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Unit 2
Atoms, Molecules, Ions,
and
Chemical Nomenclature
Section 1
Laws of Chemical Combination
and Atomic Theory
Laws of Chemical Composition
Conservation of Mass - The total mass
remains constant during a chemical
reaction.
Example: Decomposition
of mercuric oxide (HgO)
HgO(s) = Hg(l) + O2(g)
Law of Definite Proportions
All samples of a compound have the same
composition; that is, all samples have the
same proportions, by mass, of the elements
present
Water always contains:
~89% oxygen
~11% hydrogen
Law of Definite Proportions
Basic Copper Carbonate
Example 2.2
The mass ratio of oxygen to magnesium in the
compound magnesium oxide is 0.6583:1. What mass of
magnesium oxide will form when 2.000 g of magnesium
is completely converted to magnesium oxide by burning
in pure oxygen gas?
Example 2.2A
What mass of magnesium oxide is formed when 1.500 g
of oxygen combines with magnesium?
Example 2.2B
When a strip of magnesium metal was burned in pure
oxygen gas, 1.554 g of oxygen was consumed and the
only product formed was magnesium oxide. What must
have been the masses of magnesium metal burned and
magnesium oxide formed?
Law of Multiple Proportions
When two or more different compounds of
the same two elements are compared, the
masses of one element that combine with the
a fixed mass of the second element are in the
ratio of small whole numbers.
Law of Multiple Proportions
• Four different oxides of nitrogen can be formed by
combining 28 g of nitrogen with:
• 16 g oxygen, forming Compound I
• 48 g oxygen, forming Compound II
• 64 g oxygen, forming Compound III
• 80 g oxygen, forming Compound IV
What is the ratio 16:48:64:80
expressed as small whole numbers?
• Compounds I–IV are N2O, N2O3, N2O4, N2O5
Law of Multiple Proportions
Dalton’s Atomic Theory
Proposed in 1803 to explain the law of conservation
of mass, law of definite proportions, and law of
multiple proportions.
•
•
•
•
•
Matter is composed of atoms: tiny, indivisible particles.
All atoms of a given element are the same.
Atoms of one element differ from atoms of other
elements.
Compounds are formed when atoms of different elements
unite in fixed proportions.
A chemical reaction involves rearrangement of atoms.
No atoms are created, destroyed, or broken apart.
Dalton’s Atomic Theory
• All matter is composed of extremely small,
indivisible particles called atoms
• All atoms of a given element are alike in mass and
other properties, but atoms of one element differ
from the atoms of every other element
• Compounds are formed when atoms of different
elements unite in fixed proportions
• A chemical reaction involves a rearrangement
of atoms. No atoms are created, destroyed, or
broken apart in a chemical reaction
Dalton’s Atomic Theory: Conservation of
Mass and Definite Proportions
Six fluorine atoms and four
hydrogen atoms before reaction …
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… six fluorine atoms and four
hydrogen atoms after reaction.
Mass is conserved.
HF always has one H atom
and one F atom; always
has the same proportions
(1:19) by mass.
Section 2
Subatomic Particles, Atomic
Masses, and the Periodic Table
Subatomic Particles
Protons and neutrons are located at the center of
an atom called the nucleus.
Electrons are dispersed around the nucleus.
Isotopes
Atoms that have the same number of protons
but different numbers of neutrons are called
isotopes
Atomic number (Z) = number of protons
Hydrogen has 1 proton, 0 neutrons Deuterium has 1 proton, 1 neutron Tritium has 1 proton, 2 neutrons -
Z=1
Z=1
Z=1
Other Examples of Isotopes
Carbon-14
Z=6
so 8 neutrons
Chlorine-35
Z = 17
so 18 neutrons
Uranium-234
Z = 92
so 142 neutrons
The number of neutrons = A – Z
Isotopes (cont’d)
Atoms can be represented using the element’s
symbol and the mass number (A) and atomic
number (Z):
A
Z
E
35
17
Cl
37
17
Cl
• How many protons are in chlorine-35?
• How many protons are in chlorine-37?
• How many neutrons are in chlorine-37?
Example 2.3
How many protons, neutrons, and electrons are present
in a 81Br atom?
Atomic Masses
Atomic Masses
An atomic mass unit (amu) is defined as
exactly one-twelfth the mass of a carbon-12
atom
1 u = 1.66054 × 10–24 g
The atomic mass of an element is the
weighted average of the masses of the
naturally occurring isotopes of that element
Atomic Mass (cont’d)
• Question: do all isotopes of an element have
the same mass? Why or why not?
• The atomic mass given on the periodic table
is the weighted average of the masses of the
naturally occurring isotopes of that element.
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Example 2.4
Use the data cited above to determine the weighted
average atomic mass of carbon.
Example 2.4A
There are three naturally occurring isotopes of neon.
Their percent abundance and atomic masses are neon20, 90.51%, 19.99244 u; neon-21, 0.27%, 20.99395 u;
neon-22, 9.22%, 21.99138 u. Calculate the weighted
average atomic mass of neon.
Example 2.4B
The two naturally occurring isotopes of copper are
copper-63, mass 62.9298 u, and copper-65, mass
64.9278 u. What must be the percent natural
abundances of the two isotopes if the atomic mass of
copper listed in a table of atomic masses is 63.546 u?
Example 2.5
Indium has two naturally occurring isotopes and a
weighted average atomic mass of 114.82 u. One of the
isotopes has a mass of 112.9043 u. Which is likely to
be the second isotope: 111In, 112In, 114In, or 115In?
Example 2.5A
The masses of three naturally occurring isotopes of
magnesium are Mg-24, 23.98504 u; Mg-25, 24.98584
u; Mg-26, 25.98259 u. Can you determine which of the
three is most abundant? Second most abundant?
Mendeleev’s Periodic Table
• Mendeleev arranged the known elements in order
of increasing atomic weight from left to right and
from top to bottom in groups.
• Elements that closely resembled one another were
arranged in the same vertical group.
• Gaps were left where undiscovered elements
should appear.
• From the locations of the gaps, he was able to
predict properties of some of the undiscovered
elements.
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Germanium:
Prediction vs. Observation
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Modern Periodic Table
Elements
are
divided
into two
main
classes
EOS
Chapter 2: Atoms, Molecules, and
Ions
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Modern Periodic Table
Except for
hydrogen,
those
elements to
the left of the
line are
metals
EOS
Chapter 2: Atoms, Molecules, and
Ions
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Modern Periodic Table
Elements to
the right of
the line are
nonmetals
EOS
Chapter 2: Atoms, Molecules, and
Ions
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Modern Periodic Table
Elements
around the
line are
referred to as
metalloids
EOS
Chapter 2: Atoms, Molecules, and
Ions
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