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Learning Objectives
A checklist of concepts to learn and skills to master in this section.
Before You Start
Prerequisite skills and concepts to review.
Lecture Slides
Lecture Notes
Internet sites and paper references for further exploration.
Frequently Asked Questions
Find an answer, or ask a question.
Terms and definitions from the glossary are marked with an asterisk (*).
Quiz: Classifying compounds
Classify compounds as ionic or molecular.
/chem/senese/101/compounds/compound-classify-quiz.shtml (8/24/98)

Quiz: Names and formulas of polyatomic ions
Write the formulas of polyatomic ions from their names, and vice versa.
/chem/senese/101/compounds/polyatomic-quiz.shtml (7/29/98)

Quiz: Interpreting formulas
Understand the subscript notation for chemical formulas.
/chem/senese/101/compounds/interpret-formulas-quiz.shtml (8/12/98)

Isomer Construction Set
Your mission: build all the isomers of octane, starting from propane. A Chime version of this tutorial is in the works, but no plugins are required for this simple exercise.
/cgi-bin/senese/tutorials/isomer/index.cgi (4/19/98)

Learning objectives

Quiz Quiz
classifying compounds
Quiz Quiz
names and formulas of polyatomic ions
  • Explain the mole concept, and convert between grams, moles, and atoms and molecules.
  • Determine mass percent composition of a sample from experimental data.
  • Determine mass percent composition of a compound from its formula.
  • Determine empirical formula of a compound from its mass percent composition.

Before you start...

Lecture outline

Ionic and molecular compounds

What holds atoms together in a molecule?
  • Molecular compounds* are made of molecules*.
    • each molecule contains anywhere from two atoms (diatomic molecules*) to thousands (biological molecules).
    • each molecule has the same element composition and properties as the compound.
    • synonym: covalent compound
Table Table
Common molecular compounds
    • examples: H2O, CO2, C6H12O6, NH3, CH4
  • Ionic compounds* are made of positive ions (cations*) and negative ions (anions*).
    • cations combine with anions in just the right numbers to give an electrically neutral compound.
    • metals form cations easily, and nonmetals form anions, so metal/nonmetal compounds are often ionic
    • cations and anions pack into orderly arrays in solids; they become mobile when the compound melts
    • individual molecules don't normally exist!
      ion pair
      an ion pair
      ion cluster
      an ion cluster
      ion crystal
      an ion crystal
    • examples: NaCl, KBr, Na2S, MgBr2
    • synonym: salts
Quiz Quiz
classifying compounds as ionic or covalent
Table Table
A more detailed comparison of ionic and molecular compounds.
Table: Comparing ionic and molecular compounds.

Molecular compounds Ionic compounds
smallest particles molecules ions
elements present close on the periodic table widely separated on the periodic table
electrical conductivity poor good, when melted or dissolved
state at room temperature solid, liquid, or gas solid
other names covalent compounds salts

Can a compound be classified as ionic or covalent from its formula alone?

Formulas for molecular compounds

  • A molecular formula* shows the type and number of atoms in a molecule
  • type of atom indicated by element symbol*
  • number of atoms per molecule indicated by subscripts (if greater than one)
    • H2O contains 2 hydrogen atoms and one oxygen atom per molecule.
    • Carbon tetrafluoride, CF4, contains four fluorine atoms and one carbon atom per molecule.
  • atoms in formulas are sometimes grouped to show how they're connected in the molecule
    • Methanol is usually written as CH3OH to show that 3 hydrogens are bound to the carbon and another hydrogen is bound to the oxygen.
    • Acetic acid can be written as CH3COOH or as HC2H3O2 or C2H4O2. The first formula shows how the molecule is put together; the second formula emphasizes that one hydrogen is different from the others; the third formula is the least informative because it shows only the numbers and types of atoms in the molecule.
  • groups that appear more than once in the molecule are enclosed in parentheses
    • CH3(CH2)3CH3 could be written as C5H12, but all information about the structure of the molecule would be lost.
    • (CH3CH2)4P2O7 molecules contain 8 carbons, 20 hydrogens, two phosphoruses, and seven oxygens.
  • molecular weight* = sum of weights of atoms in the molecule
    • The molecular weight of CH3OH is approximately 12 + 4*1 + 16 = 32 since the carbon, hydrogen, and oxygen have atomic weights of 12, 1, and 16 u, respectively.

Formulas for ionic compounds

  • empirical formula* gives the elemental composition of a compound
    • formula lists elements present by element symbol*
    • subscripts give ratios of ions or atoms in the compound

      CuSO4 contains 4 atoms of O and 1 atom of S for every 1 atom of Cu.
      Na2CO3 contains 2 atoms of Na and 3 atoms of O for every 1 atom of C.

  • writing ionic empirical formulas
    1. write the cation formulas, including charge
    2. write the anion formulas, including charge
    3. combine enough cations with enough anions to give a total charge of zero
      • trick: swap charges as subscripts
      • don't write charges when the ions are combined
    4. use the simplest (lowest) cation-to-anion ratio possible
    5. list cations first, anions last

    Potassium ions (K+) and chloride ions (Cl-) combine to give potassium chloride, KCl
    Calcium ions (Ca2+) and bromide ions (Br-) combine to give calcium bromide, CaBr2
    Aluminum ions (Al3+) and sulfide ions (S2-) combine to give aluminum sulfide, Al2S3

  • naming ionic compounds from formulas
    1. name the anions
    2. name the cations
      • recall that the names of transition metal and main group cations must include their charge as a Roman numeral.
    3. the name of compound is cation name followed by anion name

    Na2S contains sodium ion and sulfide ion. The compound is sodium sulfide. SnCl4 contains a tin cation and four chloride ions. Each chloride carries a -1 charge, so the tin must have a +4 charge. The compound is tin(IV) chloride.

  • the formula weight* is the sum of atomic weights for atoms in the formula

    23.0 u
    + 16.0 u
    + 1.0 u
    40.0 u

    2×23.0 u
    + 12.0 u
    + 3×16.0 u
    106.0 u

    2× 14.0 u
    + 2×4×1.0 u
    + 32.0 u
    + 4×16.0 u
    132.0 u
Notes Notes
More about polyatomic ions

Polyatomic ions

  • definition: ions formed from more than one atom
  • examples: ammonium (NH4+), hydroxide (OH-), nitrate (NO3-), sulfate (SO4-)
  • polyatomic ions retain their identity within ionic compounds, and in many reactions
Table Table
common polyatomic ions by family
  • names, formulas and charges of common polyatomic ions should be memorized!
  • formulas for ionic compounds containing polyatomic ions are written as usual, except:
    • put parentheses around polyatomic ions whenever there are more than one
    • don't break up polyatomic ions (write Ca3(PO4)2, not Ca3P2O8)
Quiz Quiz
names and formulas of polyatomic ions

Structural formulas

  • definition: a map that shows how atoms are bonded within a molecule

    molecular formula structural formula molecular model
  • structural formulas of polyatomic ions are bracketed, with the charge indicated by a superscript

  • Sticks indicate shared electron pairs. There can be more than one pair shared, as in the C=O group in this acetic acid molecule.


Finding chemical formulas experimentally

  • to obtain a molecular formula from an empirical formula:
    1. you must know the molecular weight
    2. compute the ratio of molecular weight to formula weight
    3. multiply all subscripts in the empirical formula by this ratio
  • examples

    Give the molecular formula for a compound with empirical formula CH and molecular weight 78.11 daltons.

    Give the molecular formula for a compound with empirical formula CH2O and molecular weight 180 daltons.


  • release hydrogen ions when dissolved
  • leading H's in formula are acidic hydrogens
  • guidelines for naming acids
    1. name the anion within the acid
    2. change the anion ending to one of the following:
      anion ending in acid, replace with: examples
      *ide hydro*ic acid hydrochloric acid, HCl
      -ite -ous acid nitrous acid, HNO2
      -ate -ic acid sulfuric acid, H2SO4

Binary Covalent Compounds

  • naming binary covalent compounds
    1. write the name of the first nonmetal
    2. write the name of the second nonmetal with the ending changed to -ide
    3. insert prefixes into the name to reflect subscripts in the formula:
"Pentoxide" sounds a little better than "pentaoxide". Drop the "a" before an "o".
      mono- 1 hexa- 6
      di- 2 hepta- 7
      tri- 3 octa- 8
      tetra- 4 nona- 9
      penta- 5 deca- 10
    1. never start a name with mono-
  • examples
    N2S5 dinitrogen pentasulfide NO2 nitrogen dioxide
    S2Cl2 disulfur dichloride N2O4 dinitrogen tetroxide
    SF6 sulfur hexafluoride N2O5 dinitrogen pentoxides
  • common names should be used for the following compounds:
    formula common name
    H2O water
    H2O2 hydrogen peroxide
    H2S hydrogen sulfide
    N2O nitrous oxide
    NO nitric oxide
    NH3 ammonia
    N2H4 hydrazine

Addition Compounds

  • contain several compounds packed in regular way into a crystal
  • formulas of addition compounds
    • added compounds are separated by a dot
    • prefix compound with the number of times it occurs per formula unit
  • naming addition compounds
    • translate numbers after a dot into Greek prefixes
    • name each compound in order
    • cross out first redundant ion names
    • water in addition compounds is called hydrate
      • hydration waters can be driven off by strong heating
      • compound with all hydration waters driven off is called anhydrous
      • anhydrous salts usually absorb water from air to become hydrates again
  • examples
    Formula Name
    Na2SO4 · 10H2O sodium sulfate decahydrate
    CuSO4 · 5H2O copper(II) sulfate pentahydrate
    CaCO3 · MgCO3 calcium magnesium carbonate carbonate is named only once
    CaHPO4 · 2H2O calcium hydrogen phosphate dihydrate
    Ga2(SO4)3 · 18H2O gallium(III) sulfate 18-hydrate Use numbers instead of prefixes when the number is larger than 12
    ZnSO4 · (NH4)2SO4 ·6H2O zinc ammonium sulfate hexahydrate zinc is always +2, so zinc(II) is not necessary
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General Chemistry Online! Compounds

Copyright © 1997-2005 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 12/14/21.URL: http://antoine.frostburg.edu/chem/senese/101/compounds/index.shtml