Download Chapter 2 Atoms, molecules and ions

Spring 2016
Chapter 2
Atoms, molecules and
ions
Atoms,
Molecules,
and Ions
Atoms: The Greek Idea
~384 B.C.E.,
Aristotle: All
matter is
composed of four
elements and all
matter is
continuous, not
atomistic.
Aristotle declared matter
to be infinitely divisible.
Atoms,
Molecules,
and Ions
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Atoms: The Greek Idea
~ 450 B.C.E., Leucippus and
Democritus
Atomos: The point at which matter
can no longer be subdivided.
Atoms,
Molecules,
and Ions
Atomic Theory of Matter
The theory that
atoms are the
fundamental
building blocks of
matter reemerged
in the early 19th
century,
championed by John
Dalton.
Atoms,
Molecules,
and Ions
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Dalton’s Postulates
1. Each element is composed of extremely
small particles called atoms.
Atoms,
Molecules,
and Ions
Dalton’s Postulates
2. Atoms of an element cannot be created, destroyed, broken
into smaller parts or transformed into atoms of another
element
• The discovery of nuclear processes showed that it was
possible to transform atoms from one element into atoms
of another. But we don't consider processes that affect the
nucleus to be chemical processes. The postulate is still
useful. A slightly more restrictive wording is "Atoms cannot
be created, destroyed, or transformed into other atoms in a
chemical change".
Atoms,
Molecules,
and Ions
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Dalton’s Postulates
2. Atoms of an element cannot be created, destroyed, broken
into smaller parts or transformed into atoms of another
element
• The discovery of nuclear processes showed that it was
possible to transform atoms from one element into atoms
of another. But we don't consider processes that affect the
nucleus to be chemical processes. The postulate is still
useful. A slightly more restrictive wording is "Atoms cannot
be created, destroyed, or transformed into other atoms in a
chemical change".
Atoms,
Molecules,
and Ions
Dalton’s Postulates
3. All atoms of a given element are identical to one
another in mass and other properties, but the atoms of
one element are different from the atoms of all other
elements.
Actually most elements occur in nature as mixtures of
two or more kind of atoms called isotopes that have
slightly different masses but same nuclear charges.
In modern atomic theory, the postulate has been
amended to read: "Elements are characterized by the
nuclear charge of their atoms".
Atoms,
Molecules,
and Ions
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Dalton’s Postulates
3. All atoms of a given element are identical to one
another in mass and other properties, but the atoms of
one element are different from the atoms of all other
elements.
Actually most elements occur in nature as mixtures of
two or more kinds of atoms called isotopes that have
slightly different masses but same nuclear charges.
In modern atomic theory, the postulate has been
amended to read: "Elements are characterized by the
nuclear charge of their atoms".
Atoms,
Molecules,
and Ions
Dalton’s Postulates
4. When elements react, their atoms combine in simple,
whole-number ratios.
Atoms,
Molecules,
and Ions
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Dalton’s Postulates
5.When elements react, their atoms sometimes
combine in more than one simple, whole-number ratio.
Nitrogen and oxygen combine to produce more than
one product
Atoms,
Molecules,
and Ions
Atoms,
Molecules,
and Ions
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Law of Constant Composition
Joseph Proust (1754–1826)
• Also known as the law of definite
proportions.
• The elemental composition of a pure
substance never varies.
Atoms,
Molecules,
and Ions
Law of Conservation of Mass
The total mass of substances present
at the end of a chemical process is
the same as the mass of substances
present before the process took
place.
Atoms,
Molecules,
and Ions
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Law of multiple proportions
“If two elements form more than one compound
between them, then the ratios of the masses of
the second element which combine with a fixed
mass of the first element will be ratios of small
whole numbers.”
Or…
”If two elements form 2 different compounds, the
mass ratio of the elements in one compound is
related to the mass ratio in the other by a small
whole number”.
Atoms,
Molecules,
and Ions
Discovery of Electron
1897, Joseph John Thomson:
Determined the charge:mass ratio of
cathode rays (discovered electrons).
Atoms,
Molecules,
and Ions
Charge/mass = 1.76 × 108 C/g
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The Electron
Thomson used a magnetic field to bend
the cathode rays.
The amount the cathode ray bent from
the straight line using the magnetic field
allowed Thomson to calculate
the e/m ratio= 1.76  108 coulombs/g.
Atoms,
Molecules,
and Ions
CRT TV
(Cathode Ray Tube)
Atoms,
Molecules,
and Ions
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CRT TV
(Cathode Ray Tube)
Atoms,
Molecules,
and Ions
Sir William Thomson
(1824-1907)
J. J. Thomson
(1856-1940)
British mathematical physicist
British physicist
1892:
Knighted by Queen Victoria.
Titled « Baron Kelvin of Largs » (Lord Kelvin)
Atoms,
Molecules,
and Ions
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Millikan Discovery
Atoms,
Molecules,
and Ions
Millikan Discovery
Robert Millikan determined the charge on the electron to be
-1.60  10- 19 coulombs.
For convenience, electronic charge is expressed as a multiple of
that charge rather than coulombs. Thus the charge of the
electronic charge is -1.
Atoms,
Molecules,
and Ions
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Millikan Discovery
1) The drop is allowed to fall and its terminal velocity v1 in
the absence of an electric field is calculated.
The drag force (ie friction force/air resistance) acting on the
drop, Fd, is
terminal
velocity of the
falling drop
radius of the
drop
viscosity of the air
(eta)
Atoms,
Molecules,
and Ions
Millikan Discovery
W  m g
 d V  g
2) For a perfectly spherical droplet the apparent weight, W,
can be written as
density of air
radius of the
drop
gravity
acceleration
constant
density of the
Atoms,
oil
Molecules,
and Ions
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Discovery of Electron
• Streams of negatively charged particles
were found to exit from cathode tubes.
• J. J. Thompson is credited with their
Atoms,
discovery (1897).
Molecules,
and Ions
Millikan Discovery
3) At terminal velocity the oil drop is not accelerating.
This implies
W F
d
4 3
r g    air   6rv1
3
 r
9v1
2 g    air 
Atoms,
Molecules,
and Ions
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Millikan Discovery
4) Now the electrical field is turned on, and the electric force
on the drop, FE, is
FE  qE
electrical field
charge on the
oil drop
E
For parallel plates,
So:
FE  q 
V
d
V
d
potential difference
distance between
the plates
Atoms,
Molecules,
and Ions
Millikan Discovery
5) By adjusting V until the oil drop remains steady, we get
FE  W
q
V 4 3
 r g    air 
d 3
4
d
q  r 3 g    air 
3
V
charge on the
oil drop
q was found to be a multiple of -1.60 
10- 19
coulombs.
Atoms,
Molecules,
and Ions
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Electron Mass
The electron mass was determined
indirectly to be m= 9.10938 x 10-28g.
Atoms,
Molecules,
and Ions
Radioactivity:
• The spontaneous emission of radiation
by a radioactive atom.
• First observed by Henri Becquerel
(1896)
• Also studied by Marie and Pierre Curie.
Atoms,
Molecules,
and Ions
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Radioactivity
• Three types of radiation were
discovered by Ernest Rutherford:
 particles
  particles
  rays

Atoms,
Molecules,
and Ions
Radioactivity
Atoms,
Molecules,
and Ions
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Model of the atom
J. J. Thomson who discovered the
electron, proposed the “plum pudding
model” of the atom
Atoms,
Molecules,
and Ions
Rutherford Gold Foil
Experiment
Using an apparatus similar to that shown
below, Ernest Rutherford discovered the
atomic nucleus.
Atoms,
Molecules,
and Ions
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Rutherford Gold Foil
Experiment
Atoms,
Molecules,
and Ions
The Nuclear Atom
• Rutherford postulated a very small,
dense nucleus with the electrons
around the outside of the atom.
• Most of the volume of the atom is
empty space.
Atoms,
Molecules,
and Ions
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Atoms,
Molecules,
and Ions
Other Subatomic Particles
• Protons were discovered by
Rutherford (1919).
• Neutrons were discovered by James
Chadwick in (1932).
Atoms,
Molecules,
and Ions
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Subatomic Particles
• Protons and electrons are the only particles that
have a charge.
• Protons and neutrons have essentially the same
mass.
• The mass of an electron is so small we ignore it.
• 1 amu = 1.66054 x 10–24 g.
Atoms,
Molecules,
and Ions
Symbols of Elements
Elements are symbolized by one or two
letters.
Atoms,
Molecules,
and Ions
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Atomic Number
All atoms of the same element have the
same number of protons:
The atomic number (Z)
Atoms,
Molecules,
and Ions
Atomic Mass
The mass of an atom in atomic mass units
(amu) is the total number of protons and
neutrons in the atom.
Atoms,
Molecules,
and Ions
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Isotopes:
• Atoms of the same element with different
masses.
• Isotopes have different numbers of
neutrons.
11
6C
12
6C
13
6C
14
6C
Atoms,
Molecules,
and Ions
Isotopes of Hydrogen
Atoms,
Molecules,
and Ions
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Average Mass
• Average mass is calculated from the
isotopes of an element weighted by
their relative abundances.
Atoms,
Molecules,
and Ions
Example
• Naturally occurring C consists of 98.93%
12C (12 amu) and 1.07% 13C (13.00335
amu).
The average mass of C is:
• (0.9893)(12 amu) + (0.0107)(13.00335
amu) = 12.01 amu.
• 1 amu= 1.66054 x 10- 24 g
• 1 g = 6.02214 x 1023 amu
Atoms,
Molecules,
and Ions
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Atomic weight
• Atomic weight (AW) is also known
as average atomic mass.
• Atomic weights are listed on the
periodic table.
Atoms,
Molecules,
and Ions
The Mass Spectrometer
• A mass spectrometer is an
instrument that allows for direct
measure of atomic mass of
elements
• A graph of signal intensity vs.
mass of the ion is called a mass
spectrum.
Nobel prize in 1921
Atoms,
Molecules,
and Ions
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Atoms,
Molecules,
and Ions
Atoms,
Molecules,
and Ions
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Spring 2016
Atoms,
Molecules,
and Ions
Atoms,
Molecules,
and Ions
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Periodic Table
• The periodic
table is used to
organize the
elements in a
meaningful way.
Rows in the periodic table are called periods.
Columns in the periodic table are called groups.
Atoms,
Molecules,
and Ions
Periodic Table
• Several numbering conventions are used (i.e.,
groups may be numbered from 1 to 18, or from
1A to 8A and 1B to 8B).
• Some of the groups in the periodic table are
given special names.
• These names indicate the similarities between
group members.
• Examples:
• Group 1A: alkali metals
• Group 2A: alkaline earth metals
• Group 7A: halogens
• Group 8A: noble gases
Atoms,
Molecules,
and Ions
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Spring 2016
Periodicity
When one looks at the chemical properties
of elements, one notices a repeating
pattern of reactivities.
Atoms,
Molecules,
and Ions
Groups
These five groups are known by their
names.
Atoms,
Molecules,
and Ions
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Spring 2016
Periodic Table
Metals are on the left side of the chart.
Metals tend to be malleable and are good
thermal and electrical conductors.
Atoms,
Molecules,
and Ions
Periodic Table
Nonmetals are on
the right side of
the periodic table
(with the exception
of H).
Nonmetals tend to be gases, liquids or solids (brittle)
Atoms,
and do not conduct heat or electricity well. Molecules,
and Ions
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Spring 2016
Periodic Table
Metalloids border
the stair-step line
(with the exception
of Al and Po).
Elements with properties similar to both metals and
nonmetals are called metalloids
Atoms,
Molecules,
and Ions
Molecules and Molecular
Compounds
• A molecule consists of two or more
atoms bound tightly together.
• Each molecule has a chemical
formula.
Atoms,
Molecules,
and Ions
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Spring 2016
Chemical Formulas
The chemical formula
indicates :
1. which atoms are found in
the formula unit
2. in what proportion they are
found.
Atoms,
Molecules,
and Ions
Molecular Compounds
Molecular compounds
are composed of
molecules and almost
always contain only
nonmetals.
• A molecule made up of
two atoms is called a
diatomic molecule.
Atoms,
Molecules,
and Ions
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Spring 2016
Diatomic Molecules
These seven elements occur naturally as
molecules containing two atoms.
Atoms,
Molecules,
and Ions
Allotropes
• Different forms of an element, which
may have different chemical
formulas, are known as allotropes.
Allotropes differ in their chemical and
physical properties.
Atoms,
Molecules,
and Ions
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Spring 2016
Molecular Formulas
• These formulas give the actual
numbers and types of atoms in a
molecule.
• Examples: H2O, CO2, CO, CH4, H2O2,
O2, O3, and C2H4.
Atoms,
Molecules,
and Ions
Empirical formulas
• Empirical formulas give the lowest
whole-number ratio of atoms of each
element in a compound.
Atoms,
Molecules,
and Ions
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Picturing Molecules
The structural formula gives
the connectivity between
individual atoms in the
molecule
• Molecules occupy threedimensional space.
• However, we often represent
them in two dimensions.
Atoms,
Molecules,
and Ions
Ions
• When atoms lose or gain electrons, they become
ions.
– Cations are positive and are formed by
elements on the left side of the periodic chart.
– Anions are negative and are formed by
Atoms,
elements on the right side of the periodic
Molecules,
chart.
and Ions
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Spring 2016
Predicting Ionic Charges
• An atom or molecule can lose more than
one electron.
• Many atoms gain or lose enough electrons
to have the same number of electrons as
the nearest noble gas (group 8A).
• The number of electrons that an atom
loses is related to its position on the
periodic table.
Atoms,
Molecules,
and Ions
Ionic Bonds
Transfer of electrons between species. They are
generally formed between metals and nonmetals
• Example:NaCl can be formed by the transfer of an
electron from Na to Cl. By losing an electron Na and Cl
become Na+ and Cl– respectively.
• The Na+ and Cl– ions are attracted to form an ionic NaCl
lattice, which crystallizes.
Atoms,
Molecules,
and Ions
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Spring 2016
Writing Formulas For Ionic
Compounds
Writing formulas for ionic compounds:
• You need to know the ions of which it is
composed.
• The formula must reflect the electrical
neutrality of the compound.
You must combine cations and anions in a
ratio so that the total positive charge is
equal to the total negative charge.
Atoms,
Molecules,
and Ions
Example
• Because compounds are electrically
neutral, one can determine the formula
of a compound this way:
– The charge on the cation becomes the
subscript on the anion.
– The charge on the anion becomes the
subscript on the cation.
Atoms,
Molecules,
and Ions
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Common Cations
Atoms,
Molecules,
and Ions
Common Anions
Atoms,
Molecules,
and Ions
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Chemical nomenclature
• Chemical nomenclature is the
naming of substances.
• Chemical compounds are divided into
organic compounds (those containing
C, usually in combination with H, O,
N, or S) and inorganic compounds
(all other compounds).
IUPAC nomenclature
Atoms,
Molecules,
and Ions
Names and Formulas of
Ionic Compounds
Positive Ions (Cations)
• Cations formed from a metal have the
same name as the metal.
• Example: Na+ = sodium ion.
• If the metal can form more than one
cation, then the charge is indicated in
parentheses in the name.
• Examples: Cu+ = copper(I) ion;
Cu 2+ = copper(II) ion.
Atoms,
Molecules,
and Ions
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Spring 2016
Names and Formulas of
Ionic Compounds
Positive ions:
• An alternative nomenclature method uses
the endings -ous and -ic to represent the
lower and higher charged ions,
respectively.
• Examples: Cu+ = cuprous ion; Cu2+ =
cupric ion.
• Cations formed from nonmetals end in ium.
• Examples: NH4+ = ammonium ion;
Atoms,
H3O+ = hydronium ion.
Molecules,
and Ions
Names and Formulas of
Ionic Compounds
Negative Ions (Anions)
• Monatomic anions (with only one atom)
use the ending -ide.
• Example: Cl– is the chloride ion.
• Some polyatomic anions also use the -ide
ending:
• Examples: hydroxide, cyanide ions,
bromide, nitride,..
Atoms,
Molecules,
and Ions
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Spring 2016
Names and Formulas of
Ionic Compounds
• Polyatomic anions (with many
atoms) containing oxygen are called
oxyanions.
• Their names end in -ate or -ite.
(The one with more oxygen is called
-ate.)
• Examples: NO3– is nitrate; NO2– is
nitrite.
Atoms,
Molecules,
and Ions
Patterns in Oxyanion
Nomenclature
• When there are two oxyanions
involving the same elements:
– The one with fewer oxygens ends in -ite
• NO2− : nitrite; SO32− : sulfite
– The one with more oxygens ends in -ate
• NO3− : nitrate; SO42− : sulfate
Atoms,
Molecules,
and Ions
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Spring 2016
Oxyanion Nomenclature
• When there are more than two oxyanions involving the same
elements:
• The one with the fewest oxygens has the prefix hypoand ends in -ite
 ClO− : hypochlorite
• The one with the most oxygens has the prefix per- and
ends in ate
 ClO4− : perchlorate
The one with the second fewest oxygens ends in -ite
ClO2− : chlorite
The one with the second most oxygens ends in -ate
ClO3− : chlorate
Atoms,
Molecules,
and Ions
Inorganic Nomenclature
Atoms,
Molecules,
and Ions
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Spring 2016
Inorganic Nomenclature
Atoms,
Molecules,
and Ions
Acid Nomenclature
• If the anion ends in –ide,, change the
ending to -ic acid and add the prefix
hydro
– HCl: hydrochloric acid
HBr hydrobromic acid
Atoms,
Molecules,
and Ions
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Spring 2016
Acid Nomenclature
• For oxyanions ending in ite
change the ending to -ous
acid
• For oxyanions ending in
ate change the ending to ic acid
-
HClO4: perchloric acid
HClO3: chloric acid
HClO2: chlorous acid
HClO: hypochlorous acid
Atoms,
Molecules,
and Ions
Nomenclature of Binary
Compounds
• The less electronegative
atom is usually listed first.
• A prefix is used to denote
the number of atoms of
each element in the
compound.
Atoms,
Molecules,
and Ions
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Spring 2016
Nomenclature of Molecular
Compounds
• The ending on the more
electronegative element is
changed to -ide.
• CO2: carbon dioxide
• CCl4: carbon tetrachloride
• P2S5: diphosphorous pentasulfide
Atoms,
Molecules,
and Ions
Nomenclature of Binary
Compounds
If the prefix ends with a or o and
the name of the element begins
with a vowel, the two successive
vowels are often elided into one:
N2O5: dinitrogen pentoxide
Atoms,
Molecules,
and Ions
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Spring 2016
Simple Organic Compounds
Organic chemistry is the study of
carbon-containing compounds.
• Organic compounds are those that
contain carbon and hydrogen, often
in combination with other elements.
Atoms,
Molecules,
and Ions
Alkanes
• Compounds containing only carbon and
hydrogen are called hydrocarbons.
• Alkanes are hydrocarbons in which
carbon is linked up to 4 hydrogen atoms
by single bonds (i.e. they are saturated
compounds)
• Saturated hydrocarbons have the general
formula CnH2n+2. They can be:
linear (n≤3), or branched (n>3)
Atoms,
Molecules,
and Ions
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Spring 2016
Example
• C5H12
pentane
Isopentane
Neopentane
Atoms,
Molecules,
and Ions
Derivatives of Alkanes
• Functional groups are specific groups of atoms
that replace hydrogen atoms on alkanes. New
class of organic compounds are obtained:
• Alcohols are obtained by replacing a hydrogen
atom of an alkane with an –OH group (R-OH).
• Alcohol names derive from the name of the
alkane and have an -ol ending.
• Examples: methane becomes methanol (CH3OH);
ethane becomes ethanol (CH3CH2OH).
Atoms,
Molecules,
and Ions
46