Science 10 Unit 2

• In an ionic compound, positive charges must balance the negative charges.

e.g.: What is the formula for magnesium phosphide?

Magnesium is Mg2+ Phosphorous is P3-

Lowest common multiple of 2 and 3 is 6

3 Mg2+ ions and 2 P3– ions

Mg3P2

e.g.: What is the formula for calcium oxide?

Calcium is Ca2+ Oxygen is O2-

1 Ca2+ ion and 1 O2– ion

CaO

(remember, the subscript 1 is never written)

Mg2+ P3-

Mg2+ P3-

Mg2+

6+ 6-

Mg3P2

1. Write both symbols (+ion first) with ion charge

2. Criss-cross the charges and write them as subscripts

3. Reduce if possible

Mg O2+ 2-

(remember, the subscript 1 is never written)

Mg O22

MgO

Only use if you understand that the + and - charges must balance!

Magnesium oxide

Sometimes, metal ions have more than one combining capacity

Fe O2+ 2-

FeO

e.g. Cu2+ Cu1+

Fe3+ Fe2+

The name of the compound will include a roman numeral that tells you which combining capacity to use

Iron II oxidee.g. Iron III oxide

Fe O3+ 2-

Fe2O3

1+ I 2+ II 3+ III 4+ IV

5+ V 6+ VI 7+ VII

1. Write the name of the metal first.

2. Write the name of the non-metal second, and change the ending to “-ide”

3. ONLY IF THE METAL HAS 2 OR MORE ION CHARGES, Include a roman numeral to indicate which one it is.

e.g. NaCl

MgF2

CuCl2

Sodium chloride

Magnesium fluoride

copper has 2 ions: Cu 1+

Cu 2+

which one is it?

To find out which copper ion it is, look at the charge on the anion and balance the charges in reverse.

CuCl therefore must be copper I chloride

Cu Cl2+ 1-

CuCl2

Therefore it is copper II chloride

from periodic table

Cl 1-

2-

• Some groups of atoms act as individual ions

e.g. SO4-2 Sulphate ion -2

OH-1 Hydroxide ion -1

PO4-3 Phosphate ion -3

• When writing formulas and naming, pretend the polyatomic ion is just one atom and follow the rules as usual.

e.g. K+1 -3

K3PO4Potassium phosphate

Don’t change the endings of polyatomic ions

Ion Charge

PO4

Aluminum sulphate

Copper (II) sulphate

Al SO4+3 -2 Al2(SO4)3

Cu+2CuSO4

Put brackets around the SO4 to indicate 3 sulphate ions, not 43 oxygen atoms

SO4-2

Don’t forget the roman numeral since copper has more than one combining capacity