1. Atoms, Molecules and Ions
Chapter 2

2. Composition of the Atom
Understanding the structure of the atom will help to understand the properties of the elements.
Keep in mind that these, as all theories, are subject to constant refinement. The picture of the atom isn’t final.

3. Development of the Atomic Theory
Dalton’s Atomic Theory - the first experimentally based theory of atomic structure of the atom.
John Dalton
early 1800’s
Much of Dalton’s Theory is still regarded as correct today. *See starred items.*

4. Postulates of Dalton’s Atomic Theory
1. All matter consists of tiny particles called atoms.*
2. An atom cannot be created, divided, destroyed, or converted to any other type of atom.
3. Atoms of a particular element have identical properties.

5. 4. Atoms of different elements have different properties.*
5. Atoms of different elements combine in simple whole-number ratios to produce compounds (stable aggregates of atoms.)*
6. Chemical change involves joining, separating, or rearranging atoms.*
* These postulates are still regarded as true.

6. Dalton’s Atomic Theory (1808)
Elements are composed of extremely small particles called atoms.
All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements.
Compounds are composed of atoms of more than one element. In any compound, the ratio of the numbers of atoms of any two of the elements present is either an integer or a simple fraction.
A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.

7. Atoms consist of three primary particles.
electrons
protons
neutrons
Nucleus - small, dense, positively charged region in the center of the atom. Contains:
protons - positively charged particles
neutrons - uncharged particles

8. electrons - negatively charged particles
Surrounding the nucleus is a diffuse region of negative charge populated by:
electrons - negatively charged particles
Selected Properties of the Subatomic Particles
Name Charge Mass(amu) Mass (grams)
Electrons (e) x x 10-28
Protons (p) X 10-24
Neutrons (n) x 10-24

9. Dalton’s Atomic Theory Law of Multiple Proportions2
Law of Multiple Proportions

10. Law of Conservation of Mass
16 X
8 Y +
8 X2Y
Law of Conservation of Mass

11. Subatomic Particles: Thompson Experiment
Electrons, Protons and Neutrons
Electrons were the first subatomic particles to be discovered using the cathode ray tube.
Indicated that the particles were negatively charged.

12. Cathode Ray Tube

13. Joseph John Thomson (1856-1940)
Joseph John Thomson ( ). Photo courtesy of The Cavendish Laboratory.

14. Thomson’s charge/mass of e- = -1.76 x 108 C/g
Measured mass of e-
(1923 Nobel Prize in Physics)
e- charge = x C
Thomson’s charge/mass of e- = x 108 C/g
e- mass = 9.10 x g

15. (Uranium compound)

17. Protons were the next particle to be discovered.
Protons have the same size charge but opposite in sign.
Proton is 1837 times as heavy as electron.
Neutrons
Postulated to exist in 1920’s but not demonstrated to exist until 1932.
Almost the same mass as the proton.

18. (1908 Nobel Prize in Chemistry)
particle velocity ~ 1.4 x 107 m/s
(~5% speed of light)
atoms positive charge is concentrated in the nucleus
proton (p) has opposite (+) charge of electron (-)
mass of p is 1840 x mass of e- (1.67 x g)

19. Rutherford’s Model of the Atom
atomic radius ~ 100 pm = 1 x m
nuclear radius ~ 5 x 10-3 pm = 5 x m
“If the atom is the Houston Astrodome, then the nucleus is a marble on the 50-yard line.”

20. Chadwick’s Experiment (1932) (1935 Noble Prize in Physics)
H atoms - 1 p; He atoms - 2 p
mass He/mass H should = 2
measured mass He/mass H = 4
a + 9Be
1n + 12C + energy
neutron (n) is neutral (charge = 0)
n mass ~ p mass = 1.67 x g

21. mass p = mass n = 1840 x mass e-

22. Atomic number, Mass number and Isotopes
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei
Mass Number X A Z
Element Symbol
Atomic Number H 1
H (D) 2
H (T) 3 U
235 92
238

23. atomic number (Z) - the number of protons in the atom
Mass
Number
Charge of particle
Symbol of
the atom
Atomic Number
atomic number (Z) - the number of protons in the atom
mass number (A) - sum of the number of protons and neutrons

24. Calculate the Composition of an Atom
Calculate the number of protons, neutrons and electrons in each of the following:

25. Isotopes - atoms of the same element having different masses.
contain same number of protons
contain different numbers of neutrons
Isotopes of Hydrogen
Hydrogen
(Hydrogen - 1)
Deuterium
(Hydrogen - 2)
Tritium
(Hydrogen - 3)

26. Do You Understand Isotopes?
How many protons, neutrons, and electrons are in C 14 6 ?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in C 11 6 ?
6 protons, 5 (11 - 6) neutrons, 6 electrons

27. Isotopes of the same element have identical chemical properties.
Some isotopes are radioactive.
Find chlorine on the periodic table.
What is the atomic number? 17
What is the mass given?
35.45
This is not the mass number of an isotope.

28. What is this number?
It is called the atomic mass - the weighed average of the masses of the isotopes that make up chlorine.
Chlorine consists of chlorine-35 and chlorine-37 in a 3:1 ratio.
The weighted average is an average corrected by the relative amounts of each isotope present in nature.

29. Calculate the atomic mass of naturally occurring chlorine if 75
Calculate the atomic mass of naturally occurring chlorine if 75.77% of chlorine atoms are chlorine-35 and 24.23% of chlorine atoms are chlorine-37.
Step 1: Convert the percentage to a decimal fraction.
chlorine-35
chlorine-37

30. Step 2: Multiply the decimal fraction by the mass of that isotope to obtain the isotope contribution to the atomic mass.
For chlorine-35:
x amu = amu
For chlorine-37
x amu = amu
Step 3: sum to get the weighted average
atomic mass of chlorine =
26.52 amu amu = amu

31. Atomic Weights
Calculate the atomic weight of boron, B, from the following data:
ISOTOPE ISOTOPIC MASS (amu) FRACTIONAL ABUNDANCE B B

32. Atomic Weights
Calculate the atomic weight of boron, B, from the following data:
ISOTOPE ISOTOPIC MASS (amu) FRACTIONAL ABUNDANCE B B
B-10: x =
B-11: x =
= amu
( = atomic wt.)

33. Calculate the atomic mass
Nitrogen consists of two naturally occurring isotopes % nitrogen-14 with a mass of amu and 0.37% nitrogen-15 with a mass of amu. What is the atomic mass of nitrogen?

34. The Periodic Table
In 1869, Dmitri Mendeleev discovered that if the known elements were arranged in order of atomic number, they could be placed in horizontal rows such that the elements in the vertical columns had similar properties.
A tabular arrangement of elements in rows and columns, highlighting the regular repetition of properties of the elements, is called a periodic table.
Copyright © by Houghton Mifflin Company. All rights reserved. 2

35. The Periodic Table Periods and Groups
A period consists of the elements in one horizontal role of the periodic table.
A group consists of the elements in any one column of the periodic table.
The groups are usually numbered.
The eight groups are called main group (or representative) elements. 2

36. The Periodic Table Periods and Groups
The “B” groups are called transition elements.
The two rows of elements at the bottom of the table are called inner transition elements.
Elements in any one group have similar properties. 2

37. The Periodic Table Periods and Groups
The elements in group IA, often known as the alkali metals, are soft metals that react easily with water.
The group VIIA elements, known as the halogens, are also reactive elements. 2

38. The Periodic Table Metals, Nonmetals, and Metalloids
A metal is a substance or mixture that has a characteristic luster and is generally a good conductor of heat and electricity.
A nonmetal is an element that does not exhibit the characteristics of the metal.
A metalloid, or semi-metal, is an element having both metallic and nonmetallic properties. 2

39. Alkali Earth Metal
Noble Gas
Halogen
Alkali Metal
Group
Period

40. Chemistry In Action Natural abundance of elements in Earth’s crust
Natural abundance of elements in human body

41. A diatomic molecule contains only two atoms
A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical forces H2
H2O
NH3
CH4
A diatomic molecule contains only two atoms
H2, N2, O2, Br2, HCl, CO
A polyatomic molecule contains more than two atoms
O3, H2O, NH3, CH4

42. Chemical Formulas; Molecular and Ionic Substances
Although many substances are molecular, others are composed of ions.
An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons.
Sodium chloride is a substance made up of ions. 2

43. cation – ion with a positive charge
An ion is an atom, or group of atoms, that has a net positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation. Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl-
17 protons
18 electrons Cl
17 protons
17 electrons

44. Subatomic particles in ions
How many protons, neutrons and electrons are in the following ions?

45. A monatomic ion contains only one atom
Na+, Cl-, Ca2+, O2-, Al3+, N3-
A polyatomic ion contains more than one atom
OH-, CN-, NH4+, NO3-

46. Do You Understand Ions? How many protons and electrons are in ? Al27 13 3+
13 protons, 10 (13 – 3) electrons
How many protons and electrons are in ? Se 78 34 2-
34 protons, 36 (34 + 2) electrons

49. A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance
An empirical formula shows the simplest
whole-number ratio of the atoms in a substance
H2O
molecular
empirical
H2O
C6H12O6
CH2O O3 O
N2H4
NH2

50. The ionic compound NaCl
ionic compounds consist of a combination of cations and an anions
the formula is always the same as the empirical formula
the sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero
The ionic compound NaCl

51. Formula of Ionic Compounds
2 x +3 = +6
3 x -2 = -6
Al2O3
Al3+
O2-
1 x +2 = +2
2 x -1 = -2
CaBr2
Ca2+
Br-
1 x +2 = +2
1 x -2 = -2
Na2CO3
Na+
CO32-

52. Chemical Substances; Formulas and Names
Ionic compounds
Most ionic compounds contain metal and nonmetal atoms; for example, NaCl.
You name an ionic compound by giving the name of the cation followed by the name of the anion.
A monatomic ion is an ion formed from a single atom. 2

53. Chemical Substances; Formulas and Names
Rules for predicting charges on monatomic ions
Most of the main group metals form cations with the charge equal to their group number.
The charge on a monatomic anion for a nonmetal equals the group number minus 8.
Most transition elements form more than one ion, each with a different charge. 2

54. Chemical Substances; Formulas and Names
Monatomic cations are named after the element – if there is only one such ion. For example, Al3+ is called the aluminum ion.
Rules for naming monatomic ions
If there is more than one cation of an element, a Roman numeral in parentheses denoting the charge on the ion is used. This often occurs with transition elements. i.e. Fe2+ = Iron(II); Fe3+ = Iron(III)
The names of the monatomic anions use the stem name of the element followed by the suffix – ide. For example, Br- is called the bromide ion. 2

55. Naming Binary Compounds
NaF -
LiCl -
MgO -

56. Naming Binary Compounds
NaF - Sodium Fluoride
LiCl - Lithium Chloride
MgO - Magnesium Oxide

57. Chemical Substances; Formulas and Names
Polyatomic ions
A polyatomic ion is an ion consisting of two or more atoms chemically bonded together and carrying a net electric charge. 2

60. Chemical Nomenclature
Ionic Compounds
often a metal + nonmetal
anion (nonmetal), add “ide” to element name
BaCl2
barium chloride
K2O
potassium oxide
Mg(OH)2
magnesium hydroxide
KNO3
potassium nitrate

61. Transition metal ionic compounds
indicate charge on metal with Roman numerals
FeCl2
iron(II) chloride
2 Cl- -2 so Fe is +2
FeCl3
3 Cl- -3 so Fe is +3
iron(III) chloride
Cr2S3
3 S-2 -6 so Cr is +3 (6/2)
chromium(III) sulfide

62. Molecular compounds nonmetals or nonmetals + metalloids common names
H2O, NH3, CH4, C60
element further left in periodic table is 1st
element closest to bottom of group is 1st
if more than one compound can be formed from the same elements, use prefixes to indicate number of each kind of atom
last element ends in ide

63. Molecular Compounds HI hydrogen iodide NF3 nitrogen trifluoride SO2
sulfur dioxide
N2Cl4
dinitrogen tetrachloride
TOXIC!
NO2
nitrogen dioxide
N2O
dinitrogen monoxide
Laughing Gas

65. An acid can be defined as a substance that yields
hydrogen ions (H+) when dissolved in water.
HCl
Pure substance, hydrogen chloride
Dissolved in water (H+ Cl-), hydrochloric acid
An oxoacid is an acid that contains hydrogen, oxygen, and another element.
HNO3
nitric acid
H2CO3
carbonic acid
H2SO4
sulfuric acid
HNO3

69. Acid anions (provide H+ ions)
Uses of mono– and di-
Rule: to be used if oxyanion is bonded to one or more hydrogen ions.
Example:
HPO42- = monohydrogen phosphate ion
H2PO4- = dihydrogen phosphate ion

70. Prefix thio-
The prefix thio- means that an oxygen atom in the root ion name has been replaced by a sulfur atom.
Example:
SO > S2O32-
sulfate ion thiosulfate ion

71. More Practice Na2SO4 Na2SO3 Sodium Sulfate Sodium Sulfite AgCN Cd(OH)2
Silver Cyanide Cadmium Hydroxide
Ca(OCl)2 KClO4
Calcium Hypochlorite Potassium Perchlorate

72. Polyatomic Ions You Should Know
NH4+ - Ammonium
OH- - Hydroxide
CN- - Cyanide
SO42- - Sulfate
ClO4- - Perchlorate
O22- - Peroxide
PO43- - Phosphate
CO32- - Carbonate
HCO3- - Hydrogen carbonate

73. A base can be defined as a substance that yields
hydroxide ions (OH-) when dissolved in water.
NaOH
sodium hydroxide
KOH
potassium hydroxide
Ba(OH)2
barium hydroxide

74. Chemical Substances; Formulas and Names
Hydrates
A hydrate is a compound that contains water molecules weakly bound in its crystals.
Hydrates are named from the anhydrous (dry) compound, followed by the word “hydrate” with a prefix to indicate the number of water molecules per formula unit of the compound.
For example, CuSO4. 5H2O is known as copper(II)sulfate pentahydrate. 2

75. Hydrates are compounds that have a specific number of water molecules attached to them.
BaCl2•2H2O
barium chloride dihydrate
LiCl•H2O
lithium chloride monohydrate
MgSO4•7H2O
magnesium sulfate heptahydrate
Sr(NO3)2 •4H2O
strontium nitrate tetrahydrate
CuSO4•5H2O
CuSO4

76. Copper (II) sulfate.

78. Chemical Formulas; Molecular and Ionic Substances
An important class of molecular substances that contain carbon is the organic compounds.
Organic compounds
Organic compounds make up the majority of all known compounds.
The simplest organic compounds are hydrocarbons, or compounds containing only hydrogen and carbon.
Common examples include methane, CH4, ethane, C2H6, and propane, C3H8. 2

79. Organic chemistry is the branch of chemistry that deals with carbon compounds
Functional Groups C H OH C H
NH2 C H OH O
methanol
methylamine
acetic acid

80. Chemical Reactions: Equations
Writing chemical equations
A chemical equation is the symbolic representation of a chemical reaction in terms of chemical formulas.
For example, the burning of sodium and chlorine to produce sodium chloride is written
The reactants are starting substances in a chemical reaction. The arrow means “yields.” The formulas on the right side of the arrow represent the products. 2

81. Chemical Reactions: Equations
Writing chemical equations
In many cases, it is useful to indicate the states of the substances in the equation.
When you use these labels, the previous equation becomes 2

82. Chemical Reactions: Equations
Writing chemical equations
The law of conservation of mass dictates that the total number of atoms of each element on both sides of a chemical equation must match. The equation is then said to be balanced.
Consider the combustion of methane to produce carbon dioxide and water. 2

83. Chemical Reactions: Equations
Writing chemical equations
For this equation to balance, two molecules of oxygen must be consumed for each molecule of methane, producing one molecule of CO2 and two molecules of water.
Now the equation is “balanced.” 2 2

84. Chemical Reactions: Equations
Balance the following equations. 2 6 6 2 9 3 4 2

85. WORKED EXAMPLES

86. Worked Example 2.1