydrogen.
Draw a model of the molecule that results from the reaction of carbon and hydrogen.
Draw a model of the compound that results from the reaction of aluminum and hydrogen.
Draw a model of the molecule that results from the reaction of lithium and oxygen.
Wow – bonus to whoever can explain why the last molecule lithium oxide looks so much like a
water molecule.
Do p. 43 #3,4