CHAPTER 2
2.1 Dalton’s Atomic Theory
Aristotle declared that matter was continuously divisible,
with no ultimately small particle. However, John Dalton
was the first to develop the modern atomic theory, that
there exists an ultimately small particle which cannot be
further divided. Dalton called this the atom. Dalton saw
these atoms as similar to Christmas tree ornaments, with
one, two or three hooks.
H O N
Chemical reactions could be explained by unhooking and
rehooking several atoms to form new compounds.
Since the mass of individual atoms are extremely small, a
new unit, the atomic mass unit (amu or u), is used to
measure the mass of atoms.
2.2 The Nuclear Atom
As the science of electricity was developing, cathode ray
tubes were developed in which high voltages were applied
to a cathode (a negative electrode), and an anode (a
positive electrode.) As voltages increased, these cathode
ray tubes emitted light always originating at the cathode
(that’s how they got their name.)
It was determined that the light from the cathode was really
due to a negatively charged particle, called an electron,
whose symbol is e or e-. All electrons are identical in
charge (-1) and mass (0.00055 u or 0 u, for practical
purposes.)
Positive rays were also produced in cathode ray tubes, but
they varied in mass depending on what gas was present in
the cathode ray tube. The ultimate small particle of positive
charge is called a proton, whose symbol is p or p+. All
protons are identical in mass, 1 u, and in charge, +1.
Much later, the neutron, symbol n, was discovered. All
neutrons are identical in mass, 1 u, and have zero charge.
The next question was: “How are these particles arranged
in an atom?”
The Rutherford gold foil experiment demonstrated that all
of the protons of an atom are located in a tiny nucleus at
the center of each atom. The electrons form a diffuse cloud
which makes up the bulk of the space of the atom. The
neutron, discovered decades later, also is located in the
nucleus of an atom. The word nucleus comes from the
terminology of the biologic cell.
The atomic number of an atom is simply the sum of the
number of protons in an atom. It represents the total
positive charge on the nucleus of the atom.
2.3 Isotopes, Atomic Masses and Nuclear Symbols
Some atoms can have varying number of neutrons. The
number of protons is what identifies the atom. Atoms with
varying numbers of neutrons, but the same number of
protons, are called isotopes. The word isotope can also
refer to a specific atom.
The atomic mass of an atom is the sum of its protons and
neutrons, and also serves to identify specific isotopes.
The atomic weight of an element represents the average
mass of all naturally occurring isotopes of that element.
Therefore, the atomic mass is always an integer, but the
atomic weight is usually a rational number.
Specific isotopes are identified with its atomic mass as a
superscript and its atomic number as a subscript:
31
15 P
Since all phosphorus atoms have atomic number 15,
sometimes the atomic number is omitted:
31
P
2.4 The Bohr Model of the Atom
Neils Bohr proposed that electrons reside on shells, energy
levels or orbits just as the planets revolve around the sun.
The ground state occurs when the electrons of an atom are
in their lowest energy levels. If one or more electrons
occupy a higher energy level, the atom is in an excited
state.
The 2n2 rule dictates how many maximum electrons can
exist in any energy level. For energy level 1, 2 x 12 = 2,
therefore, there is a maximum of two electrons in energy
level 1. For energy level 2, 2 x 22 = 8. How many electrons
can reside in energy level 3?
Electrons are pushed into higher energy levels by pumping
heat or electricity into an atom. The electrons do not like
being in the higher orbits, so they fall back to the lowest
available energy level. The electrons must release their
extra energy and emit a photon of light. A photon is the
smallest “particle” of light just as the atom is the smallest
particle of an element. The following table illustrates these
priciples:
Element Atomic Number Shell 1 Shell 2 Shell 3
H 1 1 0 0
He 2 2 0 0
Li 3 2 1 0
Be 4 2 2 0
: : : : :
F 9 2 7 0
Ne 10 2 8 0
Na 11 2 8 1
2.5 Electronic Configurations
The Bohr model has been superceded by the quantum
mechanical model of the atom. In this model, electrons
reside in orbitals, regions of space where there is a high
probability of finding an electron. (The Bohr model had its
electrons residing on tracks, much like rail road cars on
their tracks.) The first orbital is designated by the letter “s”.
Its shape is spherical. Higher in energy is the “p” orbital.
Its shape looks like a propeller. (See page 51.)
There is only one “s” orbital, containing a maximum of two
electrons. However, there are three “p” orbitals, containing
a maximum of two electrons in each orbital, for a total of
six electrons. Likewise, there are five “d” orbitals, each
containing a maximum of two electrons in each orbital, for
a total of ten electrons. Finally, there are seven “f” orbitals,
each containing a maximum of two electrons a piece, for a
total of 14 electrons.
Essentially, quantum theory is a refinement of the Bohr
theory, where all electrons reside on a given track, at a
certain distance from the nucleus. Quantum theory has
these electrons residing in orbitals, and also breaks up the
main shells of the Bohr theory into subshells. (Table 2.2)
The Aufbau Principle states that as electrons are added to
atomic orbitals to build up an atom, they will enter the
atomic orbital with the lowest energy level that has an
opening available. (Figures 2.12 and 2.13.)
2.6 Mendeleev’s Periodic Table
In 1869 Dmitri Mendeleev organized the known elements
at that time into a periodic table, a table where the
properties of the elements repeat themselves periodically.
He arranged the elements in order of their masses, since
protons, neutrons and electrons had not yet been
discovered. He also left gaps in his periodic table where he
boldly predicted that new elements, not discovered as yet,
would fit, and even predicted their properties. The
following chart demonstrates Mendeleev’s predictions for
the element gallium which was discovered eight years
later:
Property Predicted Observed
Density, g/cm3 6.0 5.9
Melting Point low 30 oC
Boiling Point high 2000 oC
Formula of Oxide M2O3 Ga2O3
2.7 Electronic Structure and the Modern Periodic Table
The modern periodic table is arranged into rows and
columns. The rows (which extend left and right) are called
periods and the columns (which extend up and down) are
called groups or families.
Valence shell electrons consist of those electrons in the
outmost shell of the atom. It is these valence shell electrons
which will be involved in chemical reactions.
Elements in the same group on the periodic table will have
similar valence shell electron structures and similar
chemical properties. Some groups on the periodic chart
have their own names:
Group 1A Alkali Metals
Group 2A Alkaline Earth Metals
Group 7A Halogens
Group 8A Noble Gases
the B elements Transition Metals
the A elements Representative Elements
You will notice a heavy line dividing the periodic table
beginning with the element boron (B) and extending
downward and to the right. This line divides the periodic
table into metals (on the left) and non-metals (on the right.)
Metals touching the dividing line are sometimes considered
to be “semimetals” or “metalloids.”
Atoms decrease in diameter as one moves across a period.
As more electrons are added to the outermost shell, they
are more strongly attracted to the nucleus. The diameter of
the atom increases as one moves down a group, as
expected.
Atoms can gain or lose electrons (but never protons or
neutrons) in chemical reactions. An atom with a charge
imbalance is called an ion. Ionization energy represents
the energy needed to remove an electron from an atom,
while electron affinity represents the energy released
when an atom gains an electron.