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Writing Chemical Formulas. General Chemistry Mrs. Amy Nare. http://chem.pdx.edu/~wamserc/C335W00/gifs/MW2.gif. Objectives. Distinguish between molecular and ionic compounds Contrast molecular formulas and formula units Use the periodic table to determine the charge on an ion

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Writing Chemical Formulas

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writing chemical formulas

Writing Chemical Formulas

General Chemistry

Mrs. Amy Nare


  • 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
chemical bonding
Chemical Bonding
  • 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-;


chemical bonding5
Chemical Bonding
  • 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

chemical bonding7
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

representing chemical compounds
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

identifying ionic charges
Identifying Ionic Charges
  • 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
identifying ionic charges10
Identifying Ionic Charges


Charge on cations corresponds to group #.

Charge on anions is found by subtracting 8 by group number

the number 8 is used b/c it represents # of valence e- in Noble gases

naming cations and anions
Naming Cations and Anions

Monatomic Ions

  • 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

naming cations and anions12
Naming Cations and Anions

Polyatomic Ions

  • 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!

naming cations and anions13
Potassium ion

Copper (II) ion

Chloride ion

Oxide ion




Nitrite ion

Hydroxide ion

Phosphate ion




Naming Cations and Anions


Write the name or symbol w/ charge:

binary ionic compounds
Binary Ionic Compounds
  • 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+ + Cl1- CaCl2

Na1+ + Cl1- NaCl

  • To name binary compounds – write name of cation first, then anion (-ide)

ex: CaCl2 = calcium chloride

Li2O = lithium oxide

ternary ionic compounds
Ternary Ionic Compounds
  • 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


ex: Na1+ + SO32- Na2SO3

Mg2+ + OH1- Mg(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

Ag2CrO4 = silver chromate

ionic compounds






Lithium sulfide

Iron (III) phosphide

Magnesium fluoride

Barium nitrate

Aluminum hydroxide

Potassium phosphate

Ionic Compounds


Write the name or the formula for the following compounds:

Practice making ionic compounds!

binary molecular compounds
Binary Molecular Compounds
  • Two nonmetals joined by covalent bonds
  • Use prefixes for naming

1 = mono-

2 = di-

3 = tri-

4 = tetra-

5 = penta-

6 = hexa-

7 = hepta-

8 = octa-

9 = nona-

10 = deca-

binary molecular compounds18
Binary Molecular Compounds
  • 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)

molecular compounds





tetraiodine nonoxide

sulfur hexafluoride

nitrogen trioxide

carbon tetrahydride

phosphorus trifluoride

Molecular Compounds


Write the name or formula for the following compounds:

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