Chemistry Reference
In-Depth Information
1.
All matter is made up of tiny indivisible particles called atoms.
2.
Atoms of the same element have identical properties,
including identical mass.
3.
Atoms of different elements have different properties,
including different mass.
4.
Atoms combine in fixed whole number ratios to form
compounds.
5.
A specific compound is always made up of atoms in a specific
proportion.
For his work, John Dalton is often considered the “father” of modern
atomic theory.
As has the atomic theory, the atomic model has changed and devel-
oped over the years since Democritus. Democritus pictured atoms as being
solid, indestructible, and completely uniform. Two thousand years later,
J.J. Thomson proposed the “plum-pudding” model of the atom, with nega-
tively charged electrons embedded in the positively charged bulk of the
atom. Ernest Rutherford's famous “gold-foil” experiment showed that most
of the atom was empty space, leading to Niels Bohr's “planetary” model of
the atom, where the electrons orbited around the nucleus the way planets
do a star. Werner Heisenberg's “Uncertainty Principle” showed that the
planetary model of the atom was too specific. Erwin Schrodinger intro-
duced the Wave-Mechanical model of the atom, which is still the way that
most people think of atoms today. More recently, however, the idea that
even the subatomic particles are made up of smaller particles called quarks
suggests that our model of the
atom is still far from complete.
In the current model of the
atom we imagine a tiny nucleus,
which contains the vast majority
of the mass of the atom. It is in
this nucleus that we find the pro-
tons (p
+
) and neutrons (n
0
) of the
atom. The electrons (e
-
) repre-
sent the third type of subatomic
particle, and they are found out-
side of the nucleus, occupying an
area called the electron cloud.
The Wave-Mechanical Model
of the Atom
Nucleus
Electron cloud
Each atom consists of a tiny nucleus,
which contains almost all of the mass,
and one or more electrons moving
about an area called the “electron
cloud.”
Figure 3-1a
