Single atoms of an element are at relatively high potential energy

• When atoms bond with others, resulting in electron sharing or electron transfer, this energy is lowered.

• This is why it is more likely to find calcium carbonate or calcium chloride than pure calcium metal in nature.

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Writing Formulas for Ionic Compounds

• The positive ion comes FIRST.• Write the charge of each ion over each symbol to

help you decide subscripts.• The total charges must add to 0.• If the charges of the negative ion and positive ion

are equal, this is the complete formula.

K Cl KCl = potassium chloride

Ca SO4 CaSO4 = calcium sulfate3

+1 -1

+2 -2

Writing the formula for an ionic compound

• One type of positive ion (cation) is combined with one type of negative ion (anion).

• The total positive charge from the ions must equal the total negative charge from the ions.

Example: What is the formula for a compound made with strontium and bromine ions?

4

Sr2+ Br -1

Charges do not cancel with only one of each atom

Sr2+

Br -1

Br -1 +2 is now balanced by 2 x(-1)

Formula is SrBr2

Example 2: Ionic compound formula

What is the formula for a compound made with strontium and phosphorous ions?

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Sr2+ P -3

Charges do not cancel with only one of each atom

The common factor is 6. Sr2+

+6 [3x(+2)] is now balanced by -6 [2x(-3)]

Formula is Sr3P2

Sr2+

Sr2+

P -3

P -3

Writing Formulas When Ion Charge is Unequal

• The total charges must add to 0. The criss-cross method may be used to accomplish this:

Mg Cl MgCl2

• Use subscripts to indicate how many ions are needed to make the compound.

• Use parentheses for polyatomics if there is more than one.

Mg NO3 Mg(NO3 )26

+2 -1

+2 -1

Charges: Determine from Periodic Table Group

+1 ☺

H +2 +3

-3 -2 -1 He

Li Be B C N O F Ne

Na Mg

Transition Metals (Several + charges possible) Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn

Fe Co Ni

Cu Zn(+2)

Ga

Ge As Se Br Kr

Rb Sr Ag(+1)

Cd(+2)

In Sn Sb Te I Xe

Cs Ba Tl Pb At Rn

Fr Ra

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Naming CompoundsBinary Ionic Compounds• Cation (+ charge ) is listed first.• Name of cation is the chemical name.

– For many transition metals, the ion is distinguished by the addition of a roman numeral after the chemical name.

– Find charge of the anion and choose appropriate roman numeral to balance the charge.

• Name of the anion ends in –ide.Examples:• Al2O3 aluminum oxide• CuBr2 copper(II) bromide

Binary compound examples

Write formula for potassium sulfide:

K+1 + S2- K+1 + S2- K2S

( 2 x +1 ) = +2 charge for K and (1 x -2) = -2 charge for S

Write formula for aluminum sulfide:

Al+3 + S2- Al+3 + S2- Al2S3

( 2 x +3 ) = +6 charge for Al and (3 x -2) = -6 charge for S

Write formula for magnesium sulfide:

Mg+2 + S2- +2 and -2 cancel MgS 9

Naming Compounds with Transition Metals• The positive (+) charge of the metal is represented by a roman

numeral in parentheses after the name.

(I) = +1 (II) = +2 (III) = +3 (IV) = +4

Cu2+ and Cl- copper(II)chloride, or CuCl2.

Iron(III) chloride = FeCl3, in which Fe has a 3+ charge

To find the charge when you are given the formula, find the number needed to cancel out the negative charge in the compound.

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To find the charge when you are given the formula, find the number needed to cancel out the negative charge in the compound. Ex. 1. What is lead’s charge in PbCl2? 

? -1Pb | Cl2

totals | 2 x -1 = -2+2 | -2 These must add to zero. Pb has +2 charge.

 The name of this compound is lead(II) chloride. 

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 Example 2: What is the charge of copper in CuSO4?

SO4 is the formula for the polyatomic ion, SO42-.

There is one SO42- ion, so +2 is needed to cancel the -2 charge.

Therefore the one copper atom has a +2 charge.

The name of this compound is copper(II) sulfate.

Example 3: Name Cr (NO3)3 chromium (?) nitrate+? | -1 x 3+? | -3 the + charge must be 3 to cancel

The compound name is chromium(III) nitrate.

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Diatomics• Di = two• Atomic = atoms• To achieve stability of 8

electrons in the outer shell of each atoms, these elements exist in pairs.

• Also kn0wn as the “Magnificent 7,” or BrINClHOF.

N O F

Cl

Br

I

H

N2

Br2

I2

Cl2

O2H2

F2

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Naming Molecular CompoundsBinary Molecular Compounds• If more than one atom, name the first element with a

numerical prefix. – The first element is the farthest to left on periodic table.– If elements are in same group, name the lower one first.

• Name the second element with a numerical prefix and a suffix –ide.

• For prefixes, drop o or a if the element name begins with a vowel. (Examples are monoxide and pentoxide.)

• Exceptions: Do not use mono- for first element. Prefix is not needed for H as first element.Examples:N2O4 dinitrogen tetroxide OF2 oxygen difluoride

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Numerical Prefixes: Use for Molecular compounds only!

Number Prefix

1 mono-

2 di-

3 tri-

4 tetra-

5 penta-

6 hexa-

7 hepta-

8 octa-

9 nona-

10 deca-

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Polyatomic ions• Poly = many• Atomic = atoms• Entire group of atoms is an ion with a

positive or negative charge.• Within the polyatomic ion, atoms are

bound covalently.• Examples:

Sulfate ionSO4

2-

S

Carbonate ionCO3 2-

C

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Oxyanions: Polyatomics Containing Oxygen

• -ate: The most common form (ClO3-)

• -ite: One less oxygen atom (ClO2-)

• Per-: One more oxygen atom (ClO4-)

• Hypo-: One less than –ite ion (ClO-)

• Recall: Anion has negative charge. Anion of Cl is chloride.

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Naming Acids

• Definition of Acid (for now): H+ donor in H2O

• H is the cation in acids (again for now).• Need enough H+ ions to balance the charge of the anion.

Anion Acid Example

-ide hydro____ic HCl = hydrochloric acid

-ate _________ic H2SO4 = sulfuric acid

-ite _________ous HClO2 = chlorous acid

HClO = hypochlorous acid

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Naming Acids

• Binary Acid = 2 elements (HCl)– Begin with hydro-.– Use name of 2nd element and end with –ic.– HCl is hydrochloric acid.

• Oxyacid = H + O + 3rd element (H2SO4)

– Usually incorporates the polyatomic ion name into the acid name.

– Written with H first, then the polyatomic ion.

– H2SO4 is sulfuric acid.