Chemistry
Test 9/22 (A day) or 9/23 (B day)
Topics include:
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Significant figures
Dimensional Analysis
Basic Atomic Structure
Percent Error problems
Density
Atomic Mass Unit Problems
Scientific Notation
Basic SI units
Basic Lab equipment
Half-life
Radioactive decay
Accuracy vs. Precision
Nuclear Transformations
Atomic Number
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Elements are different because they contain different numbers of protons
The atomic number of an element is the number of protons in the nucleus
o Atomic number identifies an element
o Since atoms are neutral, number of protons = number of electrons
o The number of protons never changes for an element
o The number of electrons might change for an element (forms an ion)
o Elements on the periodic table are organized according to increasing atomic
number, or increasing number of protons
Mass Number
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Total number of protons and neutrons in an atom is called the mass number
Number of neutrons can be determined by
o Number of neutrons = mass number – atomic number
Shorthand notation for an element
A
Z
X
X = Element symbol from the periodic table
A = Mass Number (p + n)
Z = Atomic Number (p)
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Usually abbreviate the names of the element by saying the name of the element followed by
its atomic mass, “carbon-12.”
Isotopes
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Isotopes are atoms of the same element that have different masses. The number of protons
and electrons is the same for all isotopes of an element, but the number of neutrons is
different, causing each isotope to have a different atomic mass. Atomic number does not
change.
o Hydrogen-1 has no neutrons, mass number of 1
o Hydrogen-2 (deuterium) has one neutron, mass number of 2
o Hydrogen-3 (tritium) has two neutrons, mass number of 3
Isotopes are chemically alike because they have identical numbers of protons and electrons
which are the subatomic particles responsible for chemical behavior
Why can there be more than one possible number of neutrons in an atom? For many atoms,
there can be several different numbers of neutrons that serve to stabilize the positive charge
in the nucleus.
Atomic Mass or Atomic Weight
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Atomic mass is measured in atomic mass units (amu), defined as 1/12 the mass of a carbon12 atom. (Carbon-12 is the standard reference.)
1 amu = 1.67 x 10-27 kg
Atomic mass is not always a whole number because of the relative abundance of the
naturally occurring isotopes of the element.
Most elements occur as a mixture of two or more isotopes
Each isotope of an element has a fixed mass and natural percent abundance
The atomic mass of an element is the weighted average mass of the atoms in a naturally
occurring sample of the element.
How to calculate atomic mass:
o Need the number of stable isotopes of the element
o The mass of each isotope
o The natural percent abundance of each isotope
Example:
Chlorine-35 occurs 75.77% (34.969 amu)
Chlorine-37 occurs 24.23% (36.966 amu)
Convert percentages into decimals (75.77% = 0.7577)
(0.7577 x 34.969) + (0.2423 x 36.966) = 35.453 amu
Closer to the mass of Chlorine-35 since that’s the more abundant isotope. Note the correct
number of significant figures.
Atoms
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The number of protons in an atom determines the identity of the atom. Atomic number =
number of Protons
In a neutral atom, the number of positive protons equals the number of negative electrons.
Protons and neutrons both have a mass of 1 amu. The mass of the electron is negligible
compared to the mass of the proton and neutron. Thus the mass number, or the mass of the
atom, is the sum of the number of protons and neutrons. Mass number = number of Protons
+ number of Neutrons
Isotopes
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The number of neutrons in any specific type of atom can vary. Atoms of the same element
with different numbers of neutrons are called isotopes.
Isotopes are distinguished from each other by including the mass number with the name or
symbol.
Ions
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As we have seen, in a neutral atom, the number of protons and the number of electrons is
equal.
Atoms can gain or lose electrons to become ions. Ions are charged atoms resulting from the
difference in number of positive protons and negative electrons.
A cation is a positive ion. A cation results when an atom loses electrons. Number of Protons
> Number of Electrons
An anion is a negative ion. An anion results when an atom gains electrons. Number of
Electrons > Number of Protons
Ions are distinguished from atoms by including the ion charge as a superscript in the symbol.
Nuclear Chemistry
Symbol
Deflection in
magnetic or
electric fields
4
2𝐻𝑒
Some deflection
towards negative
plate
Paper or a few
centimetres of
air
Large deflection
towards positive
plate
Aluminium
plate 1-2 cm
thick
No deflection
Lead 1 cm thick
or 2 m of
concrete
Mass
Type of
radiation
Alpha
radiation
Charge
g mol-1
+2
4
Nature
2 protons
2 neutrons
Relative
penetration
power
(α particle)
helium nucleus
4
2𝛼
Beta
radiation
-1
1/1840
high energy
electron
0
−1𝑒
(β particle)
Gamma
radiation
0
−1𝛽
0
(  rays)
0
Short wavelength
electromagnetic
radiation

Half-Life Calculations
The definition for half-life is the time required for half the original amount of substance to decay.
This can be mass or radiation count. In this unit we will only use whole number half-lives.
Number of Half-lives
0
1
2
3
4
5
Fraction remaining
1/2
1/4 (1/2 x 1/2)
1/8 (1/2 x ½ x ½ )
1/16 (1/2 x ½ x ½ x ½ )
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% remaining
100
50
25
12.5
6.25
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