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Transcript
10/2/2013
5 Early Atomic Theory and Structure
Chapter Outline
5.1
Dalton’s Model of the Atom
5.2
Electric Charge
A. Discovery of Ions
Lightning occurs when electrons
move to neutralize charge difference
between the storm clouds and Earth.
Foundations of College Chemistry, 14th Ed.
Morris Hein and Susan Arena
5.3
Subatomic Parts of the Atom
5.4
The Nuclear Atom
5.5
Isotopes of the Elements
5.6
Atomic Mass
Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
Early Theories on the Structure of Matter
Dalton’s Model of the Atom
Early models of the atom were developed by the Greeks.
Empedocles proposed matter was composed of
four basic elements: earth, air , water and fire.
Democritus proposed matter was composed of small,
indivisible particles he called atoms.
Atoms could combine in different ways,
giving rise to the diversity of compounds we observe.
Aristotle, an influential philosopher,
supported Empedocles’ theory, so atomic theory was not
fully accepted until 2000 years later.
Dalton’s theory of atoms,
proposed in the early 1800s, states:
1. Elements are composed of small, indivisible particles
called atoms.
2. Atoms of the same element are identical in mass and size.
3. Atoms of different elements differ in their mass and size.
4. Compounds are formed by combining two or more atoms
of different elements.
5. Atoms combine to form compounds in simple whole
number ratios.
6. Atoms of two elements may combine in different ratios,
leading to formation of different compounds.
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
Dalton’s Model of the Atom
Dalton’s Model of the Atom
Revisions to Dalton’s Theory
H2O
H2O2
a. Atoms are individual particles which are different
for each element.
b./c. Atoms combine in fixed ratios to form compounds.
Two elements can combine in varying ratios
to give different compounds.
1. Elements can be decomposed under certain conditions.
2. Not all atoms of the same element have identical mass.
These are called isotopes.
3. Atoms are not indivisible.
Atoms are composed of subatomic particles.
Most of Dalton’s theory remains valid today.
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
1
10/2/2013
Electric Charge
Properties of Electric Charge
1. Charge may be either positive or negative.
2. Opposite charges (positive and negative) attract
while like charges (i.e. negative and negative) repel.
3. Charge may be transferred from one object to another,
by contact or induction.
Subatomic Parts of the Atom
A single atom is tiny (diameter of 0.1 to 0.5 nm).
Because atoms are so small, determining the presence
of subatomic particles was very difficult.
New instruments in the early 1900s permitted
detection of these particles.
4. The force of attraction between charges (F) is related to
the distance between charges by:
F=
kq1q2
r2
where q1 and q2 are the charges,
r is the distance between charges, and k is a constant.
A scanning tunneling microscope (STM) image shows an array of Cu atoms.
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
Subatomic Parts of the Atom
Electrons and Protons
A Crooks tube permits generation of cathode rays,
which are streams of electrons.
Electrons (e–):
A particle with negative electrical charge
(assigned a relative charge of –1).
Electrons have a very small mass
(9.110 x 10–28 g) and size (<10–12 cm).
Protons (p):
A Crooks (cathode) ray tube. The stream of electrons passes between the electrodes.
The electron beam is deflected by both electric
and magnetic fields, indicating it has charge.
A particle with positive electrical charge
(assigned a relative charge of +1).
Protons have a much larger mass
(~1837 times the mass of an electron).
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
The Effect of Subatomic Particles
The Effect of Subatomic Particles
Thomson’s work demonstrated the atom is composed
of smaller, charged particles.
Dalton’s theory of the atom then had to be revised.
Atoms can become ions by gaining or losing
electrons from this sphere.
Thomson’s Model of the Atom
Electrons are negatively charged particles which are
embedded in a positively charged atomic sphere.
Electrons are lost from atoms to give cations.
Electrons
+ charged
sphere
Thomson’s “plum pudding”
model of the atom.
Electrons are gained from atoms to give anions.
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
2
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Neutrons
The last subatomic particle was discovered
by Chadwick in 1932.
Summary of Subatomic Particles
Atoms are composed of three smaller, subatomic
particles: electrons, protons and neutrons.
Neutrons (n)
A particle with no electrical charge.
Neutrons have a mass similar to that of a proton.
Chemical properties of atoms can be described
based on the electrons, protons and neutrons.
Though other subatomic particles are now known,
the theories of atomic structure are based
only on these 3 subatomic particles.
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
Nuclear Model of the Atom
Nuclear Model of the Atom
In 1911, Ernest Rutherford established the nuclear model
of the atom by bombarding gold atoms with α particles.
Because most of the particles were not deflected,
this suggested most of the atom is empty space.
Most of the particles passed through the gold foil,
but some were deflected and some even bounced back!
Protons and neutrons are located in the nucleus.
Electrons are dispersed throughout the remainder
of the atom (mainly open space).
Neutral atoms contain the same number of protons
and neutrons to maintain charge balance.
This suggested the gold atoms must have a densely,
positively charged nucleus to affect the path of an
α particle (a positively charged He atom).
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
Atomic Number
Isotopes of the Elements
Atomic Number: Number of protons in the
nucleus of an atom.
After discovery of the nuclear model of the atom,
the mass of almost all atoms was found to be larger than
expected, based on the number of protons and electrons.
This led to the discovery of neutrons.
The atomic number determines the identity of the atom.
Atomic numbers for every element are above the
element’s symbol in the periodic table.
27
Atomic Number
Co
© 2014 John Wiley & Sons, Inc. All rights reserved.
Though all atoms of the same element have the same
number of protons, atoms of the same element may have
different numbers of neutrons.
Isotopes: atoms of an element with the same atomic
number but different numbers of neutrons.
© 2014 John Wiley & Sons, Inc. All rights reserved.
3
10/2/2013
Isotopes of the Elements
Isotopes of the Elements
Example: Isotopes of Hydrogen
Protium
1 proton
0 neutrons
Deuterium
Tritium
1 proton
1 neutron
1 proton
2 neutrons
Standard Isotopic Notation
Mass Number
A
Atomic Number
Z
E
Element Symbol
Practice:
How many protons, neutrons, and electrons are found
in each of the following isotopes?
64
Cu
29
Mass number: Total number of protons and neutrons
for an element.
Atomic Number: 29 protons (therefore 29 electrons)
# Neutrons = Mass Number – Atomic Number
64 – 29 = 35 neutrons
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
Let’s Practice!
Atomic Mass
Which isotope corresponds to an element with
24 protons and 28 neutrons?
Because the mass of a single atom is so small,
it is inconvenient to use this as a mass unit.
Instead, relative atomic mass units (amu) are used.
a.
28
52
Cr
Solution:
b.
52
24
Cr
c.
52
28
Ni
# protons = Atomic Number = 24
Element: Cr
d. 128 Te
52
e.
24
52
Mass Number = protons + neutrons
= 24 + 28 = 52
12
Using carbon-12, 6 C , as a standard, 1 atomic mass unit
is equal to 1/12th the mass of a carbon-12 atom.
1 amu = 1.6606 x 10-24 g
All periodic tables use atomic masses based on
the carbon-12 isotope.
Cr
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
Atomic Mass and Isotope Distribution
Atomic Mass and Isotope Distribution
Since most elements are a mixture of isotopes,
the atomic mass for an element is the weighted average
of all naturally occurring isotopes of the element.
Example:
The atomic mass of Cu is 63.546 amu.
Cu exists as 2 major isotopes, Cu-63 and Cu-65.
Cu-63 is more abundant,
as the atomic mass is very close to 63 amu.
Calculating average atomic mass:
Sum of the atomic mass of each isotope
multiplied by its % abundance.
© 2014 John Wiley & Sons, Inc. All rights reserved.
Average atomic mass of Cu:
(62.9298) x (0.6909) + (64.9278) x (0.3091) = 63.55 amu
Atomic Mass % Abundance
Atomic Mass % Abundance
Measuring Cu isotope abundances
by using mass spectrometry.
© 2014 John Wiley & Sons, Inc. All rights reserved.
4
10/2/2013
Atomic Mass Practice
Silver exists as two isotopes with atomic masses of
106.9041 and 108.9047 amu.
Determine the average atomic mass for silver if the
% abundance for each isotope is 51.82 and
48.18%, respectively.
Average atomic mass of Ag:
(106.9041) x (.5182) + (108.9047) x (0.4818) = 107.8680 amu
Let’s Practice!
Chlorine exists as two isotopes, Cl-37 (36.96590 amu)
and Cl-35. If the percent abundance of each isotope is
24.47 % and 75.53 %, what is the atomic mass of
Cl-35 if the average atomic mass is 35.46 amu?
a. 36.95690
d. 34.96885
b. 36.57823
e. 33.56438
c. 35.64544
Solution:
Solve for a:
(36.96590) x (.2447) + (a) x (0.7553) = 35.46 amu
Atomic Mass % Abundance Atomic Mass % Abundance
9.046 + (a) x (0.7553) = 35.46 amu
(a) x (0.7553) = 26.41 amu
a = 34.97 amu
© 2014 John Wiley & Sons, Inc. All rights reserved.
© 2014 John Wiley & Sons, Inc. All rights reserved.
Learning Objectives
Learning Objectives
5.1 Dalton’s Model of the Atom
Describe Dalton’s model of the atom and compare it to the
earlier concepts of matter.
5.2 Electric Charge
Use Coulomb’s Law to calculate the force between particles
and distinguish between a cation and anion.
5.3 Subatomic Parts of the Atom
Describe the three basic subatomic particles and
how they changed Dalton’s model of the atom.
© 2014 John Wiley & Sons, Inc. All rights reserved.
5.4 The Nuclear Atom
Explain how the nuclear model of the atom differs
from the Dalton and Thomson models.
5.5 Isotopes of the Elements
Define the terms atomic number, mass number
and isotope.
5.6 Atomic Mass
Define the relationship between the atomic mass of an
element and the masses of its isotopes.
© 2014 John Wiley & Sons, Inc. All rights reserved.
5