Formulas
Chapter 3
Pure Substances
• Pure substances = samples of matter where the entire sample
is the same throughout
• They have a homogeneous composition
• Ex: elements and compounds
Elements
• Cannot be decomposed by chemical means
• Everything on the Periodic Table
• First letter is always capitalized; additional letters are
lowercase
Compounds
• Substances composed of two or more elements that are
chemically combined
• Elements are in a definite ratio (water is always H2O)
• Law of definite proportions
• Can be decomposed by chemical means
Mixtures
• A physical combination of pure substances
• Do not need to have the same composition throughout
• Heterogeneous composition
• Do not follow the law of definite proportions
Mixtures, cont’d
Type of
Matter
Element
Compound
Mixture
Pure
Substance?
Varied
Composition?
Can Be
Decomposed?






Can be both
 (chemical means)
 (physical means)
Figure 3.1: (a) A monatomic element. (b) A diatomic element.
(c) A compound. (d) A mixture.
Chemical Formulas
• Tells you two things:
• Which elements are in the compound
• How many of each element is present
• The symbols must be written as they are on the Periodic Table
• The numbers are always subscripts
• “1” is always invisible
Correct
AlBr3
Incorrect
ALBR3
Al1Br3
Albr3
AlBr3
albr3
AlBr3
Figure 3.2: Avoid the incorrect ways to write
formulas. Be careful.
Types of Formulas
• Molecular formula  shows you exactly how many of each
element is in the compound
• Empirical formula  shows you the simplest ratio of
elements in the compound
• For some compounds, both formulas are the same
Ionic Charges
• Atoms and compounds  always neutral; no charges
• Equal numbers of protons and electrons
• Ions  either gained or lost electrons; charged
• Charges are superscripts
• “1” is invisible
Polyatomic Ions
• An ion made of two or more atoms bonded together
• Elements are usually nonmetals
• Table E
Types of Compounds
• Ionic compounds  made of a positive ion and a negative
ion
• Metal + nonmetal
• Metal + polyatomic ion
• Polyatomic ion + nonmetal
• Polyatomic ion + polyatomic ion
Types of Compounds, cont’d
• Covalent compounds  made of nonmetals, only
• Also called molecular compounds
• Never have metals or polyatomic ions
Making Ionic Compounds—Cancelling • If the charges have the same number but opposite signs, they
cancel
Making Ionic Compounds—Criss‐cross
• If the charges have the different numbers and opposite signs,
they criss-cross
Naming Ionic Formulas
• Metal + nonmetal
• Keep the name of the metal and change the ending of the
nonmetal to –ide
• Metal + polyatomic ion
• Keep the name of both—do not change anything
Stock System
• Used if the metal can have more than one charge
• Roman numerals indicate the charge
Ions
Fe3+ and O2–
Fe2+ and O2–
Compound
Fe2O3
FeO
Name
Iron (III) oxide
Iron (II) oxide
Ions
Cr3+ and S2–
Cr6+ and S2–
Pb2+ and OH–
Pb4+ and OH–
Compound
Cr2S3
Cr2S6
Pb(OH)2
Pb(OH)4
Name
Chromium (III) sulfide
Chromium (VI) sulfide
Lead (II) hydroxide
Lead (IV) hydroxide
Metal + Nonmetal
Metal has more than one charge
Metal has only one charge
Name of Charge of metal in
Name of
metal
Roman numerals nonmetal + –ide
Name of
metal
Name of
nonmetal + –ide
Naming Covalent Compounds
• Contain only nonmetals
• Use prefixes to show the number
of each element
• Don’t use mono- for the first
element
• Second element still ends in –ide
Nonmetal + Nonmetal
Prefix
Name of 1st
element
Name of 2nd
Prefix
element + –ide