Writing Chemical
Formulas
General Chemistry
Mrs. Amy Nare
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http://chem.pdx.edu/~wamserc/C335W00/gifs/MW2.gif
Objectives
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Distinguish between molecular and ionic
compounds
Contrast molecular formulas and formula units
Use the periodic table to determine the charge on
an ion
Define monatomic and polyatomic ion and name
the charges of common polyatomic ions
Write the formulas and names for binary and
ternary ionic compounds
Write formulas and names for molecular
compounds
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Inspiration Concept Map
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Chemical Bonding
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Atoms – same number of protons (+) and
electrons (-); electrically neutral
Ions – atoms w/ a (+) or (-) charge; have
lost or gained electrons (e-)
* Cations: (+) charge; has lost e-; metals
* Anions: (-) charge; has gained e-;
nonmetals
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Chemical Bonding
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Types of Compounds
A. Molecular – atoms
bonded together by shared
pairs of e- (covalent bonds);
formed between nonmetals
ex: C6H12O6
B. Ionic – ions attracted
to each other by opposite
charges; formed between a
metal and a nonmetal
ex: NaCl
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Chemical Bonding
Compound
Molecular
Ionic
Smallest unit
Molecule
Formula unit
Types of
elements
Nonmetals
Physical state @
room. temp.
Solid, gas, or
liquid
Metal cations
Nonmetal anions
Solid
Boiling point
High, >300oC
Low, <300oC
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Chemical Bonding
Chemical Formulas – show the type and
number of atoms in smallest unit of
substance
 Molecular Formula – type and number of
atoms joined to form a molecule
ex: C2H5OH
 Formula Unit – smallest ratio of ions in an
ionic compound
ex: CaF2
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Representing Chemical Compounds
Law of Definite Proportions – in any
sample of a compound, the elements
are always combined in the same
proportions
ex: H2O and H2O2
H2O – water – H:O ratio always 2:1
H2O2 – hydrogen peroxide – H:O ratio
always 1:1
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Identifying Ionic Charges
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Group A elements – use the periodic
table to determine ionic charge
* elements in same group have
same ionic charge
* Group 4A and Noble gases –
almost never form ions
Group B elements – many have
more than one ionic charge
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Identifying Ionic Charges
http://wps.prenhall.com/wps/media/objects/476/488316/ch04.html
Charge on cations corresponds to group #.
Charge on anions is found by subtracting 8 by group number
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the number 8 is used b/c it represents # of valence e- in Noble gases
Naming Cations and Anions
Monatomic Ions
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Ions formed by one element
Cations
* for Group A elements – just write
element’s name
ex: calcium ion (Ca2+) = calcium
* for Group B elements – write element’s
name, then Roman numerals in
parentheses to denote charge
ex: Fe2+ = Iron (II) and Fe3+ = Iron (III)
Anions – drop the end of the element’s name &
add “–ide” ending
ex: chlorine ion (Cl1-) = chloride
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Naming Cations and Anions
Polyatomic Ions
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Ions formed by more than one type of element
Atoms of different elements held together by
covalent bonds
Atoms always stay together and collectively
have a single charge
Do not always have “-ide” ending
ex: NH41- = ammonium ion
CO32- = carbonate ion
Learn names, formulas, and charges of
polyatomic ions!
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Naming Cations and Anions
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Potassium ion
Copper (II) ion
Chloride ion
Oxide ion
Ba2+
S2Au3+
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Nitrite ion
Hydroxide ion
Phosphate ion
SO42CrO42ClO3213
Binary Ionic Compounds
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Compounds composed of 2 different monatomic
elements
To write binary formulas – write cation first, then
anion
*criss-cross charges to determine how
many of each ion you need
*use subscripts to denote number of ions
ex: Ca2+ + Cl1CaCl2
Na1+ + Cl1NaCl
To name binary compounds – write name of cation
first, then anion (-ide)
ex: CaCl2 = calcium chloride
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Li2O = lithium oxide
Ternary Ionic Compounds
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Compounds containing at least one polyatomic ion; at least 3
different elements
To write ternary formulas: write cation first, then anion
*criss-cross charges to determine how
many of each ion you need
*use subscripts to denote number of ions
*must use parentheses around polyatomic if more than one is
needed!!!
ex: Na1+ + SO32Na2SO3
Mg2+ + OH1Mg(OH)2 [not same as MgOH2]
To name ternary compounds: write name of cation, then name of
anion (not all end in “-ide”)
**be careful with transition metals (more than one charge)**
ex: CaCO3 = calcium carbonate
PbSO4 = lead (II) sulfate
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Ag2CrO4 = silver chromate
Ionic Compounds
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NaNO3
CaSO4
(NH4)2O
CuSO3
Fe(OH)3
NaF
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Lithium sulfide
Iron (III) phosphide
Magnesium fluoride
Barium nitrate
Aluminum hydroxide
Potassium phosphate
Practice making ionic compounds!
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Binary Molecular Compounds
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1
2
3
4
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Two nonmetals joined by covalent bonds
Use prefixes for naming
=
=
=
=
=
monoditritetrapenta-
6 = hexa7 = hepta8 = octa9 = nona10 = deca-
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Binary Molecular Compounds
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To name binary molecular compounds:
*first element gets a prefix if there is more than
one
*second element ALWAYS gets prefix, and “-ide”
ending
ex: N2O3 = dinitrogen trioxide
CO = carbon monoxide (not monocarbon)
If element begins with vowel and prefix ends in “a”
or “o”, then drop last vowel on prefix to form the
name
ex: Cl2O7 = dichlorine heptoxide (not heptaoxide)
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Molecular Compounds
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P2O5
N2O
NO2
CBr4
CO2
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tetraiodine nonoxide
sulfur hexafluoride
nitrogen trioxide
carbon tetrahydride
phosphorus trifluoride
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Assess what you learned. Log on to
the Internet and take the quiz.
http://school.discovery.com/quizzes31/amy_trauth/
FormulasQuiz.html
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Fordham Preparatory
School Online Chemistry
University of North Carolina Chapel Hill Chemistry
Fundamentals Program
General Chemistry Online,
Frostburg State University
ScienceGeek.net
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Aver a ge
To t a l
Cu r r en cy
Desi gn
Au t h o r i t y
Lo g o
Accu r a cy
Web si t e
Co n t en t
Websites on Writing Formulas
6.6
This website by Greg Curran at Forham Prep is an online resource that
includes lecture notes, labs, worksheets, quizzes, and links to other useful
sites. Website is fairly current and thoughtfully designed, making it easy to
navigate and find necessary information quickly.
5.0
A very basic website that outlines some fundamental concepts in general
chemistry. Very few links to other websites are provided. Website has not
been updated since 1998. Overall design is lacking and content is
satisfactory for a university. This site does provide some interactive
activities that could be useful for students to practice skills.
7.6
This website designed for general chemistry undergrads clearly indicates
objectives of lessons, provides pre-tests and post-tests, and short, but
informative narratives to explain major concepts. Graphics are are clear and
useful, mostly line drawings and molecular representations of compounds.
Fred Senese is co-author of a general chemistry textbook for undergrads,
making him a well-known authority on the subject.
7.6
The Chemistry Pages on this website gives high school chemistry students
access to PowerPoint notes, labs, and worksheets. Some interactive activities
are also provided for students to review major concepts. Few graphics are
provided, but this fact does not detract from the utility of the website. The
site is nicely designed and easy to navigate. All the links tested were
functional.
General Chemistry, Purdue
University
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Chem Team
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Pr o j ec t Go a l s
Ru b r i c u sed f o r Web si t e Ev a l u a t i o n
Rev i ew
Designed as a tutorial for undergrad chemistry students, this website
provides many useful tools and interactive activities to help students grasp
key concepts. Many unique features, such as a molecular library that shows
the shape and bonding patterns of over 400 different molecules. Many
practice problems on the nomenclature of inorganic compounds. Joint
authorship by some students ans faculty in the Department of Chemistry.
This website, written for high school chemistry students, has most of the
general topics covered elswhere. However, the author provides links to many
other sites on the subject and provides nice tutorials for practice and review.
Few graphics are provided, but are educational when present. Information on
author's background and links to his credentials are difficult to locate. Page
design is simple, logical, and satisfactory in its design.
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