AP Chemistry
Mr. Markic
Page 1 of 12
Chapter 2 - Atoms, Molecules, and Ions
Atomic Theory by John Dalton
1.
Elements are composed of extremely small particles called atoms.
2. 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.
3. 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.
4. A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not
result in their creation or destruction.
The Structure of the Atom
 Atoms – the basic unit of an element that can enter into chemical combination
 Atoms possess an internal structure (subatomic structure):
 Electrons
 Protons
 Neutrons
(a) According to Dalton’s atomic theory, atoms of the same
element are identical, but atoms of one element are different
from atoms of other elements (Law of Conservation of Mass)
(b) Compounds formed from atoms of elements X and Y. In this
case, the ratio of the atoms of element X to the atoms of
element Y is 2:1
An illustration of the law of multiple proportions
Review of Concepts
The atoms of elements A (blue) and B (orange) form two compounds shown here. Do these compounds obey
the law of multiple proportions?
AP Chemistry
Mr. Markic
Page 2 of 12
The Electrons
 Negatively charged
 Discovered by using a cathode ray tube in the 1890’s –
J.J. Thompson
Cathode Tube
Millikan’s Experiment
 Measured mass of e e- charge = -1.60 x 10-19 C
 Thomson’s charge/mass of e- = -1.76 x 108 C/g
 e- mass = 9.10 x 10-28 g
Radioactivity
 Radioactivity – spontaneous emission of particles and/or radiation
 Three types of rays are produced by the decay, or breakdown, or radioactive substances such as
uranium
1. Alpha (α) rays – consist of positively charged particles (deflected by positively charged plate)
2. Beta (β) rays – electrons (deflected by the negatively charged plate)
3. Gamma (γ) rays – high energy rays, has no charge, and is not affected by an external field
Radiation
Discovery of the Atomic Structure
1. Cathode ray tubes and e Discovered by J.J. Thomson
2. Radioactivity
 Rutherford revealed 3 types of radiation: alpha (α), beta (β), and gamma (γ)
3. The Nuclear Atom
 Rutherford’s Gold Foil Experiment
AP Chemistry
Mr. Markic


Page 3 of 12
Most of the mass of the atom and all of its positive charge reside in a small, dense region called the
nucleus
The total volume of the atom is empty space in which the e- move around the nucleus
The Proton and the Nucleus
 Atoms were first thought of electrically neutral and it contained electrons
 To be negative, an atom must contain equal number of positive and negative
charges
 Thompson thought of the atom as a uniform positive sphere of matter in
which electrons are embedded like raisins in a cake (“Plum Pudding” Model)
 This model was accepted for a number of years
Rutherford’s Gold Foil Experiment
1. atoms positive charge is concentrated in the nucleus
2. proton (p) has opposite (+) charge of electron (-)
3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)
The Neutron
 Electrically neutral particles having a mass slightly greater than that of protons (discovered by
Chadwick)
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
The Isotopes of Hydrogen
𝐴
𝑍𝑋
Element symbol
Atomic number
1
1𝐻
2
1𝐻
235
92𝑈
238
92𝑈
(D)
3
1𝐻
(T)
AP Chemistry
Mr. Markic
Page 4 of 12
Sample Exercise
Give the number of protons, neutrons, and electrons in each of the following species:
(a)
20
11𝑁𝑎
(b)
22
11𝑁𝑎
(c) O17
(d) Carbon – 14
How many protons, neutrons, and electrons are in the following isotope of copper: Cu 63?
Review of Concepts
(a) What is the atomic number of an element if
one of its isotopes has 117 neutrons and a
mass number of 195?
(b) Which of the following two symbols
provides more information? 17O or 8O.
The Periodic Table
 A chart in which elements having similar chemical and physical properties are grouped together
 Elements are arranged by atomic number
 Elements with similar properties are placed in vertical columns (groups), periods are horizontal
rows
 Majority of elements are metallic (2/3)
 Metals can conduct heat and electricity, have luster, malleable, ductile, and are solids at room
temperature (except Hg)
 Nonmetals can be gaseous, liquid, or solid at room temperature
AP Chemistry
Mr. Markic
Page 5 of 12
Review of Concepts
In reviewing the periodic table, do chemical properties change more markedly across a period or down a
group?
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

Diatomic Molecules Br2 I2 N2 Cl2 H2 O2 F2
A polyatomic molecule contains more than two atoms:
O3, H2O, NH3, CH4
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.
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
A monatomic ion contains only one atom: Na+, Cl-, Ca2+, O2-, Al3+, N3A polyatomic ion contains more than one atom: OH-, CN-, NH4+, NO3-
AP Chemistry
Mr. Markic
Page 6 of 12
Review of Concepts
(a) What does S8 signify? How does it differ from 8 S?
(b) Determine the number of protons and electrons for the following ions:
(a) P3- and (b)Ti4+
Chemical Formulas


Are used to express the composition of molecules and ionic compounds in terms of chemical symbols
In this chapter we are concerned with two types of formulas: molecular and empirical
Molecular Formulas
 Shows the exact number of atoms if each element in the smallest unit of a substance (i.e. H 2O)
 Allotrope – is one of two or more distinct forms of an element (i.e. O2 and O3 ; diamond and graphite)
 Molecular Models – an effective means of visualizing molecules
 There are two types of molecular models: ball-and-stick models and space-filling models.
 The molecular models help us understand the bond angle of the structure
Empirical Formulas
 Tells us which elements are present and the simplest whole-number ratio of
their atoms
 Does not always tell us the actual number of atoms in a given molecule (i.e.
N2H4)
 For many molecules, the molecular formula and the empirical formula are
one and the same; H2O, CO2, CH4, NH3
Molecular
H2O
C6H12O6
O3
N2H4
Empirical
H2O
CH2O
O
NH2
AP Chemistry
Mr. Markic
Page 7 of 12
Sample Exercise
Write the chemical formula of methanol, an organic solvent and antifreeze, from its
ball-and-stick model, shown in the margins
Write the molecular formula of chloroform, which is used as a solvent and a cleansing
agent. The ball-and-stick model of chloroform is shown in the margin
Sample Exercise
Write the empirical formula for the following molecules:
a) Acetylene (C2H2), which is used in welding torches
b) Glucose (C6H12O6), a substance known as blood sugar
c) Nitrous oxide (N2O), a gas that is used as an anesthetic gas (“laughing gas”) and as an aerosol
propellant for whipped cream
Write the empirical formula for nicotine (C10H14N2), an addictive drug present in tobacco.
Formula of Ionic Compounds
 Formulas are usually the same as their empirical formulas
 Ions are arranged in a three-dimensional network
 Ionic compounds do not exist as molecules
 The sum of the charges on the cation and anion in each formula unit must be zero (ionic compounds
are neutral)
 The subscript of the cation is numerically equal to the charge on the anions, and the subscript of the
anion is numerically equal to the charge on the cation (zinc iodide, aluminum oxide)
(a) Structure of solid NaCl
(b) In reality, the cations are in contact with the anions. In both (a)
and (b), the smaller spheres represent Na+ ions and the larger
spheres, Cl- ions.
(c) Crystals of NaCl
Sample Exercise
Write the formula of magnesium nitride, containing the Mg2+ and N3− ions.
AP Chemistry
Mr. Markic
Page 8 of 12
Practice Exercise
Write the chemical formulas of the following ionic compounds:
(a) Chromium sulfate (containing the Cr3+ and SO42(b) Titanium oxide (containing the Ti4+ and O2- ions)
ions)
Review of Concepts
Match each of the diagrams shown here with
the following ionic compounds: Al2O3, LiH,
Na2S, Mg(NO3)2. (Green spheres represent
cations and red spheres represent anion.
Chemical Nomenclature
• Ionic Compounds
– Often a metal + nonmetal
Anion (nonmetal), add “-ide” to element name
–
•
Transition metal ionic compounds
– indicate charge on metal with Roman numerals
FeCl2 2 Cl- -2 so Fe is +2
iron(II) chloride
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
BaCl2
K 2O
Mg(OH)2
KNO3
barium chloride
potassium oxide
magnesium hydroxide
potassium nitrate
AP Chemistry
Mr. Markic
Sample Exercise
Name the following compounds
(a) Cu(NO3)2
(b) KH2PO4
Page 9 of 12
(c) NH4ClO3
(d) V2O5
Write chemical formulas for the following compounds:
(a) Mercury (I) nitrite
(c) Calcium phosphate
(b) Cesium sulfide
(e) Li2SO3
(e) Barium hydroxide
(d) Rubidium sulfate
Molecular compounds
• Nonmetals or nonmetals + metalloids
• Common names
− H2O, NH3, CH4
• Element furthest to the left in a period and closest to the bottom of a group on
periodic table is placed first in formula
• If more than one compound can be formed from the same elements, use
prefixes to indicate number of each kind of atom
• Last element name ends in -ide
AP Chemistry
Mr. Markic
Page 10 of 12
Molecular Compounds
HI
NF3
SO2
N2Cl4
NO2
N 2O
hydrogen iodide
nitrogen trifluoride
sulfur dioxide
dinitrogen tetrachloride
nitrogen dioxide
dinitrogen monoxide
Sample Exercise
Name the following compounds
(a) SiCl4
(c) NF3
(b) P4O10
(d) Cl2O7
Write the chemical formulas for the following molecular compounds:
(a) Carbon disulfide
(c) Sulfur tetrafluoride
(b) Disilicon hexabromide
(d) Dinitrogen pentoxide
AP Chemistry
Mr. Markic
Page 11 of 12
Acids and Bases
Naming Acids
An acid can be defined as a substance that yields hydrogen ions (H+) when dissolved in water.
For example: HCl gas and HCl in water
• Pure substance, hydrogen chloride
•
Dissolved in water (H3O+ and Cl−), hydrochloric acid
Oxyacids
• Acids that contain hydrogen, oxygen, and another element (the central element)
• Formulas are written with the H first, followed by the central element and then O:
HNO3 – nitric acid
H2CO3 – carbonic acid
H2SO4 – sulfuric acid
HClO3 – chloric acid
Starting with the oxoacids whose names end with “-ic” we use the following rules to name these compounds
1. Additions of one O atom to the “-ic” acids: The acid is called “per…-ic” acid.
 Adding an O atom to HClO3 changes chloric acid to perchloric acid
2. Removal of one O atom from the “-ic” acid: The acid is called “-ous” acid.
 Nitric acid, HNO3, becomes nitrous acid, HNO2
3. Removal of two O atoms from the “-ic” acid: The acid is called “hypo…-ous” acid.
 When HBrO3 is converted to HBrO, the acid is called hypobromus acid
 Rules for naming oxoanions
1. When all the H ions are removed from the “-ic” acid, the anion’s name ends with “-ate”.
 The anion CO3-2 derived from H2CO3 is called carbonate
2. When all the H ions are removed from the “-ous” acid, the anion’s name ends with “-ite”.
 Thus the anion ClO2- derived from HClO2mis called chlorite
AP Chemistry
Mr. Markic
Page 12 of 12
3. The names of anions in which one or more but not all hydrogen ions have been removed must indicate
the number of H ions present
 Anions derived from phosphoric acid
• H3PO4 phosphoric acid
• H2PO4- dihydrogen phosphate
• HPO42- hydrogen phosphate
• PO43- phosphate
Sample Exercise
Name the following oxoacid and oxoanion:
(a) H3PO3
(b) IO4-
(c) HBrO4
(d) HSO4-
Naming Bases
A substance that yields hydroxide ions (OH-) when dissolved in water
NaOH – sodium hydroxide
KOH – potassium hydroxide
Ba(OH)2 – barium hydroxide
Hydrates
• Compounds that have a specific number of water molecules attached to them
• Anhydrous – the compound no longer has water molecules associated with it
BaCl2•2H2O
barium chloride dihydrate
LiCl•H2O
lithium chloride monohydrate
MgSO4•7H2O
magnesium sulfate heptahydrate
Sr(NO3)2 •4H2O
strontium nitrate tetrahydrate