Chapter 2: Atoms, Molecules, and Ions
The Atomic Theory of Matter
John Dalton (1766-1844):
• Elements are made of tiny
particles called atoms
• all atoms of one element are
identical
• atoms are not created nor
destroyed in chemical processes
• atoms of different elements
can combine to form compounds
Chapter 2: Atoms, Molecules, and Ions
The Discovery of Atomic Structure
J.J. Thomson (1856-1940):
• discovered the electron
• atoms of all elements contain
electrons
• electron: negatively charged
particles of very small mass
Chapter 2: Atoms, Molecules, and Ions
The Discovery of Atomic Structure
R. Millikan (1868-1953):
• measured electron charge (oil-drop experiment)
− 19
electron charge = 1.60× 10
C
Chapter 2: Atoms, Molecules, and Ions
The Discovery of Atomic Structure
Ernest Rutherford (1871-1937):
Gold-foil experiment:
(a) the atom has a positive charge
concentrated in the nucleus
(b) the electrons orbit around the
positive nucleus
(c) that the atom is mostly empty
space
Chapter 2: Atoms, Molecules, and Ions
The Discovery of Atomic Structure
Rutherford's Model:
+
Chapter 2: Atoms, Molecules, and Ions
The Modern View of Atomic Structure
The nucleus is structured:
PROTONS: positively charged (+1)
NEUTRONS: no charge
Chapter 2: Atoms, Molecules, and Ions
The Modern View of Atomic Structure
The mass of an atom is concentrated in its nucleus:
MASS
Charge
•proton:
1 atomic mass unit (1amu)
+1
•neutron:
1 amu
0
•electron:
negligible mass
(5.486 x 10-4 amu)
1 amu = 1.66054 x 10-24 g
-1
(1.6 x 10-19C)
Chapter 2: Atoms, Molecules, and Ions
The Modern View of Atomic Structure
Elements differ in their number of protons:
Hydrogen:
1 proton
Helium:
2 protons
2 neutrons
1
1H
4
2 He
Lithium:
3 protons
4 neutrons
7
3 Li
the mass number counts protons and neutrons
the atomic number counts the number of protons
Beryllium:
4 protons
5 neutrons
9
4 Be
Chapter 2: Atoms, Molecules, and Ions
The Modern View of Atomic Structure
How about the neutrons?
• an element is defined by the number of protons
• atoms of one element can exist in different 'versions' :
all atoms must still have the same number of protons
but they may have a different number of neutrons
Chapter 2: Atoms, Molecules, and Ions
The Modern View of Atomic Structure
Isotopes of an element:
same number of protons but different number of neutrons
1
1
H
Hydrogen
or Protium
H
1
3
Deuterium
Tritium
2
1
H
Chapter 2: Atoms, Molecules, and Ions
Periodic Table

Elements are sorted according to atomic number
6
C
12.01
Atomic weight
Atomic Number
Chapter 2: Atoms, Molecules, and Ions
Periodic Table

Elements with similar properties fall in vertical groups

A row in the periodic table is called a period
Chapter 2: Atoms, Molecules, and Ions
Average Atomic Mass = Atomic Weight
70amu
30amu
Average Mass = fraction of heavy x mass of heavy + fraction of light x mass of light
= 3/4 x 70amu + 1/4 x 30amu
= 0.75 x 70amu + 0.25 x 30amu = 60amu
Chapter 2: Atoms, Molecules, and Ions
Average Atomic Mass = Atomic Weight
For example: Lithium AW = 6.939 amu
6Li
Exact mass: 6.015 amu
7Li
Exact mass: 7.016 amu
Do the two isotopes exist in equal amounts ?
No, because the simple average of the isotope masses is:
(6.015 + 7.016) / 2 =
0.5 x 6.015 + 0.5 x 7.016 = 6.516
which is not the correct atomic weight
Chapter 2: Atoms, Molecules, and Ions
Average Atomic Mass = Atomic Weight
For example: Lithium AW = 6.939 amu
6Li
Exact mass: 6.015 amu
7Li
Exact mass: 7.016 amu
Which isotope is more abundant?
It must be 7Li because the average mass
Is closer to the mass of the 7Li isotope
The average is weighted in favor of the
more abundant isotope
Chapter 2: Atoms, Molecules, and Ions
Average Atomic Mass = Atomic Weight
For example: Lithium AW = 6.939 amu
6Li
Exact mass: 6.015 amu
7Li
Exact mass: 7.016 amu
What are the fractions of the individual isotopes ?
X  6.015 + Y  7.016 = 6.939
X+Y=1
X  6.015 + (1-X)  7.016 = 6.939
6.015 X + 7.016 – 7.016 X = 6.939
- 1.001 X = - 0.077
7.69% 6Li
X = -0.077/ -1.001 = 0.0769
92.31 % 7Li
Chapter 2: Atoms, Molecules, and Ions
How can you tell how many electrons an element has?
number of electrons = number of protons = atomic number
Hydrogen:
1 proton
Helium:
2 protons
2 neutrons
Lithium:
3 protons
4 neutrons
Beryllium:
4 protons
5 neutrons
1 electron
2 electrons
3 electrons
4 electrons
1
1H
4
2 He
7
3 Li
9
4 Be
Chapter 2: Atoms, Molecules, and Ions
number of electrons number of protons number of neutrons
16
16
32 – 16 = 16
80Br-
35 + 1 = 36
35
80 – 35 = 45
40Ca2+
20 - 2 = 18
20
40 – 20 = 20
32S
Chapter 2: Atoms, Molecules, and Ions
Periodic Table

Most elements are metals
VIII A
I A II A
Alkaline Earth Metals
III A IV A VA VI A VIIA
Transition metals
Alkali Metals
Halogens
Noble
Gases
Chapter 2: Atoms, Molecules, and Ions
Molecules and Molecular Compounds

Some elements exist as diatomic molecules at room temp.
H2
N2
O2
F2
Cl2
Br2
I2
Chapter 2: Atoms, Molecules, and Ions
Molecules and Molecular Compounds
The periodic table helps predict how elements combine
Empirical Formulas
Only gives relative number
of atoms in compound

Subscripts are smallest
whole-number ratios

Molecular Formulas
Show actual number and
types of atoms in a molecule

Chapter 2: Atoms, Molecules, and Ions
Empirical Formulas
Molecular Formulas
HO
H2O2
Hydrogen peroxide
C4H10
C2H5
Butane
C3H8
C3H8
Propane
Chapter 2: Atoms, Molecules, and Ions
Drawing Molecules: methane
Space-filling model
Perspective
drawing
Structural formula
Ball-and-Stick model
Chapter 2: Atoms, Molecules, and Ions
Ions and Ionic Compounds
Ions are formed when a neutral atom
- gains electrons or
- loses electrons
e-
Li
Li
Cation
-
e-
Br
+
Br
Anion
Chapter 2: Atoms, Molecules, and Ions
How can you tell how many electrons an element will gain/lose?
X1 would lose 2 e- to get to the Nb configuration: X12+
X2 would gain 3 e- to get to the Nb configuration: X23X3 would gain 1 e- to get to the Ne configuration: X3X4 would lose 1 e- to get to the Nc configuration: X4+
NOBLE GASES
Na
Nb
X2
Nc
X1
X4
Nd
X3
Ne
Nf
Atoms tend to gain/lose the number of electrons needed
to achieve the electron configuration of the closest noble gas

Metals tend to form Cations
 Nonmetals tend to form Anions

Chapter 2: Atoms, Molecules, and Ions
Cations and Anions can combine to from Ionic Compounds
Li
Li
+
Cation
e-
Br
Br
Anion
Chapter 2: Atoms, Molecules, and Ions
Cations and Anions can combine to from Ionic Compounds
Ionic Crystal – not a discrete molecule
Chapter 2: Atoms, Molecules, and Ions
Empirical Formulas for Ionic Compounds:
(A) determine charge of ions formed
(B) add ions so that compound is neutral overall
Na, O =>
Na+
O2-
Na+
2+
Al, O =>
Al3+
O2-
Al3+
O2O2-
20
Na2O
6+
60
Al2O3
Chapter 2: Atoms, Molecules, and Ions
Li, Br =>
Li
+
Br
1+
-
Mg, Cl =>
Mg
2+
Cl
Cl
10
LiBr
2+
20
MgCl2
-
Chapter 2: Atoms, Molecules, and Ions
Empirical Formulas for Ionic Compounds:
Al
3+
Mg
Ca
O
2-
2+
Cl 1
2+
2-
O
-
Al2O3
MgCl2
Ca2O2
CaO
smallest possible ratio!
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds: cations come first
(Ia) Monoatomic Cations:

Metal cations keep the name of the element:
Na+ Sodium ion
Li+ Lithium ion
Zn2+ Zinc ion
If a metal can form different kinds of cations (transition metals),
the charge is indicated by a Roman Numeral:

Fe2+
Fe3+
Co2+
Iron(II) ion
Iron(III) ion
Cobalt(II) ion
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds
(Ib) Cations formed from Nonmetals:

end in -ium:
NH4+ Ammonium ion
+
H3O+
Hydronium ion
+
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds
(IIa) Monoatomic and Simple Polyatomic Anions

Are derived from the element name by replacing the ending with -ide
N3-
Nitride ion
OH-
Hydroxide ion
O2-
Oxide ion
CN-
Cyanide ion
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds
(IIb) Oxyanions (polyatomic anions containing oxygen)




The most common oxyanion of an element ends in -ate
One more oxygen: per-.....-ate
One fewer oxygen: -ite
Two fewer oxygen: hypo-.....-ite
Perchlorate
ClO3-
Chlorate
Chlorite
Hypochlorite
ClO4-
ClO2ClO-
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds
(IIb) Oxyanions (polyatomic anions containing oxygen)




The most common oxyanion of an element ends in -ate
One more oxygen: per-.....-ate
One fewer oxygen: -ite
Two fewer oxygen: hypo-.....-ite
SO52-, S2O82- ]
[ Persulfate
SO42-
Sulfate
SO32-
Sulfite
Hyposulfite
SO22-
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds
Common Oxyanions:
Carbonate
CO32-
Nitrate
NO3Phosphate
PO43-
Chlorate
ClO3Sulfate
SO42-
Bromate
BrO3Iodate
IO3-
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds
You must know these!
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds
HCO3¯
You must know these!
Hydrogen carbonate ion
or Bicarbonate ion
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds: chemical formula → name
Identify the ions
Cation
Anion
monoatomic or
Group 1A, 2A, 3A metal?
simple polyatomic
anion?
Transition metal ?
Element name
-ide
Nonmetal ion?
Memorize
e.g. Ammonium
Element name
and charge in
Roman numerals
oxyanion?
per
...
...
hypo
...
ate
ite
...
ate
ite
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds: chemical formula → name
Mg2+
MgSO4
SO4
2-
Fe3+
FeCl3
NaClO
NH4OH
-
→ simple cation: magnesium
→ oxyanion: sulfate
→ transition metal cation: Iron(III)
3 Cl
→ monoatomic anion: chloride
Na+
→ simple cation: Sodium
ClO-
→ oxyanion: hypochlorite
NH4+
→ cation: Ammonium
OH
-
Magnesium sulfate
→ anion: hydroxide
Iron(III) chloride
Sodium hypochlorite
Ammonium hydroxide
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds: name → chemical formula
Identify the ions
Anion
Cation
Determine charge
Determine charge
Monoatomic,
nonmetal?
Transition metal ?
Charge is given in
Roman numerals
Group 1A, 2A, 3A metal?
Determine charge from
Periodic Table
(Slide #25)
Determine charge from
Periodic Table
(Slide #25)
Polyatomic
cation?
Memorize...
Ammonium: NH4+
Polyatomic
anion?
Memorize...
Hydroxide: OHCyanide: CNOxyanions!
combine cations & anions so that overall compound is neutral!
Chapter 2: Atoms, Molecules, and Ions
Naming Ionic Compounds: name → chemical formula
Iron(II) chloride
Fe2+
Cl- x 2
Mg2+
Magnesium hydroxide
OH
K+
Potassium cyanide
Sodium sulfite
CN
Na+
SO3
x
2-
Remember to balance charge!
FeCl2
-
2
-
x
2
Mg(OH)2
KCN
Na2SO3
More than one
polyatomic anion
(or cation)
=> in parantheses!
Chapter 2: Atoms, Molecules, and Ions
What ions does ClO2 dissociate into?
(A) Cl- and 2 O2(B) Cl- and O22(C) Cl+ and O2-
(D) doesn’t form ions
Chapter 2: Atoms, Molecules, and Ions
Naming Molecular Compounds
ClO2
does not dissociate into ions!
Metal cations + Nonmetal anions
=>
Nonmetal cations + Nonmetal anions
(e.g. NH4+)
=>
Nonmetal + Nonmetal
=>
ionic compounds
ionic compounds
molecular compounds
Before you start naming, determine what kind of compound you have!
Chapter 2: Atoms, Molecules, and Ions
Naming Binary Molecular Compounds
ClO2

Chlorine dioxide
Name of the element farther to the left in the P.T. comes first
- except oxygen, which is usually named last

If both elements are from the same group, the heavier one is named first

The second element is given an -ide ending

Greek prefixes indicate number of atoms of each element involved
[mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona- , deca-]
[mono-prefix is never used with 1st element]
Chapter 2: Atoms, Molecules, and Ions
Naming Compounds
NiO
1) Ionic or molecular?
=> ionic
=> Ni2+
and
O2-
=> Nickel(II) oxide
SO
2 a) Ionic: what are the ions?
b) molecular: how many atoms?
=> molecular
=> Sulfur monoxide
KMnO4 => ionic
=> K+ and
MnO4-
=> Potassium permanganate
BF3 => molecular
=> Boron trifluoride
Chapter 2: Atoms, Molecules, and Ions
Naming Compounds
Calcium bromate
Ionic or molecular compound?
Empirical formula?
Ions:
Ca2+ BrO3-
Ca(BrO3)2
SO3
Ionic or molecular compound?
Name?
Sulfur trioxide
Charge on oxygen?
N/A
You can only assign “real” charges to ions in ionic compounds
You cannot determine the empirical formula of a molecular
compound simply by consulting the periodic table
SO3
SO
SO2