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John E. McMurry Robert C. Fay
Lecture NotesAlan D. EarhartSoutheast Community College Lincoln, NE
General Chemistry: Atoms First
Chapter 2The Structure and Stability of Atoms
Copyright 2010 Pearson Prentice Hall, Inc.
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/3
Conservation of Mass and the
Law of Definite Proportions
2Hg2HgO
chemical formula
O2+
chemical equation
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Chapter 2/4
Conservation of Mass and the
Law of Definite Proportions
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/5
Conservation of Mass and the
Law of Definite Proportions
Law of Conservation of Mass: Mass is neither
created nor destroyed in chemical reactions.
HgI2(s)+2KNO3(aq)Hg(NO3)2(aq)+2KI(aq)
4.55 g+2.02g = 6.57 g
Aqueous solutions of mercury(II) nitrate and
potassium iodide will react to form a precipitate ofmercury(II) iodide and aqueous potassium iodide.
3.25 g+3.32g = 6.57 g
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/6
Conservation of Mass and the
Law of Definite Proportions
Law of Definite Proportions: Different samples of
a pure chemical compound always contain the
same proportion of elements by mass.
By mass, water is: 88.8 % oxygen
11.2 % hydrogen
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/7
Law of Multiple Proportions and
Daltons Atomic Theory
Law of Multiple Proportions: Elements can combine
in different ways to form different compounds, with
mass ratios that are small whole-number multiples of
each other.
8 grams oxygen per 7 grams nitrogen
16 grams oxygen per 7 grams nitrogen
nitric oxide:
nitrous oxide:
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Law of Multiple Proportions and
Daltons Atomic Theory
Law of Multiple Proportions: Elements can combine
in different ways to form different compounds, with
mass ratios that are small whole-number multiples of
each other.
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/9
Law of Multiple Proportions and
Daltons 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 differentmasses.
Chemical combination of elements to make different
chemical compounds occurs when atoms join together
in small whole-number ratios.
Chemical reactions only rearrange the way that atoms
are combined in chemical compounds; the atoms
themselves dont change.
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Atomic Structure: Electrons
Cathode-Ray Tubes: J. J. Thomson (18561940)
proposed that cathode rays must consist of tiny,
negatively charged particles which we now call
electrons.
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Atomic Structure: Electrons
Millikans Oil Drop Experiment
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/12
Atomic Structure: Protons and
Neutrons
Atomic Nucleus: When Ernest Rutherford (1871
1937) directed a beam of alpha () particles at a
thin gold foil, he found that almost all the particles
passed through the foil undeflected. A very small
number, however, (about 1 in every 20,000) were
deflected at an angle and a few actually bounced
back toward the particle source.
Rutherford explained his results by proposing thata metal atom must be almost entirely empty space
and have its mass concentrated in a tiny central
core that he called the nucleus.
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Atomic Structure: Protons and
Neutrons
Rutherfords Scattering Experiment
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/15
Atomic Structure: Protons and
Neutrons
The mass of the atom is
primarily in the nucleus.
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/17
Atomic Numbers
Atomic Number (Z): Number of protons in an
atoms nucleus. Equivalent to the number of
electrons around the atoms nucleus.
Mass Number (A): The sum of the number of
protons and the number of neutrons in an atoms
nucleus.
Isotope: Atoms with identical atomic numbers butdifferent mass numbers.
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/18
Atomic Numbers
carbon-14
C14
6
atomic number
mass number
carbon-12
C
12
6
atomic number
mass number
6 protons
6 electrons
8 neutrons
6 protons
6 electrons6 neutrons
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/20
Atomic Masses and the Mole
carbon-12: 98.89 % natural abundance 12 amu
carbon-13: 1.11 % natural abundance 13.0034 amu
Why is the atomic mass of the element carbon 12.01 amu?
= 12.01 amu
mass of carbon = (12 amu)(0.9889)+(13.0034 amu)(0.0111)
= 11.87 amu + 0.144 amu
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The Mole
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Mole(mol) - the amount of a substance that
contains the same number of entities as
there are atoms in exactly 12g of carbon-12.
This amount is 6.022x1023. It is called
Avogadros number and is abbreviated as N.
One mole (1 mol) contains 6.022x1023
entities (to four significant figures)
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Molar mass is the mass of 1 mole of in grams
eggsshoes
marbles
atoms
For any element
atomic mass (amu) = molar mass (grams)
1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g
1 12C atom = 12.00 amu
For any molecule
molecular mass (amu) = molar mass (grams)
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Water
18.02g
CaCO3100.09 g
Oxygen32.00 g
Copper
63.55 g
One mole of common substances.
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Molecular mass (or molecular weight) is the sum of
the atomic masses (in amu) in a molecule.
SO2
1S 32.07 amu
2O +2 x 16.00 amu
SO2 64.07 amu
1 molecule SO2 = 64.07 amu1 mole SO2 = 64.07 g SO2
6.022 x 1023 Molecules of SO2
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/28
Atomic Masses and the Mole
Avogadros Number (NA): One mole of any element
contains 6.022 141 x 1023 atoms.
Molar Mass: One mole of any element is the amount
whose mass in grams is numerically equivalent to itsatomic mass.
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/29
Atomic Masses and the Mole
1 mole = 28.0855 g
6.022 141 x 1023 molecules = 28.0855 g
Silicon:
Avogadros Number (NA): One mole of any element
contains 6.022 141 x 1023 atoms.
Molar Mass: One mole of any element is the amount
whose mass in grams is numerically equivalent to itsatomic mass.
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Chapter 2/30
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Chapter 2/31
Nuclear Chemistry
Nuclear Chemistry: The study of the properties and
changes of atomic nuclei.
Nuclear Reaction: A reaction that changes an atomic
nucleus.
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/32
Nuclear Chemistry
A nuclear reaction changes an atoms nucleus. A chemical
reaction only involves a change in the way that different
atoms are combined.
Different isotopes of an elements have essentially the
same behavior in chemical reactions, but often have
completely different behavior in nuclear reactions.
The energy change accompanying a nuclear reaction is far
greater than that accompanying a chemical reaction.
Comparisons Between Nuclear and Chemical Reactions
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Nuclear Chemistry
Radioactivity: The spontaneous decay and emission
of radiation from an unstable nucleus.
Radioisotope: A radioactive isotope.
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Chapter 2/34
Nuclear Chemistry
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Nuclear Reactions and
RadioactivityBeta () Radiation
A beta particle is an electron.
Beta particle, -
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Nuclear Reactions and
Radioactivity
Positron Emission
Gamma () Radiation
A gamma particle is a high-energy photon
A positron has the same mass as an electron but anopposite charge. It can be thought of as a positive
electron.
Positron, +
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Nuclear Reactions and
RadioactivityElectron Capture
A process in which the nucleus captures an inner-
shell electron, thereby converting a proton to a
neutron.
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Chapter 2/39
Nuclear Reactions and
Radioactivity
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/41
Nuclear Stability
Every element in the periodic table has at least one
radioactive isotope.
Hydrogen is the only element whose most abundant
stable isotope, hydrogen-1, contains more protons (1)than neutrons (0).
The ratio of neutrons to protons gradually increases,
giving a curved appearance to the band of stability.
All isotopes heavier than bismuth-209 are radioactive,
even though they may decay slowly and be stable
enough to occur naturally.
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Copyright 2010 Pearson Prentice Hall, Inc. Chapter 2/44
Nuclear Stability
These processes increase the neutron/proton ratio:
NeutronElectron capture: Proton + Electron
This process decreases the neutron/proton ratio:
Proton + -Beta emission: Neutron
Proton + +
Alpha emission:
NeutronPositron emission:
XZ
A
YZ - 2
A - 4
+ He2
4