Chapter 5: Molecules and Compounds
Date:____________
Section 1: Compounds and Chemical Formulas - Notes
Objectives:
 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.
Forming Compounds:

_____________________ is an extremely reactive metal that dulls almost instantly upon
exposure to air.

_____________________ is a yellow gas with a pungent odor. It is highly reactive and
poisonous.

The compound formed by sodium and chlorine is __________________________.

The properties of a compound are, in general, _______________________ from the properties
of the elements that compose it.

In a compound, the elements combine in ____________________________________________.
o
The law of _______________________________________ (Proust)

Also known as the law of _____________________________________________
Chemical Formulas:

A chemical formula indicates the ______________________ present in a compound and the
relative number of ______________ of each.
o
For example, ___________ is the chemical formula for water; it indicates that water
consists of _______________ and ______________ atoms in a ________ 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 _____ is omitted.

What are the element ratios for these common chemical formulas:
o

NaCl
o
CO2
o
C12H22O11
The _________________________ in a chemical formula are part of the compound’s definition if they change, the formula no longer specifies the ________________________________.

Chemical formulas list the most _______________________ elements first.
o
The formula for table salt is ___________, not ______________.

In compounds that do not include a metal, the more _________________________ element is
listed first.

Among nonmetals, those to the ____________ in the periodic table are more metal-like than
those to the _____________ and are normally listed first.
o

We write _________ and _________, not _________ and _________.
Within a single column in the periodic table, elements toward the ______________________
are more metal-like than elements toward the ___________.
o

We write _________, not _________.
There are a few historical exceptions in which the most metallic element is not listed first, such
as the _______________________________, which is written as __________.

Practice: Write a chemical formula for each compound:
o
The compound containing two aluminum atoms to every three oxygen atoms
o
The compound containing three oxygen atoms to every sulfur atom
o
The compound containing four chlorine atoms to every carbon atom
Polyatomic Ions:

Some chemical formulas contain groups of atoms that act as a ___________. When several
groups of the same kind are present, their formula is set off in ___________________________
with a subscript to indicate the number of that group.
o
_____________________ indicates a compound containing ___________ magnesium
atom (present as the Mg2+ ion) and ___________ NO3– groups.

Many of these groups of atoms have a __________________ associated with them and are
called _________________________________.

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.

Practice:
o
o
Mg(NO3)2
Mg3(PO4)2
o
Al2(SO4)3

Mg = ____

Mg = ____

Al = ____

NO3 = ____

P = ____

S = ____

O = ____

O = ____
 N = ____
 O = ____
Types of Chemical Formulas:

An ___________________________________ gives the relative number of atoms of each
element in a compound.

A ___________________________________ gives the actual number of atoms of each element
in a molecule of the compound.

A ___________________________________ uses lines to represent chemical bonds and shows
how the atoms in a molecule are connected to each other.


For hydrogen peroxide:
o
Molecular formula: _____________
o
Empirical formula: _____________
The molecular formula is always a whole-number _________________ of the empirical formula.
o
For many compounds, such as ______________, the molecular formula is the
______________ as the empirical formula.

A ___________________________________ uses lines to represent chemical bonds and shows
how the atoms in a molecule are connected to each other.
o

For hydrogen peroxide:
________________________________: three-dimensional representations of molecules that
are used to represent compounds.

We use two types of molecular models:
o
________________________________
o
________________________________

In ball-and-stick models, we represent _______________________________ and
__________________________________________.

The balls and sticks are connected to represent the molecule’s ________________. The balls are
color coded, and each element is assigned a color.

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 _______________________.

Let's take a look at _______________________:
Identifying Substances:

Pure substances may be either ____________________ or ______________________.

Elements may be either ________________ or _____________________.

Compounds may be either ____________________ or __________________.

Atomic Elements:

o
Elements that occur as ___________________________
o
___________________________
o
___________________________
Molecular Elements:
o
Elements that occur ___________________
o
________________________
o
Seven elements
o

Molecular compounds are compounds formed from two or more
nonmetals.

Ionic compounds contain one or more _______________ paired with one or more __________.
o
In most cases, the cations are ________________ and the
anions are ______________________.

When a metal combines with a nonmetal, one or more
_____________________ transfer from the metal to the nonmetal,
creating positive and negative ions that are __________________
to each other.

A compound composed of a _______________________________________ is considered ionic.

The basic unit of ionic compounds is the _____________________________.

Unlike molecular compounds, ionic compounds do not contain individual molecules but rather
cations and anions in an __________________________________________________________.
Practice:

Classify each substance as an atomic element, molecular element, molecular compound, or ionic
compound:
o
Krypton
o
CoCl2
o
Nitrogen
o
SO2
o
KNO3