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Chapter 5
Electrons in Atoms
Chemistry
Section 5.1 Light and Quantized Energy
• At this point in history, we are in the early
1900’s. Electrons were the 1st subatomic
particle to be discovered.
• Chemists have a Rutherford Model of the
atom. There is a small, dense, positively
charged center of the atom called a
nucleus. Electrons move around outside
the nucleus.
The Atom and Unanswered Questions
• In Rutherford's model, the atom’s mass
is concentrated in the nucleus and
electrons move around it.
• The model doesn’t explain how the
electrons were arranged around the
nucleus.
• The model doesn’t explain why
negatively charged electrons aren’t
pulled into the positively charged
nucleus.
• In the early 1900s, scientists observed
certain elements emitted visible light when
heated in a flame.
• Analysis of the emitted light revealed that
an element’s chemical behavior is related
to the arrangement of the electrons in its
atoms.
The Wave Nature of Light
• Visible light is a type of electromagnetic
radiation, a form of energy that exhibits
wave-like behavior as it travels through
space.
• All waves can be described by several
characteristics.
• The wavelength (λ) is the shortest distance
between equivalent points on a wave.
• The frequency (ν) is the number of waves
that pass a given point per second.
• The amplitude is the wave’s height from the
origin to a crest.
What relationship do you see between
λ, v, and c?
• The speed of light (3.00  108 m/s) is the
product of it’s wavelength and frequency.
c = λν
What relationship do you see between
λ and ν?
• Sunlight, or visible light, contains a
continuous range of wavelengths and
frequencies.
• A prism separates sunlight into a
continuous spectrum of colors – pg. 138.
• The separation of white light by a prism
• The electromagnetic spectrum includes all
forms of electromagnetic radiation – pg.
139.
The Electromagnetic Spectrum
Visible Light …
Note the trends: Blue light has shorter λ, higher v, and
more energy. Red light has longer λ, lower v, and less
energy.
Brain Pop - The EM Spectrum
• The wave model of light cannot explain all
of light’s characteristics.
• An example is the photoelectric effect ,
when electrons are emitted from a metal’s
surface when light of a certain frequency
shines on it (how solar calculators work).
• Photoelectric Effect Simulation
• Albert Einstein proposed in 1905
that light has a dual nature.
• Einstein suggested a beam of light
has wavelike and particlelike
properties.
• A photon is a particle of
electromagnetic radiation with no
mass that carries a quantum of
energy.
Ephoton = h
Ephoton represents energy, h is Planck's constant
(6.626 x 10-34 J-s), &  represents frequency.
Atomic Emission Spectra
• Light in a neon sign is produced when
electricity is passed through a tube filled
with neon gas and excites the neon atoms.
• The excited atoms emit light to release
energy.
Emission Spectrum for Hydrogen
• The atomic emission spectrum of an element is the
set of frequencies of the electromagnetic waves
emitted by the atoms of the element.
• Each element’s atomic emission spectrum is
unique – they have their own fingerprints!
Absorption and Emission Spectra's
Section 5.2 Quantum Theory and the
Atom
Bohr's Model of the Atom…
• Bohr suggested that an
electron moves around the
nucleus only in certain
allowed circular orbits Planetary Atomic Model.
• The lowest allowable energy
state of an atom is called its
ground state.
• When an atom gains energy,
it is in an excited state.
Planetary Atomic Model
Bohr Model
• Bohr assigned a quantum number for each
principal energy level.
• The highest quantum number can be found
by the period the element is in on the
Periodic Table.
• He tried to predict the spectral lines for
elements following Hydrogen, but was not
successful.
• The behavior of electrons is still not fully
understood, but it is known they do not
move around the nucleus in circular orbits.
Big Discoveries…
• Louis de Broglie
hypothesized that
particles, including
electrons, could also have
wavelike behaviors.
• Werner Heisenberg
showed it is impossible to
take any measurement of
an object without
disturbing it.
• The Heisenberg uncertainty principle
states that it is fundamentally impossible
to know precisely both the velocity and
position of a particle at the same time.
• The only quantity that can be known is
the probability for an electron to occupy
a certain region around the nucleus.
Our Current Atomic Theory…
• Erwin Schrödinger treated electrons as
waves in a model called the quantum
mechanical model of the atom. Most
people call this the Electron Cloud Model.
• This model applied to all elements!!!
• The Electron Cloud is a 3-D arrangement of
electrons around the nucleus.
• Within the cloud, electrons are arranged by
energy level, sublevel, and orbital shape.
• When combined, a spherical shape is the
result.
• There is an attraction between the
positively charged nucleus and the
negatively charged electrons in the electron
cloud.
Orbital Shapes…
Section 5.3 Electron Configuration
Valence Electrons
• Valence electrons are defined as electrons
in the atom’s outermost orbitals—those
associated with the atom’s highest principal
energy level.
• An element’s valence electrons determine
the chemical properties of the element.
• The number of valence electrons can be
found using its group number on the
periodic table.
• Electron-dot structure consists of the
element’s symbol representing the
nucleus and inner electrons,
surrounded by dots representing the
element’s valence electrons.