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PreAP Chemistry Chapter 3
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Democritus was the early (around
400BC) Greek philosopher who is
credited with the concept of the atom
(atomos) –which means invisible
Dalton (around 1800AD) is an English
school teacher who proposed the law of
conservation of mass, the law of
definite proportions, and the law of
multiple proportions.
His many experiments
with gases proved
these laws are true, if
atoms exist.
Dalton is also known as the Father of
the (Modern) Atomic Theory
Dalton’s atomic theory:
1.All matter is composed of very small
particles called atoms
Dalton’s atomic theory:
1.All matter is composed of very small
particles called atoms
2.Atoms of a given element are
identical in size, mass, and other
properties; atoms of different
elements differ in these properties.
3.Atoms cannot be subdivided, created,
or destroyed
3.Atoms cannot be subdivided, created,
or destroyed
4.Atoms of different elements combine
in simple whole-number ratios to form
chemical compounds.
3.Atoms cannot be subdivided, created,
or destroyed
4.Atoms of different elements combine
in simple whole-number ratios to form
chemical compounds.
5.In chemical reactions, atoms are
combined, separated, or rearranged.
Two aspects of Dalton’s atomic theory
proven to be incorrect:
a.We now know atoms are divisible.
Two aspects of Dalton’s atomic theory
proven to be incorrect:
a.We now know atoms are divisible.
b. Atoms of the same element can have
different masses.
Atom --smallest particle of an element
that retains the properties of that
element.
J. J. Thomson is the man credited with
the discovery of the electrons in the
late 1800’s, using cathode ray tubes
Millikan calculated the mass of the
electron (very, very small)
Knowledge of electrons led to two
inferences about atomic structure:
1.Because atoms are electrically
neutral, they must contain positive
charge to balance the negative
electrons.
Knowledge of electrons led to two
inferences about atomic structure:
1.Because atoms are electrically
neutral, they must contain positive
charge to balance the negative
electrons.
2. Because electrons have so little
mass, atoms must contain other
particles to account for most of their
mass
Nucleus of the atom—discovered by
Lord Ernest Rutherford
Nucleus of the atom—discovered by
Lord Ernest Rutherford
Gold foil experiment—actually done
by Hans Geiger and Ernest Marsden
Observations:
a.Majority of the alpha (α) particles
penetrated foil undeflected.
b.About 1 in 20,000 were slightly
deflected
c.About 1 in 20,000 were deflected
back to emitter
Conclusions:
1. Mass of the atom and the positive
charge are concentrated in small
regions called nucleus
Conclusions:
1. Mass of the atom and the positive
charge are concentrated in small
regions called nucleus
2. Most of the atom is empty
Conclusions:
1. Mass of the atom and the positive
charge are concentrated in small
regions called nucleus
2.Most of the atom is empty
3.Magnitude of charge on the nucleus
is different for different atoms
4. Number of electrons outside the
nucleus = number of units of nuclear
charge (to account for the fact that the
atom is electrically neutral)
4. Number of electrons outside the
nucleus = number of units of nuclear
charge (to account for the fact that the
atom is electrically neutral)
Atoms are electrically neutral because
they contain equal numbers of
protons and electrons
A couple years later Rutherford
presented evidence for a neutral
particle which was also in the nucleus
and contained a similar mass to that
of a proton – called a neutron
Mass of one proton = mass of neutron
= mass of 1837 electrons
Mass of one proton = mass of neutron
= mass of 1837 electrons
Thus the total mass of an atom is
basically the sum of the protons and
neutrons, called the atomic mass or
mass number, abbreviated A
Atomic number—the number of protons
in the nucleus of the atom.
Atomic number—the number of protons
in the nucleus of the atom.
--number of protons identifies the
element and is equal to the number of
electrons (of a neutral atom)
--symbol is Z
Isotopes are atoms of the same
element that have different masses
because they have different numbers
of neutrons but they still have similar
chemical properties
Isotopes of Carbon
Mass Number of
Isotope
Number of Protons
Number of
Neutrons
8
6
2
9
6
3
10
6
4
11
6
5
12
6
6
13
6
7
14
6
8
15
6
9
16
6
10
17
6
11
18
6
12
19
6
13
20
6
14
Isotopes of Carbon
Mass Number of Carbon
Isotopes
Name of
Isotopes
8
carbon-8
9
carbon-9
10
carbon-10
11
carbon-11
12
carbon-12
13
carbon-13
14
carbon-14
15
carbon-15
16
carbon-16
17
carbon-17
18
carbon-18
19
carbon-19
20
carbon-20
Nuclide is the general term for any
isotope of any element
Nuclide is the general term for any
isotope of any element
Atomic Mass Unit (amu) is exactly 1/12
the mass of a carbon-12 atom
Average atomic mass is the weight
average of the atomic masses of the
naturally occurring isotopes of an
element.
Ave. Atomic mass = %abundance(mass
of isotope 1) + %abundance(mass of
isotope 2) +…..
Example 1:
Element Sciencium has two isotopes.
Sciencium-301 has an abundance of
59.5%, and Sciencium-304 is the
other. What is the average atomic
mass?
301 amu x .595 = 179 amu
304 amu x .405 = 123 amu
302 amu
Example 2:
Element Pepsium has an average
atomic mass of 335. Two isotopes of
Pepsium exist. If Pepsium-327 is
30.5% abundant, then what is the
second isotope?
327 amu x 0.305 = 99.7 amu
? amu x 0.695 =
? amu
335 amu
1 – 0.305 = 0.695
Example 2:
Element Pepsium has an average
atomic mass of 335. Two isotopes of
Pepsium exist. If Pepsium-327 is
30.5% abundant, then what is the
second isotope?
327 amu x 0.305 = 99.7 amu
? amu x 0.695 =
? amu
335 amu
Step 1: Find the missing weighted mass
Example 2:
Element Pepsium has an average
atomic mass of 335. Two isotopes of
Pepsium exist. If Pepsium-327 is
30.5% abundant, then what is the
second isotope?
327 amu x 0.305 = 99.7 amu
? amu x 0.695 = 235 amu
335 amu
335 – 99.7 = 235.3
Example 2:
Element Pepsium has an average
atomic mass of 335. Two isotopes of
Pepsium exist. If Pepsium-327 is
30.5% abundant, then what is the
second isotope?
327 amu x 0.305 = 99.7 amu
? amu x 0.695 = 235 amu
335 amu
Step 2: Find the missing mass
Example 2:
Element Pepsium has an average
atomic mass of 335. Two isotopes of
Pepsium exist. If Pepsium-327 is
30.5% abundant, then what is the
second isotope?
327 amu x 0.305 = 99.7 amu
339 amu x 0.695 = 235 amu
335 amu
235.3 ÷ 0.695 = 339
Unstable Nuclei and
Radioactive Decay
1) When referring to nuclear
reactions people commonly
think of nuclear fission (the
splitting of large atoms into
smaller pieces)
1) and nuclear fusion (the
combining of small atoms into
one large one), but on earth
these reactions do not occur
naturally.
2) Naturally occurring nuclear
reactions result from the unusual
number of neutrons of an isotope
which makes it unstable
(unusually high in energy). This
often results in the isotope
changing from one element into
another element in an attempt to
become more stable (lower in
energy).
A) These reactions are called
nuclear reactions, as they
involve changes in the
nucleus.
A)These reactions are called
nuclear reactions, as they
involve changes in the
nucleus.
B)During these nuclear
reactions, rays and
particles are given off,
which is called radiation.
C) Sometimes an unstable
nucleus will change into
many different elements as
it tries to become more
stable. This is called
radioactive decay.
3) When radioactive decay
occurs, there are three
different types of radiation
that can be given off. Each
type has a different mass,
and sometimes a charge.
A) The first type of radiation
to be discovered was called
alpha radiation and came
from alpha particles.
i) Because the mass
numbers must be equal,
226 = 222 + x. So the
mass of the alpha particle
must be
i) Because the mass
numbers must be equal,
226 = 222 + x. So the
mass of the alpha particle
must be 4.
ii) Because the atomic
numbers must be equal,
88 = 86 + x. So the atomic
number of the alpha
particle must be.
ii) Because the atomic
numbers must be equal,
88 = 86 + x. So the atomic
number of the alpha
particle must be 2.
iii) The element Helium has a
mass of 4 and an atomic
number of 2, so the alpha
particle is just like a helium
atom without any electrons;
4
4
He or 
2
2
B) The second type of
radiation to be discovered
was called beta radiation
and came from beta
particles.
i) Because the mass numbers
must be equal, 14 = 14 + x.
So the mass of the beta is
i) Because the mass numbers
must be equal, 14 = 14 + x.
So the mass of the beta is
zero.
ii) Because the atomic
numbers must be equal,
6 = 7 + x. So the atomic
number must be
ii) Because the atomic
numbers must be equal,
6 = 7 + x. So the atomic
number must be -1.
iii) The electron has a mass
of zero and a charge of -1,
so the beta particle is just
like an electron;
0
β
-1
C) The last type of radiation
to be discovered was called
gamma radiation and came
from gamma particles.
i) Because the mass
numbers must be equal,
238 = 234 + 4 + x. So the
mass of the gamma particle
must be
i) Because the mass
numbers must be equal,
238 = 234 + 4 + x. So the
mass of the gamma particle
must be zero.
ii) Because the atomic
numbers must be equal,
92 = 90 + 2 + x. So the
atomic number must also
be zero.
iii) The gamma particle was
the last to be found
because it has no mass
and no charge;
0
0
γ
Geiger Counter
Alpha, Beta, and
Gamma
Radiation