Download Arrangement of Electrons in Atoms

Arrangement of
Electrons in Atoms
Chapter 4
“Energy travels through space as waves,
but can be thought of as a stream of
particles .“
- Albert Einstein

Scientist Review
◦ Rutherford – Gold Foil Experiment
◦ Nucleus – hard, dense, small center of atom containing protons & neutrons
◦ Most of the atom is empty space in which the electrons move randomly
◦ Did not explain how the electrons filled the empty space around the nucleus.
◦ Bohr – Solar System Model
◦ Electrons orbit the nucleus maintaining a fixed amount of energy, at constant
speeds
◦ Energy levels are the circular paths that the electrons travel.
◦ Energy levels are arranged like a ladder. Rungs are not evenly spaced…they get
closer together the farther from the ground, nucleus.
◦ Electrons at higher energy levels have greater energy
◦ The amount of energy required by an electron to move from one level to another
is a quantum of energy.
◦ A quantum leap is the abrupt change in level by the electron.
◦ Schrodinger – Quantum Mechanical Model
◦
◦
◦
◦
Also called the Electron Cloud Model
The electrons travel randomly with no definite path.
Electrons are particles that act like waves.
The model is based on the mathematical 90% probability of finding an electron
within a certain volume of space, called an orbital.
◦ Heisenberg Uncertainty Principle – it is impossible to determine both the position
and velocity of an electron simultaneously.
Development of New Atomic Model

Wave Description of Light
◦ Visible light is a kind of electromagnetic radiation,
which is a form of energy that exhibits wavelike behavior
as it travels through space.
◦ Together, all forms of electromagnetic radiation form the
electromagnetic spectrum.
 Consists of radiation over a broad range of wavelengths
◦ Radio waves
◦ Radar waves
◦ Microwaves
◦ Infrared waves
◦ Visible light waves
◦ Ultraviolet waves
◦ X-rays
◦ Gamma rays
Wave-Particle Duality of Motion

Wave Description of Light
◦ Amplitude
 The height of the wave from origin to
crest
◦ Wavelength (λ)
 The distance between waves crests.
 Units are the same as length, cm.
◦ Frequency (v)
 The number of wave cycles that pass a
given point per unit of time.
 Units of frequency are cycles/second or
hertz.
◦ Wavelength and frequency are
inversely proportional, when one
goes up the other goes down.
◦ The speed of all forms of
electromagnetic radiation in a
vacuum is constant at 3.0 x 108 m/s
◦ Formula c = λ · v
We will do a bunch of calculations for
frequency and wavelength at the end of class
today!!
Wave-Particle Duality of Motion

Particle Description of Light
◦ When certain frequencies of light strike
a metal, electrons are emitted.
 The photoelectric effect refers to the
emission of electrons from a metal
when light shines on the metal.
Electrons are ejected by metals when
light shines on them – light waves act
like particles
 The energy of the light has to be a
certain minimum. Light can be
absorbed or emitted by matter.
 Calculators that require light energy as it’s
power source.
 Planck, German Physicist, proposed radiant
energy is transferred in units (or quanta) of
energy called photons.
 He wasn’t expected to come up with this –
people thought that energy was emitted in
waves.
◦ Like an hour-glass – small increments
Wave-Particle Duality of Motion
Photoelectric Effect
energy
p+
absorption
spectrum
no
e-
ground
state e-
When a specific or quantized
amount of energy is exposed to
the atom, the electron jumps
from its “ground” or original
state to an “excited” state
Photoelectric Effect
energy
photon
p+
no
When the “excited” electron
returns to lower energy
levels, it releases energy in
the form of light
e-
excited
state e-
emission
spectrum!
travels at the
speed of light
(3.00 x 108 m/s)

Particle Description of Light
◦ A quantum of energy is the
minimum quantity of energy that
can be lost or gained by an atom.
◦ Planck proposed the following
relationship between quantum
energy and the frequency of
radiation
E = hv
E is the energy, in Joules
v is the frequency, in s-1
h is the constant, 6.626x10-34 J·s
We will do a bunch of calculations at the
end of class on this!!!
◦ Planck’s constant shows
mathematically that the
electromagnetic energy is directly
proportional to frequency.

In 1905, Einstein expanded
Planck’s theory by introducing the
radical idea that electromagnetic
radiation has a dual wave-particle
nature.
◦ Light exhibits both wavelike
properties, but it can also be
thought of as a stream of
particles.
◦ Each particle carries a quantum or
energy.
◦ These particles, called photons,
is a particle of electromagnetic
radiation having zero mass and
carrying a quantum of energy.
 Frequency is the energy per photon
 Intensity is the number of photons
Wave-Particle Duality of Motion

Electrons exist in very specific energy
states.
◦ The lowest energy sate of an atom is its
ground state.
◦ A state in which an atom has a higher
potential energy than it has in its ground
state is an excited state.
 There are many possible excited states, each with a
unique energy, but only one ground state energy for
atoms of a given element.
◦ When an excited atom returns to its ground
state or lower energy excited state, it gives off
the energy it gained in the form of
electromagnetic radiation.
 Every element emits a unique atomic spectrum, the
pattern of frequencies obtained by passing energy
emitted by atoms of an element in the gas phase
through a prism, producing an emission line
spectrum.
◦ Scientists expected to observe the
emission of a continues range of
frequencies of electromagnetic radiation,
or the continuous spectrum.
 Attempts to explain emission spectrum led to
an entirely new atomic theory called the
quantum theory.
Wave-Particle Duality of Motion
Bohr Model
e-
p+
no
Niels Bohr
Electrons circle around the
nucleus on their energy level
Energy
levels
The Bohr Model of the Atom
Electron Orbits, or Energy Levels
 Electrons can circle the nucleus only in allowed
paths or orbits
 The energy of the electron is greater when it is in
orbits farther from the nucleus
 The atom achieves the ground state when atoms
occupy the closest possible positions around the
nucleus
 Electromagnetic radiation is emitted when
electrons move closer to the nucleus.

The Bohr Atomic Model
Energy transitions



Energies of atoms are
fixed and definite
quantities
Energy transitions occur
in jumps of discrete
amounts of energy
Electrons only lose
energy when they move
to a lower energy state
Shortcomings of the Bohr Model


Doesn't work for atoms larger than hydrogen
(more than one electron)
Doesn't explain chemical behavior