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Chapter 2
Elements and Compounds
Setting the Stage – Search for
Water
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A key molecule we search for on other
planets is water, as its presence suggests
the possibility of life
Water is only one example of matter (albeit
an important one)
Matter is anything that has mass and
occupies space
Chemistry is the study of matter and the
changes that it undergoes
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Setting a Goal - Part A
The Elements and Their
Composition
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You will become familiar with the
basic components of matter and the
properties that make each type of
matter unique
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Objectives for Section 2-1
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Distinguish between elements and
compounds
Match the names and symbols of the
common elements
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2-1 The Elements
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An element is the most basic form of
matter that exists under ordinary
circumstances
A compound is a unique substance
that is composed of two or more
elements that are chemically
combined
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Elements Free in Nature
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Only a few elements are found in
their free state (not combined with
other elements)
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Elemental Distribution
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The Earth has 88 elements present in
measurable amounts, but 10
constitute ~98.9% of the crust
There are 112+ known elements (26
spontaneously decay into other
elements)
The human body is 93% carbon,
hydrogen and oxygen
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Distribution of Elements – the
Earth
Ti 0.86%
K 1.68%
H, Mn, P, all other
elements 1.1%
Na 2.32%
Mg 2.77%
Ca 5.06%
Fe 5.8%
O 45.2%
Al 8%
Si 27.2%
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Distribution of Elements - Human
O 64.6%
C 18.0%
H 10.0%
N 3.1%
Ca 1.9%
P 1.1%
Trace elements
(Cu, Fe, Zn etc) 0.1%
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Cl, K, Mg, Na, S 1.2%
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Names and Symbols of the
Elements
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Symbol - usually the first and second
letters of the name
 For two letter symbols, the first letter is
capitalized but the second is not
e.g. Li, Be, Ca, Ba, He, Ne
 Two letters are used for most elements.
Exceptions: B, C, F, H, I, K, N, O, P,
S, U, V, W, Y
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Names and Symbols of the
Elements….continued
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When the names of two or more elements
begin with the same two letters, later
letters are used in the symbols.
Examples
Ruthenium (Ru) Rubidium (Rb)
Cerium (Ce) Cesium (Cs)
Thorium (Th) Thalium (Tl) (Ta is Tantalum)
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Names and Symbols of the
Elements….continued.

When the name and symbol use different letters,
that means the element was known in antiquity,
and the Latin (or other ancient language) name is
used as the basis of the symbol. The only exception
is Tungsten (W), which uses the German name
(Wolfram)
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Examples
Ag (Argentium; silver)
Au (Aurum; gold)
Cu (Cuprum; copper)
Fe (Ferrum; iron)
Hg (Hydragyrum; mercury)
K (Kalium; potassium)
Na (Natrium; sodium)
Pb (Plumbum; lead)
Sb (Stibonium; antimony)
Sn (Stannum; tin)
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Objective for Section 2-2
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List the postulates of the Atomic
Theory
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2-2 The Composition of Elements:
Atomic Theory of John Dalton
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Matter is composed of small indivisible
particles called atoms
Atoms of the same element are identical
and have the same properties
Chemical compounds are composed of
atoms of different elements combined in
small whole-number ratios
Chemical reactions are merely the
rearrangement of atoms into different
combinations
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John Dalton (1766-1844),
founder of the modern
atomic theory of matter
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Molecules and Compounds
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Atoms are the smallest fundamental
particle of an element that have the
properties of that element
Molecules - formed by the chemical
combination of two or more atoms
Molecular compounds - molecules of
atoms of two different elements
Covalent bonds - the force holding the
atoms together in a molecular compound
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Objective for Section 2-3
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List the components of an atom, their
relative masses, charges, and
location in the atom
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2-3 Composition of the Atom
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Atoms are composed of three subatomic
particles
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proton - particle with a +1 charge; p
electron - particle with a -1 charge; e
neutron - particle with no charge; n
The proton and the neutron have roughly
the same mass (~1 amu or 1.67  10-24 g)
The mass of the electron is substantially
smaller than the masses of the proton or
the neutron
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Electrical Nature of Matter
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Electrostatic forces
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Ions are charged atoms or molecules
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attraction between opposite charges
repulsion between same charges
cations - positively charged
anions - negatively charged
Compounds consisting of ions are known
as ionic compounds, and the forces
holding them together are called ionic
bonds
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Nuclear Model of the Atom
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It was shown that the protons in the atom
are found in a very small part of the atom
called the nucleus
The protons and the neutrons of the atom
are found in the nucleus (and are therefore
termed nucleons)
The electrons are found outside of the
nucleus
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Nuclear Model of the Atom
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Objectives for Section 2-4
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Define the terms atomic number, mass
number, and isotope
Using the table of elements, determine the
number of protons, neutrons, and
electrons in any isotope of an element
Demonstrate how the atomic mass of an
element is determined from isotopes and
their percent abundance
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2-4 Atomic Number, Mass Number,
and Atomic Mass
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Atomic number, Z, is the number of
protons in the nucleus
The mass number, A, is the total number
of nucleons
An isotope is an atom of a specific
element with a specific mass number
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Isotopes
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Atoms may have the same atomic number
but different mass numbers (meaning the
same number of protons but different
numbers of neutrons)
The atomic number determines the
identity of the element
Atoms of the same element with differing
mass numbers are termed isotopes
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Isotopes
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Most elements occur naturally as a mixture of
isotopes
The specific isotope is written using isotopic
notation
This isotope will be referred to by the mass
number
 e.g. copper-63
63
29
Cu or more
simply
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Cu
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Isotopic Mass
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Mass of an isotope compared to a standard
The accepted standard is 12C, which is defined as
having a mass unit of exactly 12 amu
An amu is defined as one-twelfth the mass of an
atom of 12C
The atomic mass of an element listed in the
periodic table is the weighted average of the
atomic masses of all isotopes present in nature
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Calculating Atomic Masses
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For boron, 19.9% occurs as 10B and 80.1%
occurs as 11B. The isotopic mass of 10B is
10.013 amu and 11B is 11.009 amu
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0.199  10.013 amu = 1.99 amu
11B: 0.801  11.009 amu = 8.82 amu
Atomic mass of boron = 10.81 amu
10B:
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Calculating Atomic Masses (2)
From the following data, determine the
atomic mass of lithium.
Isotope
Mass (amu) %Abundance
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6Li
6.015
7.42
7Li
7.016
92.58
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Atomic mass = 6.015 x 0.0742 + 7.016 x
0.9258 = 6.941
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Calculating Atomic Masses (3)
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Bromine has two natural isotopes, 79Br
and 81 Br. From its atomic mass, estimate
the abundance of the two isotopes.
From the front cover, the atomic mass of
bromine is 79.904, hence the abundance is
approximately 50% each.
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Setting a Goal - Part B
Compounds and Their
Composition
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You will learn to distinguish between
ionic and molecular compounds
based upon their chemical structure
and general properties
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Objectives for Sections 2-5 and 2-6
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Describe the difference between a
molecular and ionic compound
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2-5 Molecular Compounds
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A molecule is formed by the chemical
combination of two or more atoms
Molecules are the basic particles of
molecular compounds
The atoms in a molecule are held together
by a force called the covalent bond
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Formulas
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A compound is represented by using the
symbols for the elements of which it is
composed
Subscripts are used to indicate how many
atoms of a particular element exist in the
compound
If there is only one atom of a particular
element, the one is assumed
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Formulas, cont’d
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Note that changing the subscripts
changes the compound
 consider H2O and H2O2; SO2 and SO3
Two different compounds can, however,
share the same molecular formula
 dimethyl ether and ethyl alcohol
(ethanol) both have the formula C2H6O
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Ethyl Alcohol and Dimethyl Ether
H
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H
H
C
C
H
H
H
O
H
H
C
H
H
O
C
H
H
Ethyl alcohol on the left; dimethyl ether on the
right
These species are termed structural isomers
Formulas that show the order and arrangement of
specific atoms are known as structural formulas
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Isomers
Isomers are different compounds (with
different structural formulas) but sharing
the same molecular formula
 E.g. H-O-CN cyanic acid
H-N=C=O isocyanic acid
H-CNO fulminic acid
Each has the molecular formula HCNO
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Molecular Elements: Allotropes
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Hydrogen, nitrogen, oxygen, fluorine, chlorine,
bromine, and iodine, exist as diatomic
molecules
Some elements exist in a variety of forms,
called allotropes
e.g. Carbon – graphite; diamond;
buckminsterfullerene (etc)
Phosphorus - red (Pn) and white (P4)
Sulfur – S8 and S4
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Objective for Section 2-6
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Write the formulas of simple ionic
compounds given the charges on the ions
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2-6 Ionic Compounds
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Recall that some atoms and molecules
can exist as charged species
Cations are positively charged ions
Anions are negatively charged ions
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How Charged Species Arise
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Neutral atoms and molecules have the
same number of protons and electrons
Cations have fewer electrons than protons
Anions have more electrons than protons
Cation
Na
Cl + e
Na+ +
e-
ClAnion
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Ionic Compounds
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Formula unit - simplest wholenumber ratio of ions in an ionic
compound
For example: Ca2+ + Br
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the resulting formula should be
electrically neutral
two Br- are needed for each Ca2+
the resulting formula is CaBr2
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Polyatomic Ions
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These are cations or anions consisting of
groups of atoms that are covalently
bonded to each other
Examples are NO3-, SO42-, ClO4-, MnO4When more than one appears in a formula
unit, the polyatomic ion is put in between
parentheses, and a subscript is used to
indicate the number of the ions that
appear in the formula unit
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Examples of Polyatomic Ions
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Ba(ClO4)2, CaCO3
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Further Examples of Polyatomic
Ions
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NO2+ (nitronium ion: important in the
nitration of organic compounds)
C6H5CH2+ (benzyl cation; a carbocation)
VO2+ (vanadyl cation; an important cation
of vanadium)
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Formulas of Ionic Compounds
with Polyatomic Ions
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These are determined the same way as for
monoatomic ions
The total charge must be zero
For Fe3+ and NO31 Fe3+ + 3 NO3- = Fe(NO3)3
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Further Examples
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For Fe2+ and PO433 Fe2+ and 2 PO43- = Fe3(PO4)2
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For Fe3+ and C2O42- (oxalate)
2 Fe3+ and 3 C2O42- = Fe2(C2O4)3
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Pb2+ and N3- (azide)
Pb2+ and 2 N3- = Pb(N3)2
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