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226
Chapter Nine:
Atomic Structure
Px
Z
1s
2s
2p
3s
3p
3d
4s
4p
4d
4f
5s
5p
5d
5f
6s
6p
6d
6f
7s
7p
7d
X
Y
Py
Z
FIGURE 9.21
A matrix showing the order in which the orbitals are filled. Start at
the top left, then move from the head of each arrow to the tail of
the one immediately below it.This sequence moves from the
lowest-energy level to the next higher level for each orbital.
X
Y
Z
Pz
of energy sublevels and obtain the electron configuration for all
the elements. The exclusion principle limits the number of electrons in any orbital, and allowances will need to be made for the
more complex behavior of atoms with many electrons.
The energies of the orbital are not fixed as you progress
through the atomic numbers, and there are several factors that
influence their energies. The first orbitals are filled in a straightforward 1s, 2s, 2p, 3s, then 3p order. Then the order becomes contrary
to what you might expect. One useful way of figuring out the order
in which orbitals are filled is illustrated in figure 9.21. Each row of
this matrix represents a principal energy level with possible energy
sublevels increasing from left to right. The order of filling is indicated by the diagonal arrows. There are exceptions to the order of
filling shown by the matrix, but it works for most of the elements.
X
SUMMARY
Y
FIGURE 9.20
There are three possible orientations of the p orbital, and these are
called px, py, and pz. Each orbital can hold two electrons, so a total of
six electrons are possible in the three orientations; thus the
notation p6.
Attempts at understanding matter date back to ancient Greek philosophers, who viewed matter as being composed of elements, or simpler
substances. Two models were developed that considered matter to be
(1) continuous, or infinitely divisible, or (2) discontinuous, made up of
particles called atoms.
During the 1600s, Robert Boyle provided experimental evidence
to reject the ancient Greek ideas of continuous matter made up of
earth, air, fire, and water. Boyle reasoned that there were elements,
which could not be broken down to anything simpler, and compounds,
which were made up of combinations of elements.
In the early 1800s, Dalton published an atomic theory, reasoning
that matter was composed of hard, indivisible atoms that were joined
together or dissociated during chemical change.
Chapter Nine:
Atomic Structure
227
The Quark
S
ome understanding about how matter is
put together came with the discovery of
the electron, proton, and neutron—
three elementary particles that make up an
atom. In the early 1900s, a particle outside the
atom, the photon, was verified by experimental evidence. Two other particles were verified
in the 1930s, the neutrino (“little neutral one”)
and the positron (a positively charged electron). By the mid-1930s a total of six elementary particles were known. Since that time,
high-energy accelerator experiments have
made it possible to collide particles with great
violence, probing the inner parts of atoms and
how they are put together. A multitude of elementary particles are now known to exist.
There are now thought to be twelve elementary particles that make up matter. They
can be divided into three main groups: (1) leptons, which exist independently, (2) quarks,
which exist together, making up a third group,
and (3) hadrons.
Leptons are a group of fundamental
particles that include the familiar electron,
the muon (an overweight relative of the electron), and three types of neutrinos. In
radioactive decay, an electron (beta particle)
is emitted with an electron neutrino. For
each lepton there is a corresponding antiparticle, or antilepton, with the same mass but
opposite electric charge.
Hadrons are a group of composite particles with an internal structure, so they are not
elementary particles. There are two subgroups
of hadrons: (1) the mesons (meaning “intermediate mass,” between electrons and protons) and (2) the baryons (meaning “greater
mass”). Hundreds of short-lived hadrons have
been identified that exist briefly after highenergy collisions. Among the more stable are
the baryons named protons and neutrons.
Hadrons are composed of different combinations of fundamental particles called
quarks. Five kinds of quarks, fancifully called
flavors, have been identified, and a sixth is
believed to exist. The existing quarks are called
up, down, sideways (or strange), charm, and
bottom (or beauty). The sixth flavor is named
top (or truth). Each flavor carries a fractional
charge that is either 1/3 or 2/3. Antiquarks
have equal but opposite charges. In order to
explain how identical quarks could combine
as observed, each flavor was assigned three
quantum states that are called color. Each flavor can carry a charge of red, green, or blue.
Antiquarks carry a corresponding anticolor,
When a good air pump to provide a vacuum was invented in 1885,
cathode rays were observed to move from the negative terminal in an
evacuated glass tube. The nature of cathode rays was a mystery. The
mystery was solved in 1887 when Thomson discovered they were negatively charged particles now known as electrons. Thomson had discovered the first elementary particle of which atoms are made and measured their charge-to-mass ratio.
Rutherford developed a solar system model based on experiments
with alpha particles scattered from a thin sheet of metal. This model
had a small, massive, and positively charged nucleus surrounded by
moving electrons. These electrons were calculated to be at a distance
from the nucleus of 100,000 times the radius of the nucleus, so the volume of an atom is mostly empty space. Later, Rutherford proposed that
the nucleus contained two elementary particles: protons with a positive
charge and neutrons with no charge. The atomic number is the number
of protons in an atom.
Bohr developed a model of the hydrogen atom to explain the
characteristic line spectra emitted by hydrogen. His model specified
that (1) electrons can move only in allowed orbits, (2) electrons do
not emit radiant energy when they remain in an orbit, and (3) elec-
for example, a red quark has an antiquark of
the color cyan, a green quark has an antiquark of the color magenta, and a blue quark
has an antiquark of the color yellow. The idea
of quark color was designed to follow the
allowable combinations of quarks and antiquarks according to the exclusion principle.
Hadrons do not have a color charge, so the
sum of the quark colors making up the
hadron must result in a white hadron.
Baryons are made up of three quarks, so a
combination of a red, a green, and a blue
quark would be acceptable, since this would
result in a white baryon. Mesons are made up
of a quark and an antiquark so a combination of a blue quark and a yellow antiquark,
would be acceptable since this would result in
a white meson.
The quark model holds that all matter
is made from combinations of six quarks
and the six independent leptons. The story
of subnuclear elementary particles is by no
means complete. There is no explanation
for why quarks and leptons exist. Are there
more fundamental particles? Answers to
this and more questions await further
research.
trons move from one allowed orbit to another when they gain or
lose energy. When an electron jumps from a higher orbit to a lower
one, it gives up energy in the form of a single photon. The energy of
the photon corresponds to the difference in energy between the two
levels. The Bohr model worked well for hydrogen but not for other
atoms.
De Broglie proposed that moving particles of matter (electrons)
should have wave properties like moving particles of light (photons).
His derived equation, h/mv, showed that these matter waves were
only measurable for very small particles such as electrons. De Broglie’s
proposal was tested experimentally, and the experiments confirmed
that electrons do have wave properties.
Schrödinger and others used the wave nature of the electron to
develop a new model of the atom called wave mechanics, or quantum
mechanics. This model was found to confirm exactly all the experimental data as well as predict new data. The quantum mechanical model
describes the energy state of the electron in terms of quantum numbers
based on the wave nature of the electron. The quantum numbers
defined the probability of the location of an electron in terms of fuzzy
regions of space called orbitals.
228
Chapter Nine:
Atomic Structure
Summary of Equations
KEY TERMS
9.1
energy Plancks constant frequency
atomic number (p. 215)
Bohr model (p. 218)
electron (p. 213)
electron pair (p. 224)
electron volt (p. 219)
element (p. 211)
excited states (p. 220)
ground state (p. 220)
Heisenberg uncertainty
principle (p. 223)
E hf
where h 6.63 10 34Js
9.2
1
1
1
constant 2 wavelength
2
number 2
1
1
1
R 2 2
2
n
where R 1.097 10 1 m
7
9.3
( )(
orbit
angular
quantum
momentum
number
mvr n
Planck’s constant
2
)
h
2
where h 6.63 1034 J·s, and n 1, 2, 3, . . . for an orbit
9.4
electrical force centripetal force
Coulomb’s law Newton’s second law for circular motion
mv 2
kq1q2
2 r
r
9.5
energy state of orbit number En energy state of innermost orbit
number squared
EL
n2
where EL 13.6 eV, and n 1, 2, 3, . . .
9.6
(
)(
energy
energy state
energy state
of =
of
of
photon
higher orbit
lower orbit
where h 6.63 hf EH EL
1034
)
J·s; EH and EL must be in joules
9.7
circumference of orbit (whole number)(wavelength)
2r n
where n 1, 2, 3, . . .
9.8
wavelength Plancks constant
mass velocity
h
mv
where h 6.63 1034 J·s
line spectrum (p. 217)
neutron (p. 215)
nucleus (p. 214)
orbital (p. 223)
Pauli exclusion
principle (p. 224)
photons (p. 216)
proton (p. 215)
quanta (p. 216)
quantum mechanics (p. 222)
APPLYING THE CONCEPTS
1. According to the modern definition, which of the following is an
element?
a. water
b. iron
c. air
d. All of the above.
2. John Dalton reasoned that atoms exist from the evidence that
a. elements could not be broken down into anything simpler.
b. water pours and flows when in the liquid state.
c. elements always combined in certain fixed ratios.
d. peanut butter and jelly could be combined in any ratio.
3. The electron was discovered through experiments with
a. radioactivity.
b. light.
c. matter waves.
d. electricity.
4. Thomson was convinced that he had discovered a subatomic
particle, the electron, from the evidence that
a. the charge-to-mass ratio was the same for all materials.
b. cathode rays could move through a vacuum.
c. electrons were attracted toward a negatively charged plate.
d. the charge was always 1.60 1019 coulomb.
5. The existence of a tiny, massive, and positively charged nucleus
was deduced from the observation that
a. fast, massive, and positively charged alpha particles all move
straight through metal foil.
b. alpha particles were deflected by a magnetic field.
c. some alpha particles were deflected by metal foil.
d. None of the above are correct.
6. According to Rutherford’s calculations, the volume of an atom is
mostly
a. occupied by protons and neutrons.
b. filled with electrons.
c. occupied by tightly bound protons, electrons, and neutrons.
d. empty space.