Chapter 2: Atoms, Molecules and
Ions
1
Homework
„
Chapter 2
„
„
25, 31, 35, 37, 41, 43, 45, 49, 51, 83
Section 2.7: 59-71 (odd)
2
The Early History of Chemistry
Before 16th Century
„
Alchemy: Attempts (scientific or otherwise)
to change cheap metals into gold
17th Century
Robert Boyle: First “chemist” to perform
quantitative experiments
„
3
18th Century
„
George Stahl: Phlogiston flows out of a
burning material.
„
Joseph Priestley: Discovers oxygen gas,
“dephlogisticated air.”
1
Law of Conservation of Mass
Discovered
by Antoine Lavoisier
Mass
is neither created nor
destroyed
Combustion
involves oxygen, not
phlogiston
4
Other Fundamental Chemical Laws
Law of Definite Proportion
A
given compound always contains
exactly the same proportion of elements
by mass.
Carbon
tetrachloride is always 1 atom
carbon per 4 atoms chlorine.
5
Other Fundamental Chemical Laws
Law of Multiple Proportions
When
6
two elements form a series of
compounds, the ratios of the masses of
the second element that combine with 1
gram of the first element can always be
reduced to small whole numbers.
The ratio of the masses of oxygen in
H2O and H2O2 will be a small whole
number (“2”).
2
Dalton’s Atomic Theory (1808)
°
Each element is made up of tiny
particles called atoms.
±
The atoms of a given element are
identical; the atoms of different
elements are different in some
fundamental way or ways.
7
Dalton’s Atomic Theory
(continued)
8
²
Chemical compounds are formed
when atoms combine with each other.
A given compound always has the
same relative numbers and types of
atoms.
³
Chemical reactions involve
reorganization of the atoms - changes
in the way they are bound together.
The atoms themselves are not
changed in a chemical reaction.
Avogadro’s Hypothesis (1811)
At the same temperature and pressure, equal
volumes of different gases contain the same
number of particles.
5 liters of oxygen
„ 5 liters of nitrogen
Same number of particles!
„
„
9
3
Early Experiments to
Characterize the Atom
+
J. J. Thomson - postulated the
existence of electrons using cathode
ray tubes.
+
Ernest Rutherford - explained the
nuclear atom, containing a dense
nucleus with electrons traveling around
the nucleus at a large distance.
10
Figure 2.7: A cathode-ray tube. The fastmoving electrons excite the gas in the tube,
causing a glow between the electrodes.
11
Figure 2.8: Deflection of cathode
rays by an applied electric field.
12
4
Figure 2.12: Rutherford's experiment on
α-particle bombardment of metal foil.
13
Figure 2.13: (a) The expected results of the
metal foil experiment if Thomson's model were
correct. (b)Actual results.
14
Atomic Structure
„
3 primary particles that make up all atoms.
„
Protons
„
Neutrons
„
Electrons
„
„
„
„
Positively charged subatomic particle
Neutral subatomic particle
Negatively charged subatomic particle
The model we use places the protons and
neutrons in a very small center called the
nucleus. The electrons surround the nucleolus
and occupy most of the volume.
15
5
Atomic Mass Unit (AMU)
„
„
„
Because atoms are so small, it is difficult
to express their mass in the standard units
of grams. Therefore atomic mass units
(amu’s) are introduced.
An atomic mass unit is equal to 1/12 of the
mass of carbon-12.
Both the neutron and proton have a mass
of approximately 1 amu.
16
17
Atomic Number, Mass Number,
and Element Symbol
„
There are 3 different parts to identify an
element.
„
Atomic Number
„
A
Z
X
„
„
Mass Number
„
„
„
23
11
Na
The number of protons in the nucleus of an atom.
Symbolized by Z
The sum of the protons and neutrons.
Symbolized by A
Element Symbol
„
The one or two letter symbol given to every
element.
18
6
Isotopes
All elements have a number of different
isotopes.
Isotopes are atoms with the same atomic
number, but different mass numbers.
„
„
„
Example: 10B and 11B
All isotopes have different natural abundances.
„
„
Example: Boron has two isotopes. One with a mass
of 10 and the other a mass of 11. The isotope with a
mass of 10 is naturally found19.91% while the isotope
with a mass of 11 is found 80.09%.
19
Atomic Weight
The atomic weight reported in the periodic table is equal
to the addition of all of the different isotopes percent of
the natural abundance multiplied by atomic weight of
that isotope.
„
 % abundance of isotope 1 
Average Atomic Mass = 
(Mass of isotope 1) +
100


 % abundance of isotope 2 

(Mass of isotope 2) + L
100


Example: Chlorine (Cl) has two isotopes.
„ 35Cl
„ 37Cl
mass = 34.96885 amu; % abundance = 75.77%
mass = 36.96590 amu; % abundance = 24.23%
 75.77 
 24.23 
Average Atomic Mass = 
(34.96885 ) + 
(36.96590 )
 100 
 100 
= 35.45 amu
20
The Periodic Table
„
„
„
The periodic table is an arrangement of all of the elements by
their atomic number and properties.
Dimitri Mendeleev (1834-1907) is considered the father of the
modern periodic table. He first arranged the elements in order
of in creasing atomic mass. When he did so, he found that the
properties of elements repeated. Based on his table,
scientists found that the properties of yet undiscovered
elements could be predicted.
Basics
„
„
„
The rows of the periodic table are called periods.
The columns of the periodic table are called groups or families.
In general, the elements within a certain family have similar properties.
21
7
Three Types of Elements
„
Metals
„
Identified by:
„
„
„
„
„
Nonmetals
„
Identified by:
„
„
Being able to conduct electricity
Usually ductile (drawn into wires)
Malleable (can be rolled into sheets)
Can form alloys
Cannot conduct electricity (except for graphite).
Metalloids
„
Identified by having characteristics of both metals and
nonmetals
22
23
24
8
Group 1A: The Alkali Metals
„
„
„
Li, Na, K, Rb, Cs, and Fr all belong to
Group 1A.
The metals of Group 1A are all very
reactive with water.
When forming ions the Group 1A elements
usually have a charge of +1.
25
Group 2A: The Alkaline Earth
Metals
„
„
„
Be, Mg, Ca, Sr, Ba, and Ra all belong to
Group 2A.
The metals of Group 2A are all very
reactive with water.
When forming ions the Group 2A elements
usually have a charge of +2.
26
Transition Elements
„
„
„
The elements that fill the fourth through the
seventh periods in the center of the periodic
table are called the Transition Elements.
All are metals.
The two rows at the very bottom of the table
accommodate the lanthanides [lanthanum (57)
through hafnium (72)] and actinides [actinium
(89) through rutherfordium (104)].
27
9
Group 3A: Boron Family
„
„
„
B, Al, Ga, In, and Tl all belong to Group
3A.
Al, Ga, In, and Tl are all metals
B is a mettalliod
28
Group 4A: Carbon Family
„
„
C, Si, Ge, Sn, and Pb all belong to Group 4A.
C is a nonmetal
„
„
Carbon is the basis of all life and is an important element.
Carbon has 3 different allotropes (a particular element that can
exist in several different and very distinct forms)
„
„
„
„
„
Graphite
Diamond
Buckyball
Si and Ge are metalliods.
Sn and Pb are metals.
29
Group 5A: Nitrogen Family
„
„
„
„
N, P, As, Sb, and Bi all belong to Group
5A.
N and P are nonmetals.
As and Sb are metalliods.
Bi is a metal
30
10
Group 6A: Oxygen Family
„
„
„
„
„
O, S, Se, Te, and Po all belong to Group
6A.
O, S, and Se are all nonmetals.
Te is a metalliod.
Po is metal.
When forming ions Group 6A element
usually have a charge of –2.
31
Group 7A: Halogens
„
„
„
F,Cl, Br, I, and At all belong to Group 7A.
All of the Group 7A elements are
nonmetals.
When forming ions, Group 7A element
usually have a charge of –1.
32
Group 8A: Noble Gases
„
„
„
He, Ne, Ar, Kr, Xe, and Rn all belong to
Group 8A.
All of these element are gases.
These elements are rare and in general
are not very reactive.
„
So, they are also known as inert gases or rare
gases.
33
11
Diatomic Molecules
„
„
„
In nature, there are 7 different elements that are
found as diatomic molecules.
A diatomic molecule is one that contains two
elements.
These special elements are:
„
„
„
„
„
„
34
„
Hydrogen (H2)
Nitogren (N2)
Oxygen (O2)
Fluorine (F2)
Chlorine (Cl2)
Bromine (Br2)
Iodine (I2)
Chemical Bonds
„The forces that hold atoms together in
compounds. Covalent bonds result from
atoms sharing electrons.
„Molecule: a collection of covalently-bonded
atoms.
„What are the different types if chemical
bonds?
35
The Chemists’ Shorthand:
Formulas
„
Chemical Formula:
„
„
Symbols = types of atoms
Subscripts = relative numbers of atoms
CO2
„
Structural Formula:
„
Individual bonds are shown by lines.
O=C=O
36
12
Ions
Cation: A positive ion
Mg2+, NH4+
Anion: A negative ion
Cl−, SO42−
Ionic Bonding: Force of attraction
between oppositely charged ions.
37
Section 2.7
Naming Ionic Compounds and
Binary Nonmetals
38
What are Molecules and
Compounds?
„
Compound
„
„
Molecule
„
„
The smallest unit of a compound that retains the properties of
that compound.
Molecular Formula
„
„
Matter that is composed of two or more kinds of atoms
chemically combined in definite proportions.
A written formula that expresses the number of atoms of each
type within one molecule of a compound.
Example
„
„
Sugar is a compound composed of many sugar molecules.
A single sugar molecule has the molecular formula of C12H22O11
39
13
Structural Formulas
„
There is a problem with molecular
formulas
„
Many different compounds can have
the same molecular formula, but the are
put together differently.
„
„
„
„
Both have the molecular formula of
C2H6O
But they look very different
In order to remedy this problem we
normally used structural formulas.
A structural formula is a variation of
a molecular formula that expresses
how the atoms in a compound are
connected.
Previous example:
„
„
H H
Example: ethanol and dimethyl ether
„
„
H H
H C C O H
These are also structural
formulas, they just show
more information about the
chemical bonds between the
elements.
H
H C
H
O
H
H
C
Ethanol Æ CH3CH2OH
Dimethyl ether Æ CH3OCH3
H
Ions and Ionic Compounds
„
„
At this point we have only very basically described
molecular compounds.
There is another class of compound called ionic
compounds.
„
„
There are two types of ions.
„
„
„
Ionic compounds consist of ions (atoms or groups of atoms that
bear a positive or negative electric charge).
Cations – positively charged ions
Anions – negatively charged ions
Ions can consist of a single atom or a group of atoms.
„
„
Single atom ion is called a monoatomic ion
An ion made up of a group of atoms is called a polyatomic ion.
41
Valence Electrons
„
„
Before we start understanding ions more
we first need to talk about valence
electrons.
Valence electrons are the outermost
electrons. These are the electrons that
participate in bonding of atoms.
42
14
About Forming Monoatomic Ions
Remember electrons are NEGATIVELY
charged.
When forming monoatomic ions, the element
to become an ion will want to “look like” a
noble gas (in regards to the number of
electrons).
„
„
„
„
That is the element will want to lose or gain
electrons to have the same number of electrons as
the closest noble gas.
Losing or gaining these electrons is what causes
the element to become an ion.
43
Group 1A and 2A Ions
„
When the elements of Group 1A form ions they
form +1 cations.
„
„
„
This is because they only have one valence electron.
They like to lose that valence electron, which causes
them to become +1.
When the elements of Group 2A form ions they
form +2 cations.
„
„
This is because they have two valence electron.
They like to lose those two valence electrons, which
causes them to become +2.
44
Group 6A and 7A Ions
„
When the elements of Group 6A form ions they
form -2 anions.
„
„
„
This is because they only have 6 valence electron.
They like to gain two more valence electrons, which
causes them to become -2.
When the elements of Group 7A form ions they
form -1 anions.
„
„
This is because they have 7 valence electron.
They like to gain one more valence electron, which
causes them to become -1.
45
15
Other Monoatomic Ions
„
Your book states that Groups 3A and 5A form
certain types of ions.
„
„
„
This is not exactly correct.
Figure 3.7 on p. 87 shows the rest of the ions that are
common.
You just remember that:
„
„
„
„
„
„
Aluminum usually forms a +3 cation.
Tin usually forms a +2 cation.
Lead usually forms a +2 cation.
Nitrogen usually forms a -3 anion.
Phosphorus usually forms a -3 anion.
Bismuth usually forms a +3 cation.
46
47
Polyatomic Ions
„
As stated before, polyatomic ions are ions
that are made up of a group of different
elements.
48
16
More about Ionic Compounds
„
„
As stated before ionic compounds are those in
which the component particles are ions.
All ionic compounds are neutral in charge.
This is accomplished by the ionic compound be
composed of cations and anions whose charges
cancel (add up to zero).
„
„
„
The formula for the ionic compound is the
smallest ratio of cations and anions.
Many ionic compounds are composed of a metal
and nonmetal.
49
Coulomb’s Law
„
Electrostatic Forces
„
These are the forces that hold ionic compounds
together.
Coulomb’s Law describes the force of attraction between
a positive and negative ion.
„
„
Forces of attraction or repulsion caused by electric charges.
Force of Attractio n = k
(n + e)(n − e)
d2
Where n+ or n- is the number of charges on the positive or negative
ion, respectively. The charge on the electron is denoted by e and d is
the distance between the ion centers.
„
The equation shows that the forces of attraction between
oppositely charged ions increases as their charge
increases and as the distance between the ions
becomes smaller.
How to Determine the Formulas for
Ionic Compounds
„
1.
In order to learn how to determine the formula of an
ionic compound, we will use two examples: combining
an aluminum cation with a fluorine anion and
combining an ammonium cation with a sulfate anion.
Determine the charge of the cation being used.
„
„
2.
Determine the charge of the anion being used.
„
„
3.
Aluminum cation has a charge of +3
Ammonium cation has a charge of +1
Fluorine anion has a charge of -1
Sulfate anion has a charge of -2
Determine the smallest ratio of cations and anions so
that the compound has a zero charge.
„
„
Al3+ + 3F- = AlF3
2NH4+ + SO4-2 = (NH4)2SO4
(+3) + 3(-1) = 0
2(+1) + (-2) = 0
51
17
Writing Formulas for Ionic
Compounds
Let us write out the molecular formula if the following
elements were used to form an ionic compound. Remember
that the charge of the compound must equal to zero.
„
„
„
„
Strontium and Phosphorus
Sr3P2
Aluminum and Sulfur
Al2S3
Silver and Sulfate
Ag2SO4
Ammonium and Phosphorous
(NH4)3P
„
„
„
„
Sodium and Chlorine
NaCl
Sodium and Oxygen
Na2O
Magnesium and Nitrate
Mg(NO3)2
Lead (II) and Chlorine
PbCl2
52
Naming Ionic Compounds
The name of an ionic compound is built from the
names of the positive and negative ions in the
compound.
The name of the positive ion is given first, followed by
the name of the negative ion.
„
„
„
Naming the positive ion
1.
2.
„
Naming the negative ion
1.
2.
53
„
„
Metal ions have the same name as the element they are derived
from.
If the metal can have more than one possible charge, then a
roman numeral is used to indicate the positive charge
Nonmetal monoatomic anions are named by dropping the suffix of
the element and adding –ide.
Polyatomic ions have there own names and must be memorized
from table 3.1 (there are some set of rule listed for anions on p.
116, but I think it is just as easy to memorize the common ones)
Put them together
Example: FeS
Iron (II) Sulfide
Binary Compounds of the Nonmetals
„
„
„
„
„
There is also another type of ionic compound that does
not include a metal. They consist of two nonmetals.
Virtually all binary, nonmetal compounds are based on a
combination of elements from Groups 4A to 7A.
The formula is generally written by putting the elements
in order of increasing group number.
When naming the compound, the number of atoms of a
given type in the compound is designated with a prefix,
such as: “di-”, “tri-”, “tetra-”, etc.
Example:
„
„
NF3
Sulfur hexafluoride
Nitrogen trifluoride
SF6
54
18
EXAMPLES
We should have enough tools now to be able to name or give
the molecular formula of the following ionic compounds
„
„
„
„
„
„
Barium acetate
Ba(CH3CO2)2
Vanadium (II) oxide
VO
Nickel (II) cyanide
Ni(CN)2
Carbon dioxide
CO2
Phosphorus triioxide
PO3
Sulfur dichloride
SCl2
„
„
„
„
„
MgBr2
Magnesium Bromide
Li2CO3
Lithium Carbonate
CuCl2
Copper (II) Chloride
SF4
Sulfur Tetraflouride
P4O10
Tetraphosphorus Decaoxide
„
ClF3
Chlorine Triflouride
Acids & Bases
„
„
„
„
There are many different ways of defining
acids and bases.
There is no clear consensus as to the
proper definitions for the terms acid and
base.
Nor is any single definition best for all
situations.
There are 7 “common” different acid base
theories.
„
We will only be discussing 3 of those.
Usanovich
Lewis
Ionotropic
BronstedLowry
Arrhenius
Solvent
System
In
1923, Lewis
Bronsted
Arrhenius
in 1887
and
Lowry
defined
first
defined
the
proposed
that
an
acids
terms
acid
and
acid isas
anhydrogen
electron
ion
and
base.
pairdonors
acceptor
and a
bases
as
hydrogen
They
are
base is
andefined
electron
ion
acceptors.
as
acid being a
pairan
donor.
hydrogencontaining species
that dissociates in
water to give, H+,
and a base as a
hydroxidecontaining species
that dissociates in
water to give OH-.
Lux-Flood
19
Naming Acids
„
„
„
We will first define acids and bases, based
upon the definition given to us by
Arrhenius.
This defines an acid as a compound that
increases the H+ concentration when that
compound is dissolved in water.
A base is a a compound the increases the
OH- concentration when that compound is
dissolved in water.
58
Naming Acids (cont’d)
„
„
Remembering that we are naming acids and that
an acid has H+ in it, we can use the monoatomic
and polyatomic anions to create acids.
A few rules
„
Monoatomic anion based acids
„
„
„
Polyatomic anion based acids
„
„
„
-ide becomes –ic
The prefix hyrdo- must be added
-ate becomes –ic
-ite becomes –ous
Make sure when you are providing the molecular
formula of an acid you add as many H+’s needed
to get a overall charge of zero.
20
Naming Compounds
Binary Ionic Compounds:
1. Cation first, then anion
2. Monatomic cation = name of the element
Ca2+ = calcium ion
3. Monatomic anion = root + -ide
Cl− = chloride
CaCl2 = calcium chloride
61
Naming Compounds
(continued)
Binary Ionic Compounds (Type II):
metal forms more than one cation
use Roman numeral in name
PbCl2
Pb2+ is cation
PbCl2 = lead (II) chloride
62
Naming Compounds
(continued)
Binary compounds (Type III):
Compounds between two nonmetals
First element in the formula is named first.
Second element is named as if it were an
anion.
Use prefixes
Never use mono
P2O5 = diphosphorus pentoxide
63
21