3. Learn the rules for ionic compounds

How ionic compounds are formed

An ionic compound involves the transfer of electrons between a metal and non-metal.

1. Metals tend to lose electrons.
2. Non-metals tend to gain electrons.
3. The atom that loses electrons becomes a positively charged ion – called a cation.
4. The atom that gains electrons becomes a negatively charged ion – called an anion

Balance charges

It’s important to learn how to balance the charges in ionic compounds

1. Use the periodic table to determine the charge in an element
2. The overall charge in an ionic compound must be neutral
3. All the negative charges must cancel out all the positive charges in the compound

Ionic charges

Note:

All elements in the first column tend to lose 1 valence electrons and form ions with 1+ charge

All elements in the 2nd column tend to lose 2 valence electrons and form ions with 2+ charge

 

Concept check

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Rules for ionic compounds

1. First, determine the formula by balancing the charges
2. The cation is always named before the anion
3. The second element ends in “IDE”
4. If transition elements are involved, you must use Roman numerals for elements with more than one oxidation state.

Rules for transition metals

• In naming the transition metal oxides, add a Roman numeral in parenthesis straight after the name of the transition metal ion.
• Do not put a space in between.
• The Roman numeral must have the same value as the charge of the ion. Then add the name of the anion – “IDE” at the end.

Ionic charges for transition metals

Transition metals’ charges vary but the charges are always positive.

Iron (Fe) has possible charges of 2+, 3+, 4+, 5+ and 6+, but it commonly forms compounds with 2+ and 3+ charges.

You must balance the charges the same way as before.

Transition metals that do not require Roman numerals

Roman numerals are only used for transition metals that have more than one oxidation state. The transition metals below that do not need a roman numeral in the names of their compounds because they have one oxidation state:

Silver, Ag

Zinc, Zn

Cadmium, Cd

Concept Check

 

To find the roman numeral for the element, first refer to the periodic table.

• For example, Cu, can have a charge of 1+ or 2+
• Sulfur will always have a charge of 2-
• The table below shows that 2 Cu ions will balance the charge of 1 S ion.
• Therefore, the Roman numeral (I) must be used to show the charge of the copper ion.

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Concept check 1

Concept check 2

Polyatomic ions