CPO Science
Foundations of Physics
Unit 9, Chapter 28
Unit 9: The Atom
Chapter 28 Inside the Atom
 28.1 The Nucleus and Structure of the
Atom
 28.2 Electrons and Quantum States
 28.3 The Quantum Theory
Chapter 28 Objectives
1.
Describe the structure of an atom.
2.
Describe the four forces acting inside an atom.
3.
Use the periodic table to obtain information about the atomic
number, mass number, atomic mass, and isotopes of different
elements.
4.
Predict whether a certain nucleus is stable or unstable and
explain why.
5.
Distinguish between and provide examples of chemical
reactions and nuclear reactions.
6.
Describe how atomic spectral lines can be explained by energy
levels and quantum states.
7.
Explain quantum theory as it relates to light and electrons.
8.
Describe the major developments in quantum theory and
identify the scientists associated with each.
Chapter 28 Vocabulary Terms
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nucleus
electron
proton
neutron
atomic mass
atomic mass unit
electromagnetic
force
strong nuclear
force
weak force
element
atomic number
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mass number
isotope
radioactive
nuclear reaction
quantum
spectral line
spectrum
quantum state
energy level
quantum numbers
Pauli exclusion
principle
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Planck’s constant
wave function
Probability
orbital
uncertainty
principle
photoelectric
effect
photon
chemical reaction
spectrometer
quantum physics
28.1 The Nucleus and Structure of the
Atom
Key Question:
What is inside an atom?
*Students read Section 28.1
AFTER Investigation 28.1
28.1 The Nucleus and Structure of the
Atom
 Atoms are made of three kinds of particles:
electrons, protons, and neutrons.
28.1 The structure of the atom
 Because the mass of a
proton is tiny by normal
standards, scientists
use atomic mass units
(amu) instead of kg.
(chart)
 One amu is 1.661 × 10-27
kg, slightly less than
the mass of a proton.
 One electron has a
mass of 0.0005 amu.
28.1 The structure of the atom
 A neutral atom has a total charge of zero.
 Because the number of electrons equals the
number of protons in a complete atom they tend
to stay neutral because electric forces are very
strong.
28.1 The nucleus of the atom
 The neutrons and protons are
grouped together in the
nucleus, which is at the center
of the atom.
 If the atom were the size of
your classroom, the nucleus
would be the size of a single
grain of sand in the center of
the room.
 Most of an atom’s mass is
concentrated in the nucleus.
28.1 The structure of the atom
 Electrons are outside the
nucleus in the electron
cloud.
 Because electrons are so
fast and light, physicists
tend to speak of the
"electron cloud" rather
than talk about the exact
location of each electron.
28.1 The nucleus of the atom
 The mass of a carbon
nucleus is 12 amu. (6p+,
6no)
 The mass of the
electrons is only 0.003
amu.
 So 99.97 % of the carbon
atom’s mass is in the
nucleus and only 0.03%
is in the electron cloud.
28.1 The structure of the atom
 Electrons are bound to the
nucleus by electromagnetic
forces.
 The force is the attraction
between protons (positive)
and electrons (negative).
 The momentum of the electron causes it to move
around the nucleus rather than falling straight in.
28.1 The structure of the atom
 The strong nuclear force attracts neutrons and
protons to each other, otherwise the positively
charged protons would repel each other.
28.1 The structure of the atom
 The weak force is weaker than both the electric force
and the strong nuclear force.
 If you leave a solitary neutron outside the nucleus, the
weak force eventually causes it to break up into a proton
and an electron.
 The force of gravity inside the atom is much weaker
even than the weak force.
 Every process we
know in the universe
can be explained in
terms of these
fundamental forces.
28.1 Review atoms and elements
 The variety of matter we find in nature (here on
Earth) is made from 92 different types of atoms
called elements.
 The atomic number of each element is the
number of protons in its nucleus.
 The periodic table arranges the elements in
increasing atomic number.
28.1 Calculate nuclear particles
 How many protons are in the
nucleus of an atom of
vanadium (V)?
28.1 The structure of the atom
 Different isotopes exist for atoms of each element.
 Different isotopes of the same element have different
mass numbers.
 The mass number is the total number of particles
(protons and neutrons) in the nucleus.
 Some isotopes occur naturally.
 Other isotopes may be created in a laboratory for
research.
28.1 The structure of the atom
 If an isotope has too many (or too few) neutrons,
the nucleus eventually breaks up and we say the
atom is radioactive.
 In a stable isotope the nucleus stays together.
28.1 Average atomic mass
 The average atomic mass
give the proportion of each
isotope by mass.
 For example, the periodic
table lists an atomic mass of
6.94 for lithium.
 On average, 94% of lithium
atoms are Li7 and 6% are
Li6.
28.1 Calculate nuclear particles
49
22
T
 How many neutrons are in the nucleus of an
atom of titanium-49?
28.1 Reactions inside and between
atoms
 Most atoms in nature are
found combined with
other atoms into
molecules.
 A molecule is a group of
atoms that are chemically
bonded together.
28.1 Reactions between atoms
 A chemical reaction rearranges the same atoms into
different molecules.
 Chemical reactions rearrange atoms into new
molecules but do not change atoms into other kinds
of atoms.
28.1 Reactions inside atoms
 A nuclear reaction is any process that changes the
nucleus of an atom.
 A nuclear reaction can change atoms of one element
into atoms of a different element.
28.2 Electrons and Quantum Theory
Key Question:
How do atoms create
and interact with light?
*Students read Section 28.2
AFTER Investigation 28.2
28.2 Electrons and Quantum Theory
 Quantum physics is the branch of
science that deals with extremely small
systems such as an atom.
 A brilliant scientist, Neils Bohr is often
called the father of quantum physics.
 Niels Bohr was the first person to put
the clues together correctly and in 1913
proposed a theory that described the
electrons in an atom.
28.2 Electrons and Quantum Theory
 An unusual feature of light was one clue that lead
to the discovery of quantum physics.
 When substances were made into gases and
electricity was passed through the gas, light was
given off in the form of lines of color.
 Since the energy of light depends on the color, the
lines in a spectrum meant that substances could
only emit light of certain energies.
28.2 Electrons and Quantum Theory
 Each individual color is called a spectral line
because each color appears as a line in a
spectrometer.
 A spectrometer is a device that spreads light into
its different wavelengths, or colors.
28.2 Balmer's formula
 The first serious clue to an explanation of the atom was
discovered in 1885 by Johann Balmer, a Swiss high
school teacher.
 He showed that the wavelengths of the light given off
by hydrogen atoms could be predicted by a
mathematical formula (Balmer’s formula).
28.2 Balmer's formula
1 = 91.16
l
[2
1 - 1
2
n2
]
Wavelength (nm)
n is an integer
greater than two
28.2 Quantum states
 Bohr proposed that electrons in the atom were
limited to certain quantum states.
 The quantum states in an atom have certain
allowed values of energy, momentum, position,
and spin.
 The number (n) in the Balmer formula is one of
four quantum numbers that describe which
quantum state an electron is in.
28.2 Quantum states
 Every quantum state in the atom is identified by a
unique combination of the four quantum numbers.
 Every electron in an atom can be completely
described by the values of its four quantum
numbers: n, l, m, and s.
 From the four quantum numbers it is possible to
calculate everything about the electron, including
its energy, angular momentum, average position,
and spin.
28.2 Quantum numbers
1. The first quantum number (n) can be any integer
bigger than zero.
2. The second quantum number (l) must be a
positive integer from zero to n-1.
— For example, if n = 1, the only possibility is l = 0. If n =
2, then l can be 0 or 1.
28.2 Quantum numbers
3. The third quantum number (m) is an integer that
can go from - l to + l.
— For example, if l = 3, m can have any of seven values
between -3 and +3 (m = -3, -2, -1, 0, 1, 2, 3).
4. The fourth quantum number (s) can only be
either +1/2 or -1/2.
28.2 Energy levels
 An electron in a hydrogen atom dropping from the third
level to the second level gives off an amount of energy
exactly equal to the red line in the hydrogen spectrum.
28.2 Quantum states
 The quantum states in an atom are grouped into
energy levels.
 Bohr explained that spectral lines are produced by
electrons moving between different energy levels.
28.2 Pauli exclusion principle
 According to the quantum theory, two electrons in
an atom can never be in the same quantum state
at the same time.
 This rule is known as the Pauli exclusion principle
after Wolfgang Pauli, the physicist who discovered
it.
 Once all the quantum states in the first level are
occupied by electrons, the next electron has to go
into a higher energy level.
28.2 Periodicity and energy levels
28.2 The "shape" of quantum states
 In chemistry, the quantum states for electrons in an
atom are called orbitals.
 The name comes from an older idea that electrons
moved in orbits around the nucleus,
 Each “orbital” shape shows the most likely locations
for a pair of electrons with matching quantum
numbers n, l, and m.
28.2 Orbital shapes for n= 1, 2, 3
28.3 The Quantum Theory
Key Question:
How can a system be
quantized?
*Students read Section 28.3
AFTER Investigation 28.3
28.3 The Quantum Theory
 The quantum theory started
between 1899 and 1905
when classical physics
disagreed with results of
new experiments.
 Max Planck and Albert
Einstein were working on
two phenomena that
couldn't be explained by
classical physics.
28.3 The Quantum Theory
 Einstein was thinking about
the photoelectric effect.
 When light falls on the
surface of a metal,
sometimes electrons are
emitted from the surface.
 If the light is made brighter,
the metal absorbs more
energy.
28.3 The Quantum Theory
 Classical physics
predicts that electrons
coming off the metal
should have more
kinetic energy when
the light is made
brighter.
 But that is NOT what
happens.
28.3 Quantum theory of light
 In 1899 Max Planck proposed that light existed in
small bundles of energy called photons.
 The smallest amount of light you could have is a
single photon.
28.3 Quantum theory of light
 Bright light consists of billions of photons per
second while dim light has very few photons per
second.
 According to Planck, the energy of a single photon
is related to its frequency.
 Higher frequency means higher photon energy.
 Like atoms, photons are such small quantities of
energy that light appears as a continuous flow of
energy under normal circumstances.
28.3 Planck's constant
 Planck’s idea was very different from the wave
theory of light.
frequency (Hz)
Energy (J)
E=hf
Planck's constant
(6.626 x 10-34 J.sec)
28.3 Quantum theory of light
 In 1905, Einstein proposed that an atom can
absorb only one photon at a time.
 An electron needs a minimum amount of energy to
break free from an atom.
 If the energy of the photon is too low there is not
enough energy to free an electron and no
photoelectric effect is observed.
 Making brighter light does not help.
28.3 Quantum theory of light
 In the quantum theory, all matter and energy have
both wavelike and particle-like properties.
 To a physicist, if something is quantized, it can
only exist in whole units, not fractions of units.
28.3 Quantum theory of light
 An electron acts like a particle when it is both free
to move and far from other electrons.
 However, if an electron is confined in a small
space (an atom), it behaves like a wave.
28.3 Planck's constant
 The wavelength of a particle (λ) depends on its
mass (m) and speed (v), according to the
DeBroglie formula.
Planck's constant
(6.626 x 10-34 J.sec)
wavelength (m)
l=h
mv
mass (kg)
speed (m/sec)
28.3 The Uncertaintly principle
 Quantum theory puts limits on how precisely we
can know the value of quantities such as position,
momentum, energy, and time.
 The uncertainty principle places a limit on how
precisely these four parameters can be measured.
 The uncertainty principle arises because the
quantum world is so small.
28.3 The Uncertainty principle
 The uncertainty principle works on pairs of variables
because measuring one always disturbs the other in
an unpredictable way.
 The uncertainty principle in position (Dx), multiplied
by the uncertainty principle in momentum (Dp) can
never be less than h/2p.
DxDp> h
2P
change in
postition
change in
momentum
Planck's constant
(6.626 x 10-34 J.sec)
28.3 Quantum theory and probability
 Calculations in quantum physics do not result in
knowing what will happen, but instead give the
probability of what is likely to happen.
 While you can never accurately predict the
outcome of one toss of the penny, you can make
accurate predictions about a collection of many
tosses.
 Quantum theory uses probability to predict the
behavior of large numbers of particles.
28.3 Quantum theory and probability
 In quantum physics, each quantum of matter or energy
is described by its wave function.
 The wave function mathematically describes how the
probability for finding a quantum of matter or energy is
spread out in space.
Application: The Laser