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GSCI 163
Lecture 3
Review:
Structure of the atom
• Atoms are made of 3 elementary
particles
Electron
Negative
charge
0.000549 (u)
Considered a
point mass
Proton
Positive
charge
1.0073 (u)
Composed of
quarks
Neutron
Neutral
charge
1.0087 (u)
Composed of
quarks
• The mass of an atom is primarily the sum
of the masses of their protons and
neutrons
Elements
• What gives the chemical properties and characteristic
of elements is the number of protons that compose
the nucleus of the elements. The number of protons is
called the atomic number. (In the periodic table, the
elements are arranged by their atomic number)
Examples:
–
–
–
–
Hydrogen has one proton (atomic number 1)
Helium has 2 protons (atomic number 2)
Carbon has 12 protons (atomic number 12)
Gold has 79 protons (atomic number 79)
Ions…
Whereas the number of protons of an element
is always the same, the number of electrons
and neutrons may vary.
• Ions: atoms of an element with a surplus
(cations) or deficiency (anions) of electrons
– Sodium Na+  11 protons and 10 electrons
– Chlorine Cl-  17 protons and 18 electrons
…and Isotopes
• Isotopes: atoms of an element with a surplus or deficit
of neutrons
– Carbon: 11C, 12C, 13C, 14C with 5, 6, 7 and 8 neutrons
respectively.
(12C, 13C are stable, abundance 99%, 1%)
– Hydrogen: 1H (protium), 2H (deuterium), 3H (tritium) with
0, 1 and 2 neutrons.
(1H (protium), 2H (deuterium) are stable, abundance
99.99%, 0.01%)
The atomic weight in the periodic table is typically the
weighted average of the masses of the elements’ isotopes.
Today’s class
• What is light?
• Where does light come from?
• Emission from atoms.
Ancient concepts of light
• Is light a particle or wave?
Evidences that light is a wave:
Diffraction
and
interference
What kind of wave is light?
When a charged particle accelerates it emits
energy in the form of electric and magnetic
waves (variations on the E&M field) just like a
moving source produces mechanical waves in
the surface of water.
Oscillating charge
Characteristics of waves
• Wavelength – distance
between crests
• Amplitude – related to
intensity
• Period – time for one full
oscillation
• Frequency – number of
oscillations per second
Relations for light
• Speed of light, c = 300,000 km/s or 3 x 108 m/s
• Frequency and wavelength f 
c

or c    f
Spectrum: range of frequencies of a wave
The EM spectrum
Peak emission of a hot object
At any given temperature every object emits electromagnetic
radiation. As the temperature increases, the range of
energy increases and so does the distribution of
frequencies (which is somewhat bell-shaped).
• At room temperature the peak is at infra-red frequencies
(we can’t see)
• At 700 K the object has dull red glow
• 1700 K is the temperature for a candle flame
• 3000 K is aprox. the temperature of incandescent light
• Sun light is emitted at about 6000 K (temperature of the
surface of the sun)
Incandescent light is inefficient
because most of the energy is
wasted in the invisible part of
the spectrum.
Spectrum
• When light that is produced by a hot object
goes through a prism, it is decomposed into
many colors (white light is the sum of many
frequencies).
Light emission from atomic vapor
• So far we saw how light is emitted from a hot
object.
• But light can also be emitted from an
incandescent gas. In this case the spectrum is
different (see next slide). Why?
Light also behaves as a particle
• Photoelectric phenomena:
Light can be imagined as smalls
particles that carry energy called
photons.
A bright and intense light has
many photons, whereas dim light
has only few.
In the photoelectric phenomena,
they interact with matter giving off
all its energy to electrons, which
make the electrical current.
Packets of energy
• Max Plank 1900 came up with the idea that vibrating molecules can
only have energy in multiples of energy in certain amounts he called
quanta (plural of quantum, or discrete).
• In 1905 Albert Einstein applied Plank’s concept to the photoelectric
phenomena with success. (He won the Nobel prize for this work,
not for relativity!)
• In summary, he found that the energy of the photon is
E = hf
where h is the value of Plank’s constant (6.63x10-34 Js)
and f is the frequency of light.
Problem
• What is the energy of a photon of red light
with a frequency of 4.00x1014 Hz?
• What is the energy of a photon of x-rays with
a frequency of 3x1018 Hz?
Back to spectral lines
• Every element has its own spectral lines.
– It is like its fingerprint.
– This how scientists can tell which elements are
present in a given celestial body.
• Change in temperature only change the
intensity of the lines of the colors.
• The line spectra may extend beyond the
visible range.
The hydrogen spectrum
In 1885, J. J. Balmer noticed that the line spectrum of
Hydrogen satisfied the following equation:
 1 1 
 R 2  2 

2 n 
1
Where, R = 1.097x107 1/m
Thus, there must be some regularity in the atom.
It was also known that H had only one electron.
Bohr atomic model (1913)
• Allowed orbits
– Electrons can revolve around an atom only in specific
allowed orbits
• Radiation orbits
– An electron in an allowed orbit does not emit radiant
energy, as long as it remains in the orbit.
• Quantum leaps
– Electrons gains or loses energy only by moving from
one allowed orbit to another. Electrons jump up when
absorb energy, and jump down by emitting energy.
How much energy?
• In the simpler case of hydrogen, the energy of
each of the allowed orbits can be found by:
E1
En  2
n
Where E1 = -13.6 eV
Problem
• An electron in a Hydrogen atom jumps from
the exited energy level n=4 to n=2. What is the
frequency of the emitted photon? To what
wave length (color) does this photon
correspond?
Back to spectral lines
• Note that the spectrum of the sun there are
some black lines. Why?
Absorption spectrum
• The colder gas in the sun absorb photons
emitted by the hot gases.
Energy levels
• Electrons do not share the same energy level
(orbits). Think as if the electron is a selfish
particle…
• But an experiment showed that there is a way
to fit 2 electrons in one energy level: if they
have different spins, up and down. Like two
pair of shoes in a box.
Atom shells
• The “electronic closet” of the atom is divided
in:
– Energy level (shoe box)
– Shells (closets)
– Orbitals (shelves within the closet)
• Energy levels are spin up and down
• Shells are 1,2,3, etc
• Orbitals are lettered “s”, “p”, “d”, “f”, “g”
Next class
Atomic shells and atomic bonding
Assignment:
– Read handout day-4, Filling shells.
– Quiz on the topic of Atomic structure, energy
levels.
– First Teaching project: Kim on the periodic table.