Transcript 5.1 Revising the Atomic Model

5.1 Revising the Atomic Model >
Chapter 5
Electrons In Atoms
5.1 Revising the Atomic
Model
5.2 Electron Arrangement in Atoms
5.3 Atomic Emission Spectra and
the Quantum Mechanical Model
1
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
Objectives
1. Describe what Bohr proposed in his
model of the atom.
2. Describe what the quantum
mechanical model determines
about the electrons in a atom.
3. Explain how sublevels of principal
energy levels differ.
4. Define the following terms: energy
level, quantum, quantum
mechanical model, and atomic
orbital.
2
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
CHEMISTRY
& YOU
Why do scientists use mathematical
models to describe the position of
electrons in atoms?
Shown here is a lifesized model of a skier,
but not all models are
physical. In fact, the
current model of the
atom is a
mathematical model.
3
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
Development of Atomic Models
Time
Scientists
Discoveries
1803
John Dalton
Atomic theory
1897
J.J. Thomson
Electrons and pudding model
1911
Ernest Rutherford
Nucleus and a lot of space in an atom
1913
Niels Bohr
Electrons moves in a circular orbit at
fixed distances from the nucleus
1926
Erwin Schrodinger Developed equations which leads to
the quantum mechanical model
1932
James Chadwick
4
Confirmed the existence of neutrons
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
Limitations of Rutherford’s Atomic Model
Rutherford proposed that in the nuclear
atom (the name of his model), the protons
and neutrons are located in the positively
charged nucleus. The electrons are
distributed around the nucleus and
occupy almost all the volume of the atom.
He did not address the exact location or
movement of the electrons.
5
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Energy Levels in Atoms
Limitations of Rutherford’s Atomic Model
• It explained only a few simple properties of atoms.
• It could not explain the chemical properties of
elements.
For example, Rutherford’s
model could not explain why
an object such as the iron
scroll shown here first glows
dull red, then yellow, and then
white when heated to higher
and higher temperatures.
6
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Energy Levels in Atoms
The Bohr Model
In 1913, Niels Bohr (1885–1962), a young
Danish physicist and a student of
Rutherford, developed a new atomic model.
• He changed Rutherford’s model to
incorporate newer discoveries about
how the energy of an atom
changes when the atom absorbs
or emits light.
7
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Energy Levels in Atoms
The Bohr Model
Bohr proposed that an electron
is found only in specific circular
paths, or orbits, around the
nucleus.
Each possible electron orbit in
Bohr’s model has a fixed energy.
8
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Energy Levels in Atoms
The Bohr Model
• The fixed energies an electron
can have are called energy
levels.
• A quantum of energy is the
amount of energy required to
move an electron from one
energy level to another energy
level.
9
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
Quantum
The values of some measurable variables of
a system, most notably the total energy of
a bounded system, can attain only certain
discrete values determined by the system.
(The smallest possible jumps in the values
of those observables are called "quanta"
(Latin quantum, quantity), hence the name
quantum mechanics.)
http://encyclopedia.kids.net.au/page/qu/Quantum_mechanics
10
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Energy Levels in Atoms
The Bohr Model
The rungs on this ladder are somewhat
like the energy levels in Bohr’s model of
the atom.
• A person on a ladder
cannot stand between
the rungs. Similarly, the
electrons in an atom
cannot exist between
energy levels.
11
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Energy Levels in Atoms
The Bohr Model
• The energy levels in
atoms are unequally
spaced, like the
rungs in this unusual
ladder. The higher
energy levels are
closer together.
12
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Bohr’s model
http://www.bing.com/images/search?q=quantum+mechanica
l+model+animation&view=detail&id=3561782DCBEED8F3F
13
8AAC1F74544E98F4757ADB6&first=1
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
How does the Bohr model improve
upon the Rutherford model?
The Rutherford model could not explain why
elements that have been heated to higher
and higher temperatures give off different
colors of light. The Bohr model explains
how the energy levels of electrons in an
atom change when the atom emits light.
14
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > The Quantum
Mechanical Model
The Quantum Mechanical Model
What does the quantum
mechanical model determine
about the electrons in an atom?
15
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > The Quantum
Mechanical Model
• Austrian physicist Erwin Schrödinger (1887–
1961) used new theoretical calculations and
experimental results to devise and solve a
mathematical equation describing the
behavior of the electron in a hydrogen atom.
• The modern description of the electrons in
atoms, the quantum mechanical model,
came from the mathematical solutions to the
Schrödinger equation.
16
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > The Quantum
Mechanical Model
• Like the Bohr model, the quantum
mechanical model of the atom
restricts the energy of electrons
to certain values.
• Unlike the Bohr model, however,
the quantum mechanical model
does not specify an exact path
the electron takes around the
nucleus.
17
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > The Quantum
Mechanical Model
The quantum mechanical model
determines the allowed energies
an electron can have and how
likely it is to find the electron in
various locations around the
nucleus of an atom.
Probability describes how likely it is to find
an electron in a particular location around
the nucleus of an atom.
18
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > The Quantum
Mechanical Model
• In the quantum mechanical
model, the probability of
finding an electron within a
certain volume of space
surrounding the nucleus
can be represented as a
fuzzy cloudlike region
(electron cloud).
• The cloud is more dense
(darker in color) where the
probability of finding the
electron is high.
19
Electron cloud
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
How are the quantum mechanical
model and the Bohr model alike?
How are they different?
Like the Bohr model, the quantum
mechanical model restricts the energy of
electrons to certain values.
Unlike the Bohr model, the quantum
mechanical model does not specify an
exact path the electron takes around the
nucleus.
20
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Atomic Orbitals
Atomic Orbitals
How do sublevels of principal
energy levels differ?
21
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Atomic Orbitals
• Solutions to the Schrödinger equation
give the energies, or energy levels, an
electron can have.
• For each energy level, the Schrödinger
equation also leads to a mathematical
expression, called an atomic orbital.
• An atomic orbital is represented
pictorially (illustrated by pictures) as a
region of space in which there is a high
probability of finding an electron.
22
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Atomic Orbitals
• The energy levels of electrons in the quantum
mechanical model are labeled by principal
quantum numbers (n).
• These numbers are assigned the values n = 1, 2,
3, 4, and so forth.
• For each principal energy level greater than 1,
there are several orbitals with different shapes and
at different energy levels.
• These energy levels within a principal energy
level constitute energy sublevels.
23
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Atomic Orbitals
Each energy sublevel
corresponds to one or more
orbitals of different shapes.
The orbitals describe where an
electron is likely to be found.
24
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Atomic Orbitals
Different atomic orbitals are denoted by
letters.
• The s orbitals are spherical.
• The p orbitals are dumbbell-shaped.
25
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Atomic Orbitals
Different atomic orbitals (sublevels under
the principal energy level) are denoted by
letters: there is one s orbital, 3 p orbitals,
and 5 d orbitals (and even f orbitals which
are more complicated then d orbitals).
26
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Atomic Orbitals
Four of the five d orbitals have the same
shape but different orientations in space.
27
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Atomic Orbitals
The numbers and types of atomic orbitals
(sublevels) depend on the principal energy level.
Summary of Principal Energy Levels and Sublevels (atomic orbitals)
Principal
energy
level
Number
of
sublevels
n=1
1
1s (1 orbital) / 1
2
n=2
2
2s (1 orbital), 2p (3 orbitals) / 4
8
n=3
3
3s (1 orbital), 3p (3 orbitals),
3d (5 orbitals) / 9
18
n=4
4
4s (1 orbital), 4p (3 orbitals),
4d (5 orbitals), 4f (7 orbitals) / 16
32
28
Type of sublevel/ Number of
sublevels (orbitals)
Maximum number
of electrons
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Atomic Orbitals
• The principal quantum number, n, always
equals the number of sublevels within that
principal energy level.
• The number of orbitals in a principal
energy level is equal to n2.
• A maximum of two electrons can occupy
an orbital.
• Therefore, the maximum number of
electrons that can occupy a principal
energy level is given by the formula 2n2.
29
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
Calculate the maximum number of
electrons in the 5th principal energy
level (n = 5).
If n = 5, 2n2 = 50
30
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
CHEMISTRY
& YOU
Why do scientists no longer use
physical models to describe the motion
of electrons?
• Previous models of the atom were physical models
based on the motion of large objects.
• Theoretical calculations and experimental results
showed that these models did not always correctly
describe electron motion.
• Schrödinger devised a mathematical equation
describing the behavior of the electron in a hydrogen
atom. The quantum mechanical model came from the
solutions to the Schrödinger equation.
31
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
Some quotations
– Those who are not shocked when they
first come across quantum mechanics
cannot possibly have understood it.
By Niels Bohr
– God does not play dice with the
cosmos.
By Albert Einstein
http://encyclopedia.kids.net.au/page/qu/Quantum_mechanics
32
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
German physicist
and one of the
founding fathers of
Quantum Theory,
Max Planck,
http://www.quantumjumping.com/lp/subconscious?sr=1&cid=Display-CampaignOptimizer&aid=QuantumJumping&placement=encyclopedia.kids.net.au&gclid=CL33saq2g7MCFcmd4AodvWsA
bg
33
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
34
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Key Concepts
Bohr proposed that an electron is found only in
specific circular paths, or orbits, around the
nucleus.
The quantum mechanical model determines
the allowed energies an electron can have and
how likely it is to find the electron in various
locations around the nucleus of an atom.
Each energy sublevel corresponds to one or
more orbitals of different shapes, which
describe where the electron is likely to be
found.
35
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Glossary Terms
• energy level: the specific energies an
electron in an atom or other system can
have
• quantum: the amount of energy needed to
move an electron from one energy level to
another
36
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model > Glossary Terms
• quantum mechanical model: the modern
description, primarily mathematical, of the
behavior of electrons in atoms
• atomic orbital: a mathematical expression
describing the probability of finding an
electron at various locations; usually
represented by the region of space around
the nucleus where there is a high probability
of finding an electron
37
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
BIG IDEA
Electrons and the Structure of Atoms
• The quantum mechanical model of the atom
comes from the solutions to the Schrödinger
equation.
• Solutions to the Schrödinger equation give
the energies an electron can have and the
atomic orbitals, which describe the regions
of space where an electron may be found.
38
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
5.1 Revising the Atomic Model >
END OF 5.1
39
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.