Chemical Formulas: Chemical Formulas: The BasicsThe Basics

Subscripts in Chemical FormulasSubscripts in Chemical Formulas

Numbers inside formulas.Numbers inside formulas. Smaller than the font size & lowered below Smaller than the font size & lowered below

the line of type.the line of type. Subscripts show the number of atoms of Subscripts show the number of atoms of

each element in a each element in a representative unitrepresentative unit..

Rules for Subscripts in FormulasRules for Subscripts in Formulas No subscript – it’s understood to be 1.No subscript – it’s understood to be 1.

Subscripts refer to the element immediately Subscripts refer to the element immediately preceding them. preceding them. Remember – every element symbol starts Remember – every element symbol starts with an upper case letter.with an upper case letter.

Subscripts after parentheses refer to Subscripts after parentheses refer to everything inside ( ).everything inside ( ).

Chemical FormulasChemical Formulas

HH22O has 2 hydrogens and 1 oxygenO has 2 hydrogens and 1 oxygen

NHNH33 has 1 nitrogen and 3 hydrogens has 1 nitrogen and 3 hydrogens

CHCH44 has ? has ?

COCO22 has ? has ?

AgNOAgNO33 has ? has ?

Al(NOAl(NO33))33 has ? has ?

1 carbon and 4 hydrogens1 carbon and 2 oxygens

1 silver, 1 nitrogen, 3 oxygens1 aluminum, 3 nitrogens, and 9 oxygens

No. of atoms in formulasNo. of atoms in formulas

Al(NOAl(NO33))33

Al =Al = N =N = O =O = Total =Total =

Subscript outside ( ) X subscript inside ( ) =# of atoms

Hint: Look for uppercase letters!

11

1 X 3 = 31 X 3 = 3

3 X 3 = 93 X 3 = 9

1313

No. of atoms in formulasNo. of atoms in formulas

MgMg33(PO(PO44))22

Mg =Mg = P = P = O = O = Total =Total =

332 X 1 = 22 X 1 = 2

4 X 2 = 84 X 2 = 8

1313

CoefficientsCoefficients

Numbers in front of formulasNumbers in front of formulas

Refer to Refer to everythingeverything in the formula in the formula immediately followingimmediately following

No coefficient? It’s understood to be 1.No coefficient? It’s understood to be 1.

CoefficientsCoefficients

2H2H22O means 2(HO means 2(H22O)O)

To count atoms take coefficient X To count atoms take coefficient X subscriptsubscript

2 X 2 = 4 H’s2 X 2 = 4 H’s 2 X 1 = 2 O’s2 X 1 = 2 O’s

HydratesHydrates

A group of salts that have water A group of salts that have water molecules stuffed in the empty spacesmolecules stuffed in the empty spaces

Formulas are distinctiveFormulas are distinctive Ex: CuSOEx: CuSO445H5H22OO

means “is associated with” or PLUSmeans “is associated with” or PLUS Not a true chemical bond, but the structure is Not a true chemical bond, but the structure is

definite so the number of empty spaces is definite so the number of empty spaces is definite and the number of water molecules is definite and the number of water molecules is definitedefinite

CuSOCuSO445H5H22OO

Count up the atoms!Count up the atoms!

1 X Cu1 X S4 X O10 X H5 X O

55[[CuSOCuSO445H5H22OO]]

Count up the atoms!Count up the atoms!

5 X Cu5 X S20 X O50 X H25 X O

Coefficients X Subscripts

Representative UnitRepresentative Unit

Depends on the structure of the Depends on the structure of the substance.substance.

For covalent substances, the For covalent substances, the representative unit is the molecule.representative unit is the molecule.

For ionic substances, which make a For ionic substances, which make a crystal lattice, the representative unit is the crystal lattice, the representative unit is the unit cell.unit cell.

Covalently bonded substances make molecules.

If you have more raw materials, you make more molecules.

There is one Na+ for every Cl-.They are arranged in a repeating pattern.

NaCl is an ionic compound.

The ions are arranged in a crystal lattice.

The more raw materials you have, the larger the crystal.

It is not useful to have a “molecular” formula for ionic compounds – every crystal in the world is different.

Use empirical formulas instead – give smallest whole number ratio.

This diagram shows the relative sizes of the two ions.

When all the ions are in thecorrect location, you can get a large “single crystal.”

This model shows the position of the ions but not the relative sizes.

Covalent vs. IonicCovalent vs. Ionic

All Non-metalsAll Non-metals MoleculesMolecules Definite sizeDefinite size Molecular formula Molecular formula

gives exact gives exact composition of composition of moleculemolecule

Metal & NonmetalMetal & Nonmetal Crystal LatticeCrystal Lattice Indefinite sizeIndefinite size Empirical formula Empirical formula

gives smallest whole gives smallest whole number ratio of number ratio of elementselements