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Chemical FormulasChemical Formulas

Writing Binary CompoundsTim Bass

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Chemical Formulas

A shorthand method of representing the makeup of chemicals using chemical symbols and oxidation numbers. Positive Ions always go 1st in the formula. Negative Ions always go last.

Ionic Bonded chemical formulas are always written as empirical formulas. Empirical Formula: The smallest whole

number ratio of elements in a compound.

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Golden Rule For Formulas:

The sum of the oxidation #s = zero!!

Mg+2 Cl-1

K+1 O-2

Sum of the oxidation numbers always equal zero!!!!2

+2 + -2 = 0

2

+2 + -2 = 0

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Charged Particles

Ion: a particle with an electrical charge. Cation: a positively charged ion. Anion: a negatively charged ion.

Cation always goes 1st in the formula. Anion always goes last in the formula.

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Oxidation Number

Oxidation Number: The apparent charge on an atom. The charge on an ion. Also called the valence.

Example: Ca+2 Cl-1 Calcium:

20P20N18 e-

Chlorine:17p18N18e-

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Noble Gas

The greater the stability the less active a substance becomes.

The noble gasses are the least active substances (therefore are very stable and low energy). When the outer shells of an element are

full the atom is at a much lower energy level.

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Shorthand Notation

10Ne: 1s2 2s2 2p6

• (8 valence electrons)

18Ar:1s2 2s2 2p6 3s2 3p6

• (8 valence electrons)

36Kr:1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6

• (8 valence electrons)

All noble gases (except He) have 8 valence electrons.

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Noble Gas (Octet) Law

Noble Gas Law: When forming compounds, elements tend to gain or lose electrons until they have an electron configuration similar to one of the noble gases. Metals tend to lose electrons (become

cations) Nonmetals tend to gain electrons

(become anions)

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Metals and Nonmetals

Metals

Nonmetals

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Oxidation Examples

Na + Cl - Na+1Cl-1

Mg + Cl Mg+2Cl-1

Al + Cl Al+3Cl-1

Mg + 2Cl Mg+2Cl2-1

Al + 3Cl Al+3Cl3-1

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Oxidation Numbers

The sum of the oxidation numbers in a chemical formula must equal zero.

This is the golden rule of writing formulas for inorganic compounds!There is only one category that does not follow this rule. They follow the rule also, but the rules change a little to apply to them.

Nonmetals bonded to nonmetals.

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SubscriptsSubscript: Small numbers to the

lower right of a symbol. Represent the number of atoms of that

element in the compound. Affects only the element immediately in

front of the subscript.Coefficient: Large number in front of

a chemical formula. Affects everything following the coefficient.

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H2OHydrogen:Oxidation # X Subscript

+ 1 X 2 = + 2Oxygen:Oxidation # X Subscript

- 2 X 1 = - 2 Sum of oxidation numbers = 0

( + 2 ) + ( - 2 ) = 0

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A ShortcutA Shortcut

M a k e t h e o x i d a t i o n o f o n ee l e m e n t t h e s u b s c r i p t o ft h e o t h e r e l e m e n t .

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A ShortcutA ShortcutThere is one problem!

These subscripts will reduce. Remember: Ionic bonded compounds are

always written as an empirical formula.

C a O+ 2 - 2

C a 2 O 2

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Polyatomic Ion

An ion made up of more than one atom that behaves like one atom.

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Polyatomic Ion

Never change subscripts within a polyatomic ion. (Changes the polyatomic ion.)

If more than one polyatomic ion is needed, put parenthesis around the ion and add the subscript needed outside the parenthesis. NH4OH Ammonium Hydroxide (NH4)2O Ammonium Oxide Al(MnO4)3 Aluminum Permanganate

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Polyatomic Ion

The sum of the oxidation numbers in an ion is equal to the charge on the ion.

You must learn the names, formulas, and charges of the most common ions. MnO4

-1 Permanganate OH-1 Hydroxide CN-1 Cyanide NH4

+1 Ammonium

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Warnings

1. Once the sum of the oxidation #s = 0, do not change the formula.

2. Parenthesis are only for polyatomic ions, and only if they are needed two times or more in a chemical formula.

3. Always make sure that the final chemical formula is an empirical formula!

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Chemical FormulasChemical Formulas

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