Chapter 5 Molecules and Compounds Learning Goals Write chemical formulas Determine the total number of each type of atom in a chemical formula Classify elements as atomic or molecular Classify ID: 718998
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Slide1
Section 1: Compounds and Chemical Formulas
Chapter 5: Molecules and CompoundsSlide2
Learning Goals
Write
chemical formulas.
Determine
the total number of each type of atom in a chemical formula.
Classify
elements as atomic or molecular.
Classify
compounds as ionic or molecular. Slide3
Forming Compounds
Sodium
is
an
extremely reactive metal that dulls almost instantly upon exposure to air. Slide4
Forming Compounds
Chlorine
is a yellow gas with a pungent odor. It is highly reactive and poisonous.Slide5
Forming Compounds
The
compound formed
by
sodium and chlorine
i
s
table salt.
The properties of a compound are, in general, different from the properties of the elements that compose it.Slide6
Forming Compounds
In
a compound, the elements combine in fixed, definite proportions
.
The law of definite proportions (Proust)
Also known as the law of constant compositionSlide7
Chemical Formulas
A
chemical formula indicates the elements present in a compound and the relative number of atoms of each.
For example, H
2
O is the chemical formula for water; it indicates that water consists of hydrogen and oxygen atoms in a 2:1 ratio.
The formula contains the symbol for each element, accompanied by a subscript indicating the number of atoms of that element. By convention, a subscript of 1 is omitted. Slide8
Chemical Formulas
What are the element ratios for these common
chemical
formulas:
NaCl
CO
2
C
12
H22O
11
Slide9
Chemical Formulas
The subscripts in a chemical formula are part of the compound’s definition—if they change, the formula no longer specifies the same compound. Slide10
Chemical Formulas
Chemical formulas list the most metallic elements first.
The
formula for table salt is
NaCl
, not ClNa
.
In compounds that do not include a metal, the more metal-like element is listed first
.Slide11
Chemical Formulas
Among
nonmetals, those to the left in the periodic table
are
more metal-like than those to the right and are normally listed first.
We
write NO
2
and NO, not O
2
N and ON.
Within a single column in the periodic table, elements toward the bottom are more metal-like than elements toward the top. We
write SO
2
, not O
2
S
.Slide12
Chemical Formulas
There
are a few historical exceptions in which the most
metallic
element is
not listed first,
such as the hydroxide ion, which is written as OH
–
. Slide13
Practice
Write a chemical formula for each compound:
The compound containing two aluminum atoms to every three oxygen atoms
The compound containing three oxygen atoms to every sulfur atom
The compound containing four chlorine atoms to every carbon atomSlide14
Polyatomic Ions
Some chemical formulas contain groups of atoms that act as a unit.
When
several groups of the same kind are present, their formula is set off in parentheses with a subscript to indicate the number of that group.
Mg(NO
3
)
2
indicates a compound containing one magnesium atom (present as the Mg
2+
ion) and two NO
3
–
groups. Slide15
Polyatomic Ions
Many
of these groups of atoms have a charge associated with them and are called polyatomic ions.
To determine the total number of each type of atom in a compound containing a group within parentheses, multiply the subscript outside the parentheses by the subscript for each atom inside the parentheses.Slide16
Practice
Mg(NO
3
)
2
Mg = ____
NO3 = ____
N = ____
O = ____Slide17
Practice
Mg
3
(PO
4
)2
Mg = ____
P
= ____
O = ____Slide18
Practice
Al
2
(SO
4
)3
Al = ____
S = ____
O = ____Slide19
Types of Chemical Formulas
An
empirical formula
gives the relative number of atoms of each element in a compound.
A
molecular formula
gives the actual number of atoms of each element in a molecule of the compound.
A
structural formula
uses lines to represent chemical bonds and shows how the atoms in a molecule are connected to each other. Slide20
Types of Chemical Formulas
For hydrogen peroxide:
Molecular formula: H
2
O
2
Empirical formula: HO
The
molecular formula is always a whole-number multiple of the empirical formula.
For many compounds, such as H2O, the molecular formula is the same as the empirical formula
.Slide21
Types of Chemical Formulas
A
structural formula
uses lines to represent chemical bonds and shows how the atoms in a molecule are connected to each other
.
For hydrogen peroxide:
H O O H Slide22
Types of Chemical Formulas
Molecular
models: three-dimensional
representations of
molecules that are
used to represent compounds.
We use two types of molecular
models:
ball-and-stick
space-fillingSlide23
Types of Chemical Formulas
In
ball-and-stick models, we represent atoms as balls and chemical bonds as sticks.
The balls and sticks are connected to represent the molecule’s shape. The balls
are
color coded, and each element is assigned a
color.Slide24
Types of Chemical Formulas
In
space-filing models, atoms fill the
space
between each other to more closely represent our best idea for how a
molecule
might appear if we could scale
it
to a visible size
.Slide25
Types of Chemical Formulas
Let’s take a look at methane:Slide26
Identifying Substances
Pure substances may be either elements or compounds.
Elements may be either atomic or molecular.
Compounds may be either molecular or ionic.Slide27Slide28
Identifying Substances
Atomic Elements:
Elements that occur as single atoms
Monoatomic
Most elementsSlide29
Identifying Substances
Molecular Elements:
Elements that occur in pairs
Diatomic
Seven elementsSlide30Slide31
Identifying Substances
Molecular compounds are compounds formed from two or more nonmetals. Slide32
Identifying Substances
Ionic
compounds contain one or
more
cation
paired with one or
more
anion.
In most cases, the cations are metals and the anions are nonmetals. Slide33
Identifying Substances
When a
metal combines
with a
nonmetal, one
or more electrons transfer from the metal to the nonmetal, creating positive and negative ions that are attracted to each other.
A compound composed of a metal and a nonmetal is considered ionic. Slide34
Identifying Substances
The
basic unit of ionic compounds is the
formula unit
.
Unlike molecular compounds, ionic compounds do not contain individual molecules but rather cations and anions in an alternating three-dimensional array. Slide35
Practice
Classify each substance as an atomic element, molecular element, molecular compound, or ionic compound:
Krypton
CoCl
2
Nitrogen
SO
2
KNO
3