Download Chapter 2 Atoms, Molecules, and Ions

John E. McMurry • Robert C. Fay
C H E M I S T R Y
Sixth Edition
Chapter 2
Atoms, Molecules, and Ions
Conservation of Mass and the Law
of Definite Proportions
Law of Conservation of Mass: Mass is neither
created nor destroyed in chemical reactions.
Conservation of Mass and the Law
of Definite Proportions
3.25 g + 3.32 g = 6.57 g
Hg(NO3)2(aq) + 2KI(aq)
HgI2(s) + 2KNO3(aq)
4.55 g + 2.02 g = 6.57 g
Conservation of Mass and the Law
of Definite Proportions
Law of Definite Proportions: Different samples of a pure chemical
substance always contain the same proportion of elements by mass.
By mass, water is:
88.8 % oxygen
11.2 % hydrogen
The Law of Multiple Proportions and
Dalton’s Atomic Theory
Law of Multiple Proportions: Elements can combine in different ways to
form different substances, whose mass ratios are small whole-number
multiples of each other.
nitrogen monoxide (NO):
nitrogen dioxide (NO2):
7 grams nitrogen per 8 grams oxygen
7 grams nitrogen per 16 grams oxygen
Insert Figure 2.2 p37
The Law of Multiple Proportions
and Dalton’s Atomic Theory
•
Elements are made up of tiny particles called atoms.
•
Each element is characterized by the mass of its atoms. Atoms of
the same element have the same mass, but atoms of different
elements have different masses.
•
The chemical combination of elements to make different chemical
compounds occurs when atoms join in small whole-number ratios.
•
Chemical reactions only rearrange how atoms are combined in
chemical compounds; the atoms themselves don’t change.
Atomic Structure: Electrons
Cathode-Ray Tubes: J. J. Thomson (1856-1940) proposed that cathode rays
must consist of tiny negatively charged particles. We now call them
electrons.
Atomic structure: Electrons
 The strength of deflecting magnetic electric field
 The size of the negative charge of electron
 The mass of the electron
Atomic Structure: Electrons
Millikan’s oil drop experiment
Atomic Structure: Protons and
Neutrons
Rutherford’s Gold Foil Experiment
In Rutherford’s gold foil experiment (1871-1937),positively
charged particles
•
were aimed at atoms of gold.
•
mostly went straight through the atoms.
•
were deflected only occasionally.
Conclusion:
There must be a small, dense, positively charged
nucleus in the atom that deflects positive particles that
come close.
Rutherford proposed that the atom must consist mainly of
empty space with the mass concentrated in a tiny
central core—the nucleus
Atomic Structure: Protons and
Neutrons
The mass of the atom is primarily
in the nucleus.
Atomic Structure: Protons and
Neutrons
The charge of the proton is opposite in
sign but equal to that of the electron.
Atomic Numbers
Atomic Number (Z): Number of protons in an atom’s nucleus.
Equivalent to the number of electrons around an atom’s nucleus
Mass Number (A): The sum of the number of protons and the number of
neutrons in an atom’s nucleus
Isotope: Atoms with identical atomic numbers but different mass
numbers
Atomic Numbers
Atomic Numbers
carbon-12
mass number
12
6
C
6 protons
6 electrons
6 neutrons
C
6 protons
6 electrons
8 neutrons
atomic number
carbon-14
mass number
14
6
atomic number
Atomic Masses and the Mole
Atomic Mass: The weighted average of the isotopic masses of the
element’s naturally occurring isotopes
The mass of 1 atom of carbon-12 is defined to be 12 amu.
Examples
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
B. How many neutrons are in the zinc atom?
C. What is the mass number of a zinc atom that has
Examples
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
B. Its mass number is
C. The element is
Example
Write the atomic symbols for atoms with the
following subatomic particles:
A. 8 p+, 8 n, 8 e-
___________
B. 47p+, 60 n, 47 e-
___________
Atomic Masses and the Mole
Why is the atomic mass of the element carbon 12.01 amu?
carbon-12:
98.89 % natural abundance
12 amu
carbon-13:
1.11 % natural abundance
13.0034 amu
mass of carbon = (12 amu)(0.9889) + (13.0034 amu)(0.0111)
= 11.87 amu + 0.144 amu
= 12.01 amu
Atomic Masses and the Mole
 Atomic mass unit (amu) is the mass in grams of a
single atom
 Isotopic mass
 1 amu = 1.6605 x 10-24g
Atomic Mass of Magnesium
The atomic mass of Mg
• is due to all the Mg
isotopes.
• is a weighted average.
• is not a whole number.
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Calculating Atomic Mass
The calculation for atomic mass requires the
• percent(%) abundance of each isotope.
• atomic mass of each isotope of that element.
• sum of the weighted averages.
mass of isotope(1)x (%) + mass of isotope(2) x (%) +
100
100
Calculating Atomic Mass for Cl
35Cl
has atomic mass 34.97 amu (75.76%) and 37C
has atomic mass 36.97 amu (24.24%).
• Use atomic mass and percent of each isotope to
calculate the contribution of each isotope to the
weighted average.
Atomic mass 35Cl x % abundance =
Atomic mass 37Cl x % abundance =
• Sum is atomic mass of Cl is
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Learning Check
Gallium is an element found in lasers used in compact
disc players. In a sample of gallium, there is 60.10% of
69Ga (atomic mass 68.926) atoms and 39.90% of 71Ga
(atomic mass 70.925) atoms.
What is the atomic mass of gallium?
Which isotope has more atoms?
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Atomic Masses and the Mole
Avogadro’s Number (NA): One mole of any substance contains 6.022 x 1023
formula units.
Molar Mass: The mass in grams of one
mole of any element. It is numerically
equivalent to its atomic mass.
Molar Mass from Periodic Table
Molar mass
is the atomic
mass
expressed in
grams.
1 mole of Ag
= 107.9 g
1 mole of C
= 12.01 g
1 mole of S
= 32.07 g
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Learning Check
Give the molar mass for each
A. 1 mole of K atoms
=
________
B. 1 mole of Sn atoms
=
________
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Collection Terms
A collection term states
a specific number of items.
• 1 dozen donuts
= 12 donuts
• 1 ream of paper
= 500 sheets
• 1 case
= 24 cans
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A Mole of Atoms
A mole is a collection that contains
• the same number of particles as there are carbon atoms in
12.0 g of carbon.
• 6.02 x 1023 atoms of an element (Avogadro’s number).
1 mole element
Number of Atoms
1 mole C
= 6.02 x 1023 C atoms
1 mole Na
= 6.02 x 1023 Na atoms
1 mole Au
= 6.02 x 1023 Au atoms
3
Samples of 1 Mole Quantities
1 mole of C atoms
= 6.02 x 1023 C atoms
1 mole of Al atoms
= 6.02 x 1023 Al atoms
1 mole of S atoms
= 6.02 x 1023 S atoms
5
Avogadro’s Number and the Mole
Avogadro’s Number (NA): One mole of any substance
contains 6.022 x 1023 formula units.
One mole of any substance is equivalent to its molar mass.
H
1 mole = 1.08 g
6.022 x 1023 molecules = 1.08 g
C:
1 mole = 12.01 g
6.022 x 1023 molecules = 12.01 g
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Avogadro’s Number
Avogadro’s number, 6.02 x 1023, can be written as an
equality and two conversion factors.
Equality:
1 mole = 6.02 x 1023 particles
Conversion Factors:
6.02 x 1023 particles and
1 mole
1 mole
6.02 x 1023 particles
6
Learning Check
1. The number of atoms in 2.0 g of Al atoms is
2. The number of moles of S in 1.8 x 1024 atoms of S
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Compounds and Mixtures
Variable composition
Same composition
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Compounds and Mixtures
Variable
properties
Similar
properties
The same number Cannot be
of protons
separated
physically
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Molecules and Covalent Bonds
Covalent Bond: Results when two atoms share several (usually
two) electrons. Typically a nonmetal bonded to a nonmetal
Molecules and Covalent Bonds
Covalent Bond: Results when two atoms share several (usually
two) electrons. Typically a nonmetal bonded to a nonmetal
Molecule: The unit of matter that results when two or more atoms
are joined by covalent bonds.
Molecules and Covalent Bonds
Insert Figure 2.12 p56
Ions and Ionic Bonds
Ionic Bond: A transfer of one or more electrons from one atom to
another. A strong electrical attraction between charged particles.
Typically a metal bonded to a nonmetal.
Ion: A charged particle
Cation: A positively charged particle. Metals tend to form cations.
Anion: A negatively charged particle. Nonmetals tend to form anions.
Molecules, Ions, and Chemical Bonds
In the formation of sodium chloride, one electron is
transferred from the sodium atom to the chlorine atom.
11 protons
11 electrons
17 protons
17 electrons
Na
+
Cl
Na1+
11 protons
10 electrons
+
Cl117 protons
18 electrons
Naming Chemical Compounds
Cation Charges for Typical Main-Group Ions
1+
2+
3+
Naming Chemical Compounds
Anion Charges for Typical Main-Group Ions
3-
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Naming Chemical Compounds
Ionic Compound: A neutral compound in which the total
number of positive charges must equal the total number of
negative charges.
Binary Ionic Compounds
Na+
Cl-
NaCl
magnesium oxide:
Mg2+
O2-
MgO
aluminum sulfide:
Al3+
S2-
Al2S3
sodium chloride:
Naming Chemical Compounds
Some transition metals form more than one cation
Naming Chemical Compounds
Use Roman numerals in parentheses to indicate the charge on metals
that form more than one kind of cation.
Binary Ionic Compounds
iron(III) oxide:
Fe3+
O2-
Fe2O3
tin(II) chloride:
Sn2+
Cl-
SnCl2
lead(II) fluoride:
Pb2+
F-
PbF2
Naming Ionic Compounds
To name a compound
that contains two
elements,
• identify the cation
and anion.
• name the cation first,
followed by the name
of the anion with an –
ide ending.
48
Example
Write the formula and names of the following compounds:
1) K+ and Br2) Ca2+ and O23) Pb4+ and S24) Cu2+ and F-
49
Naming Chemical Compounds
Binary Molecular Compounds
Because nonmetals often combine with one
another in different proportions to form
different compounds, numerical prefixes are
usually included in the names of binary
molecular compounds.
Naming Chemical Compounds
N2F4
The first element listed is more
cationlike and takes the name of
the element.
The second element listed is more
anionlike and takes the name of the
element with an “ide” modification to
the ending.
The prefix is added to the front of each to indicate the number of
each atom.
dinitrogen tetrafluoride
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Naming Chemical Compounds
Binary Molecular Compounds
Whenever the prefix ends in “a” or “o” and the element name
begins with a vowel, drop the “a” or “o” in the prefix.
N2O4
dinitrogen tetroxide
Whenever the prefix for the first element is “mono,” drop it.
CO2
carbon dioxide
CO
carbon monoxide
Learning Check
Write the name of each covalent compound.
P2O5
_____________________
SiCl4
_____________________
CCl4
_____________________
N2O
_____________________
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Naming Chemical Compounds
Naming Chemical Compounds
Polyatomic Ionic Compounds
Na+
OH-
NaOH
Mg2+
CO32-
MgCO3
sodium carbonate:
Na+
CO32-
Na2CO3
iron(II) hydroxide:
Fe2+
OH-
Fe(OH)2
sodium hydroxide:
magnesium carbonate:
Learning Check
Write formula and name of the following compounds:
A.
Mg2+ and NO3-
B.
Al3+ and BrO3-
C.
Fe2+ and SO42-
D.
Ba2+ and PO32-
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Examples
Write the correct formula for each.
A. aluminum nitrate
B. copper(II) nitrate
C. iron(III) hydrogen sulfite
D. tin(IV) hydroxide
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