When Ions Combine

positive 1 (+1) positive 2 (+2) positive 3 (+3)

hydrogen H+1 calcium Ca+2 aluminium Al+3

sodium Na+1 magnesium Mg+2 iron III Fe+3

potassium K+1 barium Ba+2 chromium III Cr+3

copper I Cu+1 copper II Cu+2

silver Ag+1 iron II Fe+2

ammonium NH4+

negative 1 (-1) negative 2 (-2) negative 3 (-3)

chloride Cl- oxide O-2 nitride N-3

fluoride F- sulfide S-2 phosphide P-3

bromide Br- carbonate CO3-2 phosphate PO4

-3

iodide I- sulfate SO4-2

hydroxide OH-

nitrate NO3-

ethanoate (or acetate) CH3COO-

hydrogen carbonate (or bicarbonate)

HCO3-

If you give me your spare electron then you’ll see

how attractive I am.

Phwar… You’re right. I

am very strongly

attracted to you, baby.

When a non-metal atom takes an electron from a metal atom two

ions will form.

They will be oppositely charged and attract each other.

Whenever a positive ion forms there must also be a negative ion formed as well. In other

words a metal atom will only lose its electrons if a non-metal atom is taking them. Electrons don’t just exist by themselves,

they must be associated with an atom.

The law of charges says that differently charged objects will

attract.

Which of the following pairs will attract each other (ie which of the following pairs have different charges?)

(a) a proton & an electron(b) two protons(c) two electrons(d) two sodium ions(e) an electron & a chloride ion(f) a magnesium ion and an oxygen atom(g) a magnesium ion and an oxygen ion(h) an aluminium ion and a sulfate ion.

When oppositely charged ions attract each other the total charge will always equal zero.

eg each Ca+2 ion will attract one O-2 ion. ie (+2) + (-2) = 0

and each Ca+2 ion will attract two Cl-1 ions ie (+2) + (2x(-1)) = 0

When oppositely charged ions attract, then they are held together to form ionic compounds.

We can represent these compounds with ionic formulae. The compounds in the example above are:

CaO and Ca Cl2.

How to write ionic formulae.1. The first name is the (+) ion and the second name is the (-) ion. eg: sodium bromide2. Look up and write the symbols for the ions.

Na+1 Br-1

3. The sum of the positive and negative oxidation numbers must be zero. ie: +1-1=04. If it is then write the two symbols together without the charges. ie: NaBr5. If it is not, add more positive or more negative ions until it is.6. Use a subscript number to indicate more than one ion. eg: Na2S, CaCl2.

Examples.

Name of compound

Positive ion symbol Negative ion symbol

Add up charges to see if they add to zero

Write the formula. Use subscripts to indicate more than one ion.

potassium chloride

Examples.

Name of compound




potassium chloride

K+1 Cl-1

Examples.

Name of compound




potassium chloride

K+1 Cl-1

+1-1=0

Examples.

Name of compound




potassium chloride

K+1 Cl-1

+1-1=0

KCl

Examples.

Name of compound




potassium chloride

K+1 Cl-1

+1-1=0

KCl

calcium fluoride

Examples.

Name of compound




potassium chloride

K+1 Cl-1

+1-1=0

KCl

calcium fluoride

Ca+2 F-1

Examples.

Name of compound




potassium chloride

K+1 Cl-1

+1-1=0

KCl

calcium fluoride

Ca+2 F-1

+2-1=1

Examples.

Name of compound




potassium chloride

K+1 Cl-1

+1-1=0

KCl

calcium fluoride

Ca+2 F-1

+2-1-1=0

Examples.

Name of compound




potassium chloride

K+1 Cl-1

+1-1=0

KCl

calcium fluoride

Ca+2 F-1

+2-1-1=0

CaF2

Problems. Try writing these formula yourself:

sodium fluoride

calcium chloride

aluminium bromide

What happens when radical ions are involved in an ionic compound?

The rules are the same for writing formula for compounds containing radical ions as for those containing monatomic ions. The only difference is when you need more than one radical to balance the charge of the other ion.

What happens when radical ions are involved in an ionic compound?

For example if calcium ions combined with nitrate ions:

The calcium ion (Ca+2) would need two nitrate radical ions (NO3

-1) to balance its charge.

How do we represent this compound?

If we wrote CaNO32 we would be wrong. That formula says that for every calcium

ion there are one nitrogen and 32 oxygen atoms!!!

There are no nitrate ions (ie NO3-1) in the

formula CaNO32.

How do we represent this compound?

We want the formula to represent one calcium ion for every two nitrate ions. So we write:

Ca(NO3)2

The two outside the brackets means that there is two of everything inside it.

For example:

potassium nitrate

For example:

potassium nitrate

K+1 NO3-1

For example:

potassium nitrate

K+1 NO3-1

+1-1=0

For example:

potassium nitrate

K+1 NO3-1

+1-1=0

KNO3

For example:

potassium nitrate

K+1 NO3-1

+1-1=0

KNO3

calcium hydroxide

For example:

potassium nitrate

K+1 NO3-1

+1-1=0

KNO3

calcium hydroxide

Ca+2 OH-1

For example:

potassium nitrate

K+1 NO3-1

+1-1=0

KNO3

calcium hydroxide

Ca+2 OH-1

+2-1=1

For example:

potassium nitrate

K+1 NO3-1

+1-1=0

KNO3

calcium hydroxide

Ca+2 OH-1

+2-1-1=0

For example:

potassium nitrate

K+1 NO3-1

+1-1=0

KNO3

calcium hydroxide

Ca+2 OH-1

+2-1-1=0

Ca(OH)2

Try writing these formula yourself

ammonium chloride

sodium hydroxide


magnesium sulfate

copper II ethanoate


iron III carbonate

sodium hydrogen carbonate


barium phosphate

tin IV phosphate

magnesium oxide

Mg2+ O2-

+2-2=0

MgO

sodium sulfide

Na1+ S2-

+1+1-2=0

Na2S

aluminium phosphide

Al3+ P3-

+3-3=0

AlP

iron III oxide

Fe3+ O2-

+3+3-2-2-2=0

Fe2O3

copper I chloride

Cu1+ Cl1-

+1-1=0

CuCl

copper II chloride

Cu2+ Cl1-

+2-1-1=0

CuCl2

chromium III sulfide

Cr3+ S2-

+3+3-2-2-2=0

Cr2S3

Cl-1 Br-1 O-2 S-2 P-3 N-3 OH-1 SO4-2

Na+1 NaCl NaBr Na2O Na2S Na3P Na3N NaOH Na2SO4

K+1 KCl KBr K2O K2S K3P K3N KOH K2SO4

Ag+1 AgCl AgBr Ag2O Ag2S Ag3P Ag3N AgOH Ag2SO4

Fe+2 FeCl2 FeBr2 FeO FeS Fe3P2 Fe3N2 Fe(OH)2 FeSO4

Sn+2 SnCl2 SnBr2 SnO SnS Sn3P2 Sn3N2 Sn(OH)2 SnSO4

Ba+2 BaCl2 BaBr2 BaO BaS Ba3P2 Ba3N2 Ba(OH)2 BaSO4

Fe+3 FeCl3 FeBr3 Fe2O3 Fe2S3 FeP FeN Fe(OH)3 Fe2(SO4)3

Cr+3 CrCl3 CrBr3 Cr2O3 Cr2S3 CrP CrN Cr(OH)3 Cr2(SO4)3

When Ions Combine

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