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Electrons in Atoms
Wave and Particle Models
of Light
Frequency and Unique
Characteristics of Atoms
Bohr and Quantum
Mechanical Model
Orbital Notations
• Rutherford’s explanation of the atom
was found to be incomplete.
• Questions remained as to where the
electrons in an atom were located and
why the nucleus did not pull them into
itself.
Wave Nature of Light
• Visible light is part of a range of
electromagnetic radiation, a form of
wave energy that travels through empty
space and is propagated in the form of
alternating electric and magnetic fields.
• All waves exhibit certain common
characteristics.
Parts of a Wave
Frequency
• All waves consist of a series of crests and
troughs that travel away from their source
at a velocity that is determined by the
nature of the wave and the material
through which the wave passes.
• The rate of vibration of a wave is called the
frequency and is defined as the number of
waves that pass a given point per second.
• The units of frequency is the hertz (Hz);
one hertz equals one wave per second (s-1).
Wavelength
• The velocity and the frequency of the
wave determine the wavelength of the
wave.
• The equation that expresses this
relationship is
c = ln
• In this equation , c equals the speed of
light, 3.0 X 108 m/s, l equals wavelength in
meters, and n is the frequency in hertz.
Practice
1. A helium-neon laser emits light with a
wavelength of 633 nm. What is the
frequency of this light?
2. What is the wavelength of X rays having
a frequency of 4.80 x 1017 Hz?
Particle Nature of Light
• Not only does light behave as a wave, it
also behaves as a particle.
• Einstein’s explanation of the
photoelectric effect helped display this
quality.
• The effect says that electrons are ejected
from the surface of a polished metal
plate when it is struck by light.
Photons
• Einstein found that this could only happen if
light behaved as particles.
• These particles, or photons, of light at the
high-frequency (or violet) end of the spectrum
had greater energy and could therefore
dislodge many more electrons.
• He found that the energy of a photon of a
certain frequency can be calculated by using
the equation:
Ephoton= h n
where h (Planck’s constant) = 6.626 x 10-34 J s
Practice
3. Calculate the energy of a gamma ray
photon whose frequency is 5.02 x 1020
Hz.
4. What is the difference in energy
between a photon of violet light with a
frequency of 6.8 x 1014 and a photon
of red light with a frequency of 4.3 x 10
14 Hz?
Atomic Emission Spectra
• When atoms of an element in the
gaseous phase are excited by energy,
they emit light.
• This emitted light can be broken into a
spectrum consisting of discrete lines of
specific frequencies, or colors.
• This pattern of frequencies is unique to
each element and is known as the
element’s atomic emission spectrum.
Bohr Model
• According to Bohr, hydrogen’s single
electron can only orbit at specific
distances from the atom’s nucleus.
• When close to the nucleus, it has low
energy; when farthest away it has the
highest energy possible.
• Thus the electron can only occupy
specified allowed orbits.
Quantums of Energy
• Electrons that are excited by an input of
energy only absorb the amount needed
to jump to a higher energy orbit.
• When it falls back to the lower level, it
emits a quantum of energy equal to the
difference in energy between the two
orbits.
• Because hydrogen emission spectra
contained several frequencies, Bohr
designated them using integers called
quantum numbers.
Modern Atomic Model
1. Electrons occupy the space
surrounding the nucleus and can exist
in several discrete principal energy
levels, each designated by one of the
principal quantum numbers (n) that
are integers 1, 2, 3, 4 and so on.
[This number corresponds to the row
number of the element]
Atomic Model (cont’d)
2. Electrons in successively higher
principal energy levels have greater
energy.
3. Each energy level consists of energy
sublevels that have different energy
values. These are designated by s, p, d,
and f respectively.
[The number of sublevels depends on
the principal energy level number ]
Atomic Model (cont’d)
4. Each sublevel has orbitals, each of
which van contain only 2 electrons. All
of the orbitals in the same sublevel
have the same energy.
5. Atomic orbitals are regions of space in
which there is a high probability (90 %)
of finding an electron. The electron
can be anywhere in an orbital and
there is a 10% chance they will be
outside the orbital.
Practice
5. How many electrons can the second
principal energy hold? How many
electrons can the third principal energy
level hold? Explain the difference in
these numbers of electrons.
Electron Configurations
• The number and arrangement of
electrons around the nucleus of an atom
determines its chemical properties.
• Because of this, the electron
arrangement, or electron configuration .
• The electron configuration of an atom is
written by stating the number of
electrons in each energy sublevel.
• The number of electrons in the sublevel
is shown using a superscript.
Aufbau Principle
• One rule governing electron
configurations is aufbau principle which
states that each successive electron
occupies the lowest energy orbital
available.
Orbital Filling
• For elements in the third row (third energy
level), once the s and p orbitals are filled you
may expect the d orbitals to begin filling.
• However, because the 4s sublevel is of lower
energy than the 3d sublevel, 4s fills before 3d.
• Remember that the configurations are written
by increasing energy, not in numerical order.
• The following diagram may be necessary to
write configurations correctly.
Filling Diagram
• Remember to
follow from the
tail of the arrow
to the head of
the arrow.
• For example,
francium.
Practice
• Write the electron configuration of the
following elements:
– Cesium
– Radium
– Iridium
– Holmium
Noble Gas Configuration
• Since a new principal energy level
always begins with the element
immediately following on of the noble
gases, we can use a noble gas
configuration.
• This uses the symbol of the previous
noble gas to denote all of an atom’s
inner-level electrons.
Practice
• Write the noble gas configuration of
the following elements:
a. Fluorine
b. Phosphorous
c. Calcium
d. Cobalt
e. Selenium
f. Technetium
Valence Electrons
• When elements combine chemically,
only the electrons in the highest
principal energy level are involved.
• These outermost electrons are called
valence electrons, and they determine
most of the chemical properties of an
element.
• Since bonding requires the valence
electrons, the electron-dot structure is a
useful tool.
Electron-dot Structures
• The electron-dot structure contains the
symbol of the element and dots around it.
• A single dot is used to represent each
valence electron.
• One dot is placed on each of the four sides
before any two dots can be paired
together.
Practice
• Write the electron-dot structure for the
following elements.
– Nitrogen
– Aluminum
– Neon
– Strontium
– Antimony
– Iodine
– Lead