Download Atomic masses are weighted averages.

Chapter 4
Atomic Structure
Anything in black letters = write it in
your notes (‘knowts’)
4.1 – Defining the Atom
Atom - smallest particle of an element that still
has the properties of the element
comes from the Greek word atomos
which means uncuttable or indivisible
Democritus (460 B.C. – 370 B.C.)
one of the first to propose the idea of the atom;
based on pure speculation
John Dalton (~1800)
proposed 1st atomic theory
Daltons Atomic Theory (~1800) p. 103
1. All elements are composed of tiny indivisible particles called atoms.
2. Atoms of the same element are identical. The atoms of any one
element are different from those of any other element.
3. Atoms of different elements can physically mix together or can
chemically combine in simple whole-number ratios to form compounds.
4.
Chemical reactions occur when atoms are separated from each other, joined,
or rearranged in different combinations. Atoms of one element are never
changed into atoms of another element as a result of a chemical reaction.
Atoms of
element A
Atoms of
element B
Mixture of atoms of
elements A and B
Compound made by
atoms of elements A
and B
What we know now of Dalton’s Atomic Theory
1. All elements are composed of tiny indivisible particles called atoms.
Atoms are not indivisible – they are made of subatomic particles
2. Atoms of the same element are identical. The atoms of any one
element are different from those of any other element.
Every atom has at least one isotope; one atom’s isotope is NOT
identical to another isotope of the same atom.
3. Atoms of different elements can physically mix together or can
chemically combine in simple whole-number ratios to form compounds.
This is known as the Law of Definite Proportions – very important.
4.
Chemical reactions occur when atoms are separated from each other, joined,
or rearranged in different combinations. Atoms of one element are never
changed into atoms of another element as a result of a chemical reaction.
Atoms of one element can change into an atom of another element
as a result of a nuclear reaction.
4.2 – Structure of the Nuclear Atom
Subatomic Particles
Electron – negatively charged particle in
all atoms, discovered in 1897 by J.J. Thomson
Thomson performed experiments that involved passing electric current
through gases at low pressure.
The result was a glowing beam, or cathode ray, that traveled from the cathode to
the anode.
Thomson found that a cathode ray is deflected by electrically charged metal plates.
Thompson knew that opposite charges attract and like charges repel, so he
hypothesized that a cathode ray is a stream of tiny negatively charged
particles moving at high speed; now called electrons.
To test his hypothesis,
Thompson set up an
experiment to measure
the ratio of an electron’s
charge to its mass.
Also, the charge-to-mass
ratio of electrons did not
depend on the kind of gas
in the cathode-ray tube or
the type of metal used for
the electrodes.
A cathode ray can also be deflected by a magnet.
The Atomic Nucleus
How are atoms structured?
Democritus
Dalton
Thomson’s Plum
Pudding Model
In 1911, Ernest Rutherford and others performed
the Gold Foil Experiment to test the plum pudding
model
Ernest
Rutherford
The Gold Foil Experiment
The Gold Foil Experiment
The results…
It was expected that alpha
particles would pass through
the plum pudding model of
the gold atom undisturbed.
Expected
It was observed that a small
portion of the alpha particles
were deflected, indicating a
small, concentrated positive
charge (the nucleus!)
Actual
“It was quite the most incredible event that has ever happened to
me in my life. It was almost as incredible as if you fired a 15-inch
shell at a piece of tissue paper and it came back and hit you. On
consideration, I realized that this scattering backward must be the
result of a single collision, and when I made calculations I saw that
it was impossible to get anything of that order of magnitude unless
you took a system in which the greater part of the mass of the atom
was concentrated in a minute nucleus. It was then that I had the
idea of an atom with a minute massive center, carrying a charge.”
— Ernest Rutherford
Nucleus – tiny positively charge core of an atom
Rutherford’s Nuclear
Model of the Atom
Is this the current
model of the atom?
NO…
• If an atom were the size of a football stadium,
the nucleus would be about the size of a marble
Proton – positively charged particle in the
nucleus of all atoms.
Neutron – particle with no charge in the nucleus
of all atoms except 1H
Properties of Subatomic Particles
Particle
Symbol
Relative
charge
Relative mass
(mass of proton = 1)
Actual mass
(g)
Electron
e–
1–
1/1840
9.11  10–28
Proton
p+
1+
1
1.67  10–24
Neutron
n0
0
1
1.67  10–24
ASSIGN:
Read 4.1 & 4.2
Lesson Check 4.1 & 4.2; #1-15
(page 104 & 109)
Chapter 4 Quick Quiz
1. Identify the idea or discovery these people are credited with.
Thompson,
Dalton,
Democritus,
Rutherford
2. Place the above ideas or discoveries in chronological order.
3. Describe the setup of the gold foil experiment. Also describe the
expected and actual results of this experiment. What conclusion was
made about the atom from the gold-foil experiment?
4. Name the three subatomic particles. Also give their symbol and
charge. What particles are present in the nucleus of the atom?
5. What is an atom?
4.3 – Distinguishing Among Atoms
Atomic Number (Z) The number of protons in an atom; identifies
the element.
Atoms have no net charge: # p+ = # e-
Mass Number (A) The number of protons (p+) and neutrons
(n0) in an atom.
The mass number is NOT the atomic mass.
Element
H
O
Ca
Atomic
Protons
Number (Z)
(p+)
Electrons
(e-)
Neutrons
(n0)
1
1
1
???
8
8
8
???
20
20
20
???
The number of n0 depends on the
mass number of the isotope
Isotopes Atoms of the same element that have a different
number of neutrons.
Chemical Symbols for Isotopes
A is the superscript
20
10
Ne
21
10
Ne
22
10
Z is the subscript
Ne
Determining the Composition of an Atom
How many protons, electrons, and neutrons
are in each atom?
9
4
a. Be
p+
en0
20
10
b. Ne
23
11
c. Na
Naturally Occurring Isotopes of Neon
20
10
Ne
21
10
Ne
22
10
Ne
Percent Abundance in Nature
90.48%
0.27%
9.25%
The masses of atoms are rarely expressed in grams.
The C-12 isotope has been given a mass of exactly
12 atomic mass units (amu)
The masses of all other elements are based on the
mass of the C-12 isotope.
Atomic Mass –
Weighted average of all the naturally occurring
isotopes of the element.
12
6
C
13
6
C
12.000 amu
13.003 amu
98.93 %
1.07 %
14
6
C
14.003 amu
0.0000000001 %
(12 x 0.9893)  (13.003 x 0.0107)  12.011
Atomic Mass of Carbon = 12.011 amu
12.000 amu
13.003 amu
98.93 %
1.07 %
14.003 amu
0.0000000001 %
Atomic Mass of Carbon = 12.011 amu
No atom of carbon actually weighs 12.011 amu. But
a typical carbon atom averages 12.011 amu.
Atomic masses are weighted averages.
There are 2 stable isotopes of silver
Silver-107; 106.905097 amu; 51.84%
Silver-109; 108.904752 amu; 48.16%
Calculate the atomic weight of silver.
Atomic Weight of Silver = 107.868 amu
Weighted Averages are NOT just for Atomic Masses
Your Chemistry grade is calculated as a weighted average.
50%
15%
15%
10%
10%
Quizzes/Tests,
Homework,
Lab Reports,
Quarter Project,
Respect & Participation
60/90
50/50
70/75
45/50
60/60
Calculate the un-weighted average grade
Calculate the weighted average grade
Mass Spectrometer – separates isotopes by
mass differences.
Mass Spectrum for Cadmium
Mass Spectrum for Zinc; Atomic Weight = 65.395
Despite differences in the number of neutrons,
isotopes of an element are chemically similar.
Neutrons do not determine chemical reactivity; the
electrons do.
ASSIGN:
Read 4.3 & answer #18-34 within 4.3
p. 114-119
Terms & Things to Know
Democritus, Dalton, Thomson, Rutherford
Atom
Atomic Number (Z)
Mass Number (A)
Proton, Neutron, Electron
Nucleus
Isotope
Atomic Mass (a.m.u.)
Weighted Average
Natural Abundance
How to determine the number of p+, n0, eHow to write isotope symbols
How to calculate weighted averages
Explain Gold Foil Experiment (setup, expected and
actual results, conclusion)
Different models of the atom (Democritus, Dalton,
Thomson, Rutherford)
ASSIGN:
Ch. 4 #35, 44, 47-57, 61, 64-71, 75
p. 122 -
Iso
Example…
64Zn
Calculate
65Zn
The average mass of zinc
66Zn
67Zn
68Zn
70Zn
The weighted average mass 72Zn
of zinc
NA (%)
48.6
syn
27.9
4.1
18.8
0.6
syn
Mass
63.929
64.929
65.926
66.927
67.925
69.925
71.927