Writing the Formula of Inorganic Salts (binary ionic compounds) Chemistry Tutorial

Key Concepts

Inorganic salts are simple binary⁽¹⁾ inorganic ionic compounds.

Inorganic salts are composed of two ions:
⚛ a cation (positively charged ion, M^x+)

⚛ an anion (negatively charged ion, M^y-)

Cations can be composed of
⚛ one atom (monoatomic⁽²⁾ cation)

⚛ more than one atom (polyatomic cation)

Anions can be composed of
⚛ one atom (monoatomic anion)

⚛ more than one atom (polyatomic anion)

The formula of an inorganic salt is composed of two parts:
(i) formula of the cation, but without the charge, is written first
M is written not M^x+

(ii) formula of the anion, but without the charge, is written last

X is written not X^y-

The formula of an inorganic salt represents the smallest whole number ratio of ions present.⁽³⁾
formula M_aX_b
where a and b are whole numbers

not M_2aX_2b and not M_½aX_½b

A subscript number⁽⁴⁾ written to the right of an ion's symbol tells us how many of those ions are present in the formula of the salt.
For the inorganic salt M_aX_b:
number of positive ions of M = a

number of negative ions of X = b

If there is only one cation present, no number is written to the right of the cation formula in the salt formula.
If there is only one anion present, no number is written to the right of the anion formula in the salt formula.

For example, if one M^x+ cation and b lots of X^y- anions make up an inorganic salt, then the formula of the salt is
MX_b NOT M₁X_b

If more than one polyatomic ion (ion having more that one atom in its formula) is present:
(i) parentheses or round brackets enclose the formula of the polyatomic ion
(ii) a subscript number is written to the right of the final bracket to tell us how many of that polyatomic ion are present in the formula

Example of salt composed of a ions of M^x+ and b ions of the polyatomic anion XO₃^y- :

M_a(XO₃)_b NOT M_aXO_3b

The net charge on an ionic compound is zero, so the sum of the positive charges added to the sum of the negative charges is zero:
For a salt composed of a lots of M^x+ cations and b lots of X^y- anions:
(a × x+) + (b × y-) = 0

or, put another way:

(a × x+) = -(b × y-)

You will need to know the name and formula of some monoatomic ions and some polyatomic ions as shown in the table below:

Type of Ion	Charge on ion	Monoatomic example	Polyatomic example
Cations	1+	hydrogen, H⁺ Group 1 metals lithium: Li⁺, sodium: Na⁺, potassium: K⁺, rubidium: Rb⁺, caesium⁽⁵⁾: Cs⁺ Group 11 metals silver: Ag⁺ copper(1+) or copper(I): Cu⁺	ammonium⁽⁶⁾: NH₄⁺
	2+	Group 2 metals magnesium: Mg²⁺, calcium: Ca²⁺, strontium: Sr²⁺, barium: Ba²⁺ Other metals iron(2+) or iron(II): Fe²⁺ copper(2+) or copper(II): Cu²⁺ zinc: Zn²⁺ mercury(2+) or mercury(II): Hg²⁺ lead(2+) or lead(II): Pb²⁺ tin(2+) or tin(II): Sn²⁺
	3+	Group 13 metals aluminium: Al³⁺ Other metals iron(3+) or iron(III): Fe³⁺
	4+	Group 14 Metals tin(4+) or tin(IV): Sn⁴⁺ lead(4+) or lead(IV): Pb⁴⁺
Anions	1-	hydride: H^- Group 17 non-metals fluoride: F^-, chloride: Cl^-, bromide: Br^-, iodide: I^-	hydroxide⁽⁷⁾: OH^- nitrate: NO₃^- nitrite: NO₂^- hydrogencarbonate: HCO₃^-
	2-	Group 16 non-metals oxide: O^2- sulfide: S^2-	carbonate: CO₃^2- sulfate: SO₄^2- sulfite: SO₃^2-
	3-	Group 15 non-metals nitride: N^3- phosphide: P^3-	phosphate: PO₄^3-

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Steps for Writing the Formula of a Inorganic Salt Given Its Name

The name of a salt tells us:

the name of the cation (first part of the salt's name)

the name of the anion (last part of the salt's name)

If a monoatomic cation is present, the name of the cation also includes its charge number in parentheses after the name of the metal, unless it is one of the following cations:

hydrogen cation: charge number is always 1+

group 1 cation: charge number is always 1+

silver cation: charge number is always 1+

group 2 cation: charge number is always 2+

zinc cation: charge number is always 2+

aluminium cation: charge number is always 3+

So, we can use the information given in the name of a salt in order to write the formula for the salt.

Break the name of the salt up into the name of the cation, and, the name of the anion:

Salt

Names: cation anion

Use the cation's name to determine the formula for the cation
M⁺ : Group 1 metal, hydrogen, silver, ammonium
M²⁺ : Group 2 metal, zinc
M³⁺ : aluminium
Otherwise charge on ion is given in parentheses after name

Salt

Names: cation anion

Formulae: M^x+

Use the anion's name to determine the formula for the anion
X^- : Group 17 non-metal, hydroxide, nitrate, nitrite, hydrogencarbonate
X^2- : Group 16 non-metal, carbonate, sulfate, sulfite
X^3- : Group 15 non-metal, phosphate

Salt

Names: cation anion

Formulae: M^x+ X^y-

	Salt
Names:	cation		anion

	Salt
Names:	cation	anion
Formulae:	M^x+

	Salt
Names:	cation	anion
Formulae:	M^x+	X^y-

Determine the ratio of cations to anions in the formula of the salt:

	Salt
Names:	cation		anion
Formulae:	M^x+		X^y-
Ratio of ions:	y	:	x

Note that (y × x+) + (x × y-) = 0

or, (y × x+) = -(x × y-)

Determine the lowest whole number ratio of cations to anions:

(i) If y = x then the lowest whole number ratio of cations to anions is 1:1

(ii) If y ≠ x then

(a) If y = 1 and/or x = 1 you already have the lowest whole number ratio of cations to anions, M:X is y:x
(b) Otherwise divide y by x AND divide x by y

(I) If (y ÷ x) and (x ÷ y) are both fractions, then the lowest whole number ratio of M:X is y:x
(II) If (y ÷ x) = whole number greater than 1 OR (x ÷ y) = a whole number greater than 1
then divide both x and y by the smallest number (which is either x or y)

	Salt
Names:	cation		anion
Formulae:	M^x+		X^y-
Ratio of ions:	y	:	x
Lowest whole number ratio :	a	:	b

Remove the charges from the ions to give the formula for the ions as they will be represented in the formula of the salt:

	Salt
Names:	cation		anion
Formulae:	M^x+		X^y-
Ratio of ions:	y	:	x
Lowest whole number ratio :	a	:	b
Ions as represented in salt formula :	M		X

Construct a formula for the salt:

(i) If a = b, ratio of M to X is 1:1 and the salt's general formula is MX

(ii) If a ≠ b, ratio of M to X is a:b and the salt's general formula is M_aX_b

(a) If a = 1, general salt formula is MX_b
(b) If b = 1, general salt formula is M_aX
(c) If more than one of a specific polyatomc ion is present in the salt's formula, then the formula of that polyatomic ion is enclosed in parentheses and the number of polyatomic ions written as a subscript to the right of the parentheses.

	Salt
Names:	cation		anion
Formulae:	M^x+		X^y-
Ratio of ions:	y	:	x
Lowest whole number ratio :	a	:	b
Ions as represented in salt formula :	M		X
Provisional salt formula:	M_a		X_b

Write the formula for the salt: M_aX_b

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Worked Examples of Writing the Formula for an Inorganic Salt: Monoatomic Ions

Question 1: Write the formula for sodium chloride

Solution:

Break the name of the salt up into the name of the cation, and, the name of the anion:

Salt

cation anion

Names: sodium chloride

	Salt
	cation	anion
Names:	sodium	chloride

Use the cation's name to determine the formula for the cation

M⁺ : sodium(1+), Na⁺, is a Group 1 metal, hydrogen, silver, ammonium
M²⁺ : Group 2 metal, zinc
M³⁺ : aluminium
Otherwise charge on ion is given in parentheses after name

	Salt
	cation	anion
Names:	sodium	chloride
Formulae:	Na⁺

Use the anion's name to determine the formula for the anion

X^- : chloride, Cl^-, is a Group 17 non-metal, hydroxide, nitrate, nitrite, hydrogencarbonate
X^2- : Group 16 non-metal, carbonate, sulfate, sulfite
X^3- : Group 15 non-metal, phosphate

	Salt
	cation	anion
Names:	sodium	chloride
Formulae:	Na⁺	Cl^-

Determine the ratio of cations to anions in the formula of the salt:

	Salt
	cation		anion
Names:	sodium		chloride
Formulae:	Na⁺		Cl^-
Ratio of ions:	1	:	1

Note that (1 × +) + (1 × -) = 0

or, (1 × +) = -(1 × -)

Determine the lowest whole number ratio of cations to anions:

(i) If y = x then the lowest whole number ratio of cations to anions is 1:1

(ii) If y ≠ x then

(a) If y = 1 and/or x = 1 you already have the lowest whole number ratio of cations to anions, M:X is y:x
(b) Otherwise divide y by x AND divide x by y

(I) If (y ÷ x) and (x ÷ y) are both fractions, then the lowest whole number ratio of M:X is y:x
(II) If (y ÷ x) = whole number greater than 1 OR (x ÷ y) = a whole number greater than 1
then divide both x and y by the smallest number (which is either x or y)

	Salt
	cation		anion
Names:	sodium		chloride
Formulae:	Na⁺		Cl^-
Ratio of ions:	1	:	1
Lowest whole number ratio :	1	:	1

Remove the charges from the ions to give the formula for the ions as they will be represented in the formula of the salt:

	Salt
	cation		anion
Names:	sodium		chloride
Formulae:	Na⁺		Cl^-
Ratio of ions:	1	:	1
Lowest whole number ratio :	1	:	1
Ions as represented in salt formula :	Na		Cl

Construct a formula for the salt:

(i) If a = b, ratio of M to X is 1:1 and the salt's general formula is MX

(ii) If a ≠ b, ratio of M to X is a:b and the salt's general formula is M_aX_b

(a) If a = 1, general salt formula is MX_b
(b) If b = 1, general salt formula is M_aX
(c) If more than one of a specific polyatomc ion is present in the salt's formula, then the formula of that polyatomic ion is enclosed in parentheses and the number of polyatomic ions written as a subscript to the right of the parentheses.

	Salt
	cation		anion
Names:	sodium		chloride
Formulae:	Na⁺		Cl^-
Ratio of ions:	1	:	1
Lowest whole number ratio :	1	:	1
Ions as represented in salt formula :	Na		Cl
Provisional salt formula:	Na		Cl

Write the formula for the salt:
The formula for sodium chloride is NaCl

Question 2: Write the formula for magnesium bromide.

Solution :

Break the name of the salt up into the name of the cation, and, the name of the anion:

Salt

cation anion

Names: magnesium bromide

	Salt
	cation	anion
Names:	magnesium	bromide

Use the cation's name to determine the formula for the cation

M⁺ : Group 1 metal, hydrogen, silver, ammonium
M²⁺ : magnesium, Mg²⁺, is a Group 2 metal, zinc
M³⁺ : aluminium
Otherwise charge on ion is given in parentheses after name

	Salt
	cation	anion
Names:	magnesium	bromide
Formulae:	Mg²⁺

Use the anion's name to determine the formula for the anion

X^- : bromide, Br^-, is a Group 17 non-metal, hydroxide, nitrate, nitrite, hydrogencarbonate
X^2- : Group 16 non-metal, carbonate, sulfate, sulfite
X^3- : Group 15 non-metal, phosphate

	Salt
	cation	anion
Names:	magnesium	bromide
Formulae:	Mg²⁺	Br^-

Determine the ratio of cations to anions in the formula of the salt:

	Salt
	cation		anion
Names:	magnesium		bromide
Formulae:	Mg²⁺		Br^-
Ratio of ions:	1	:	2

Note that (1 × 2+) + (2 × 1-) = 0

or, (1 × 2+) = -(2 × 1-)

Determine the lowest whole number ratio of cations to anions:

(i) If y = x then the lowest whole number ratio of cations to anions is 1:1

(ii) If y ≠ x, that is, 1 ≠ 2 then

(a) If y = 1 and/or x = 1 you already have the lowest whole number ratio of cations to anions, M:X is 1:2
(b) Otherwise divide y by x AND divide x by y

(I) If (y ÷ x) and (x ÷ y) are both fractions, then the lowest whole number ratio of M:X is y:x
(II) If (y ÷ x) = whole number greater than 1 OR (x ÷ y) = a whole number greater than 1
then divide both x and y by the smallest number (which is either x or y)

	Salt
	cation		anion
Names:	magnesium		bromide
Formulae:	Mg²⁺		Br^-
Ratio of ions:	1	:	2
Lowest whole number ratio :	1	:	2

Remove the charges from the ions to give the formula for the ions as they will be represented in the formula of the salt:

	Salt
	cation		anion
Names:	magnesium		bromide
Formulae:	Mg²⁺		Br^-
Ratio of ions:	1	:	2
Lowest whole number ratio :	1	:	2
Ions as represented in salt formula :	Mg		Br

Construct a formula for the salt:

(i) If a = b, ratio of M to X is 1:1 and the salt's general formula is MX

(ii) If a ≠ b, ratio of M to X is a:b and the salt's general formula is M_aX_b

(a) If a = 1, general salt formula is MX_b
(b) If b = 1, general salt formula is M_aX
(c) If more than one of a specific polyatomc ion is present in the salt's formula, then the formula of that polyatomic ion is enclosed in parentheses and the number of polyatomic ions written as a subscript to the right of the parentheses.

	Salt
	cation		anion
Names:	magnesium		bromide
Formulae:	Mg²⁺		Br^-
Ratio of ions:	1	:	2
Lowest whole number ratio :	1	:	2
Ions as represented in salt formula :	Mg		Br
Provisional salt formula:	Mg		Br₂

Write the formula for the salt:
magnesium bromide has the formula MgBr₂

Question 3: Write the formula for lead(4+) oxide (also known as lead(IV) oxide)

Solution:

Break the name of the salt up into the name of the cation, and, the name of the anion:

Salt

cation anion

Names: lead(4+) oxide

	Salt
	cation	anion
Names:	lead(4+)	oxide

Use the cation's name to determine the formula for the cation

M⁺ : Group 1 metal, hydrogen, silver, ammonium
M²⁺ : Group 2 metal, zinc
M³⁺ : aluminium
Otherwise charge on ion is given in parentheses after name: charge on lead(4+) is 4+, cation formula is Pb⁴⁺

	Salt
	cation	anion
Names:	lead(4+)	oxide
Formulae:	Pb⁴⁺

Use the anion's name to determine the formula for the anion

X^- : Group 17 non-metal, hydroxide, nitrate, nitrite, hydrogencarbonate
X^2- : oxide, O^2-, is a Group 16 non-metal, carbonate, sulfate, sulfite
X^3- : Group 15 non-metal, phosphate

	Salt
	cation	anion
Names:	lead(4+)	oxide
Formulae:	Pb⁴⁺	O^2-

Determine the ratio of cations to anions in the formula of the salt:

	Salt
	cation		anion
Names:	lead(4+)		oxide
Formulae:	Pb⁴⁺		O^2-
Ratio of ions:	2	:	4

Note that (2 × 4+) + (4 × 2-) = 0

or, (2 × 4+) = -(4 × 2-)

Determine the lowest whole number ratio of cations to anions:

(i) If y = x then the lowest whole number ratio of cations to anions is 1:1

(ii) If y ≠ x then

(a) If y = 1 and/or x = 1 you already have the lowest whole number ratio of cations to anions, M:X is y:x
(b) Otherwise divide y by x AND divide x by y

(I) If (y ÷ x) and (x ÷ y) are both fractions, then the lowest whole number ratio of M:X is y:x
(II) If (y ÷ x) = whole number greater than 1 OR (x ÷ y) = a whole number greater than 1 (4/2 = 2 which is a whole number greater than 1)
then divide both x and y by the smallest number (that is, divide throughout by 2)

	Salt
	cation		anion
Names:	lead(4+)		oxide
Formulae:	Pb⁴⁺		O^2-
Ratio of ions:	2	:	4
Lowest whole number ratio :	2 ÷ 2 = 1	:	4 ÷ 2 = 2

Remove the charges from the ions to give the formula for the ions as they will be represented in the formula of the salt:

	Salt
	cation		anion
Names:	lead(4+)		oxide
Formulae:	Pb⁴⁺		O^2-
Ratio of ions:	2	:	4
Lowest whole number ratio :	1	:	2
Ions as represented in salt formula :	Pb		O

Construct a formula for the salt:

(i) If a = b, ratio of M to X is 1:1 and the salt's general formula is MX

(ii) If a ≠ b, ratio of M to X is a:b and the salt's general formula is M_aX_b

(a) If a = 1, general salt formula is MX_b
(b) If b = 1, general salt formula is M_aX
(c) If more than one of a specific polyatomc ion is present in the salt's formula, then the formula of that polyatomic ion is enclosed in parentheses and the number of polyatomic ions written as a subscript to the right of the parentheses.

	Salt
	cation		anion
Names:	lead(4+)		oxide
Formulae:	Pb⁴⁺		O^2-
Ratio of ions:	2	:	4
Lowest whole number ratio :	1	:	2
Ions as represented in salt formula :	Pb		O
Provisional salt formula:	Pb		O₂

Write the formula for the salt:
The formula for lead(4+) oxide is PbO₂

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Worked Examples of Writing the Formula for an Inorganic Salt: Polyatomic Ions

Question 1: Write the formula for the inorganic salt sodium hydrogencarbonate.

Solution:

Break the name of the salt up into the name of the cation, and, the name of the anion:

Salt

cation anion

Names: sodium hydrogencarbonate

Use the cation's name to determine the formula for the cation
M⁺ : sodium(1+), Na⁺, is a Group 1 metal, hydrogen, silver, ammonium
M²⁺ : Group 2 metal, zinc
M³⁺ : aluminium
Otherwise charge on ion is given in parentheses after name

Salt

Names: sodium hydrogencarbonate

Formulae: Na⁺

Use the anion's name to determine the formula for the anion
X^- : Group 17 non-metal, hydroxide, nitrate, nitrite, hydrogencarbonate, HCO₃^-
X^2- : Group 16 non-metal, carbonate, sulfate, sulfite
X^3- : Group 15 non-metal, phosphate

Salt

Names: sodium hydrogencarbonate

Formulae: Na⁺ HCO₃^-

	Salt
	cation	anion
Names:	sodium	hydrogencarbonate

	Salt
Names:	sodium	hydrogencarbonate
Formulae:	Na⁺

	Salt
Names:	sodium	hydrogencarbonate
Formulae:	Na⁺	HCO₃^-

Determine the ratio of cations to anions in the formula of the salt:

	Salt
Names:	sodium		hydrogencarbonate
Formulae:	Na⁺		HCO₃^-
Ratio of ions:	1	:	1

Note that (1 × +) + (1 × -) = 0

or, (1 × +) = -(1 × -)

Determine the lowest whole number ratio of cations to anions:

(i) If y = x then the lowest whole number ratio of cations to anions is 1:1

(ii) If y ≠ x then

(a) If y = 1 and/or x = 1 you already have the lowest whole number ratio of cations to anions, M:X is y:x
(b) Otherwise divide y by x AND divide x by y

(I) If (y ÷ x) and (x ÷ y) are both fractions, then the lowest whole number ratio of M:X is y:x
(II) If (y ÷ x) = whole number greater than 1 OR (x ÷ y) = a whole number greater than 1
then divide both x and y by the smallest number (which is either x or y)

	Salt
Names:	sodium		hydrogencarbonate
Formulae:	Na⁺		HCO₃^-
Ratio of ions:	1	:	1
Lowest whole number ratio :	1	:	1

Remove the charges from the ions to give the formula for the ions as they will be represented in the formula of the salt:

	Salt
Names:	sodium		hydrogencarbonate
Formulae:	Na⁺		HCO₃^-
Ratio of ions:	1	:	1
Lowest whole number ratio :	1	:	1
Ions as represented in salt formula :	Na		HCO₃

Construct a formula for the salt:

(i) If a = b, ratio of M to X is 1:1 and the salt's general formula is MX

(ii) If a ≠ b, ratio of M to X is a:b and the salt's general formula is M_aX_b

(a) If a = 1, general salt formula is MX_b
(b) If b = 1, general salt formula is M_aX
(c) If more than one of a specific polyatomc ion is present in the salt's formula, then the formula of that polyatomic ion is enclosed in parentheses and the number of polyatomic ions written as a subscript to the right of the parentheses.

	Salt
Names:	sodium		hydrogencarbonate
Formulae:	Na⁺		HCO₃^-
Ratio of ions:	1	:	1
Lowest whole number ratio :	1	:	1
Ions as represented in salt formula :	Na		HCO₃
Provisional salt formula:	Na		HCO₃

Write the formula for the salt:
sodium hydrogencarbonate has the formula NaHCO₃

Question 2: Write the formula for the inorganic salt calcium nitrate.

Solution:

Break the name of the salt up into the name of the cation, and, the name of the anion:

Salt

cation anion

Names: calcium nitrate

Use the cation's name to determine the formula for the cation
M⁺ : Group 1 metal, hydrogen, silver, ammonium
M²⁺ : calcium(2+), Ca²⁺ is a Group 2 metal, zinc
M³⁺ : aluminium
Otherwise charge on ion is given in parentheses after name

Salt

Names: calcium nitrate

Formulae: Ca²⁺

Use the anion's name to determine the formula for the anion
X^- : Group 17 non-metal, hydroxide, nitrate, NO₃^-, nitrite, hydrogencarbonate
X^2- : Group 16 non-metal, carbonate, sulfate, sulfite
X^3- : Group 15 non-metal, phosphate

Salt

Names: calcium nitrate

Formulae: Ca²⁺ NO₃^-

	Salt
	cation	anion
Names:	calcium	nitrate

	Salt
Names:	calcium	nitrate
Formulae:	Ca²⁺

	Salt
Names:	calcium	nitrate
Formulae:	Ca²⁺	NO₃^-

Determine the ratio of cations to anions in the formula of the salt:

	Salt
Names:	calcium		nitrate
Formulae:	Ca²⁺		NO₃^-
Ratio of ions:	1	:	2

Note that (1 × 2+) + (2 × -) = 0

or, (1 × 2+) = -(2 × -)

Determine the lowest whole number ratio of cations to anions:

(i) If y = x then the lowest whole number ratio of cations to anions is 1:1

(ii) If y ≠ x then

(a) If y = 1 and/or x = 1 you already have the lowest whole number ratio of cations to anions, M:X is y:x
(b) Otherwise divide y by x AND divide x by y

(I) If (y ÷ x) and (x ÷ y) are both fractions, then the lowest whole number ratio of M:X is y:x
(II) If (y ÷ x) = whole number greater than 1 OR (x ÷ y) = a whole number greater than 1
then divide both x and y by the smallest number (which is either x or y)

	Salt
Names:	calcium		nitrate
Formulae:	Ca²⁺		NO₃^-
Ratio of ions:	1	:	2
Lowest whole number ratio :	1	:	2

Remove the charges from the ions to give the formula for the ions as they will be represented in the formula of the salt:

	Salt
Names:	calcium		nitrate
Formulae:	Ca²⁺		NO₃^-
Ratio of ions:	1	:	2
Lowest whole number ratio :	1	:	2
Ions as represented in salt formula :	Ca		NO₃

Construct a formula for the salt:

(i) If a = b, ratio of M to X is 1:1 and the salt's general formula is MX

(ii) If a ≠ b, ratio of M to X is a:b and the salt's general formula is M_aX_b

(a) If a = 1, general salt formula is MX_b
(b) If b = 1, general salt formula is M_aX
(c) If more than one of a specific polyatomc ion is present in the salt's formula, then the formula of that polyatomic ion is enclosed in parentheses and the number of polyatomic ions written as a subscript to the right of the parentheses.

	Salt
Names:	calcium		nitrate
Formulae:	Ca²⁺		NO₃^-
Ratio of ions:	1	:	2
Lowest whole number ratio :	1	:	2
Ions as represented in salt formula :	Ca		NO₃
Provisional salt formula:	Ca		(NO₃)₂

Write the formula for the salt:
calcium nitrate has the formula Ca(NO₃)₂

Question 3: Write the formula for the inorganic salt ammonium sulfate.

Solution:

Break the name of the salt up into the name of the cation, and, the name of the anion:

Salt

cation anion

Names: ammonium sulfate

Use the cation's name to determine the formula for the cation
M⁺ : Group 1 metal, hydrogen, silver, ammonium, NH₄⁺
M²⁺ : Group 2 metal, zinc
M³⁺ : aluminium
Otherwise charge on ion is given in parentheses after name

Salt

Names: ammonium sulfate

Formulae: NH₄⁺

	Salt
	cation	anion
Names:	ammonium	sulfate

	Salt
Names:	ammonium	sulfate
Formulae:	NH₄⁺

Use the anion's name to determine the formula for the anion

X^- : Group 17 non-metal, hydroxide, nitrate, nitrite, hydrogencarbonate
X^2- : Group 16 non-metal, carbonate, sulfate, SO₄^2-, sulfite
X^3- : Group 15 non-metal, phosphate

	Salt
Names:	cation	anion
Names:	ammonium	sulfate
Formulae:	NH₄⁺	SO₄^2-

Determine the ratio of cations to anions in the formula of the salt:

	Salt
Names:	cation		anion
Names:	ammonium		sulfate
Formulae:	NH₄⁺		SO₄^2-
Ratio of ions:	2	:	1

Note that (2 × +) + (1 × 2-) = 0

or, (2 × +) = -(1 × 2-)

Determine the lowest whole number ratio of cations to anions:

(i) If y = x then the lowest whole number ratio of cations to anions is 1:1

(ii) If y ≠ x then

(a) If y = 1 and/or x = 1 you already have the lowest whole number ratio of cations to anions, M:X is y:x
(b) Otherwise divide y by x AND divide x by y

(I) If (y ÷ x) and (x ÷ y) are both fractions, then the lowest whole number ratio of M:X is y:x
(II) If (y ÷ x) = whole number greater than 1 OR (x ÷ y) = a whole number greater than 1
then divide both x and y by the smallest number (which is either x or y)

	Salt
Names:	cation		anion
Names:	ammonium		sulfate
Formulae:	NH₄⁺		SO₄^2-
Ratio of ions:	2	:	1
Lowest whole number ratio :	2	:	1

Remove the charges from the ions to give the formula for the ions as they will be represented in the formula of the salt:

	Salt
Names:	cation		anion
Names:	ammonium		sulfate
Formulae:	NH₄⁺		SO₄^2-
Ratio of ions:	2	:	1
Lowest whole number ratio :	2	:	1
Ions as represented in salt formula :	NH₄		SO₄

Construct a formula for the salt:

(i) If a = b, ratio of M to X is 1:1 and the salt's general formula is MX

(ii) If a ≠ b, ratio of M to X is a:b and the salt's general formula is M_aX_b

(a) If a = 1, general salt formula is MX_b
(b) If b = 1, general salt formula is M_aX
(c) If more than one of a specific polyatomc ion is present in the salt's formula, then the formula of that polyatomic ion is enclosed in parentheses and the number of polyatomic ions written as a subscript to the right of the parentheses.

	Salt
Names:	cation		anion
Names:	ammonium		sulfate
Formulae:	NH₄⁺		SO₄^2-
Ratio of ions:	2	:	1
Lowest whole number ratio :	2	:	1
Ions as represented in salt formula :	NH₄		SO₄
Provisional salt formula:	(NH₄)₂		SO₄

Write the formula for the salt:
ammonium sulfate has the formula (NH₄)₂SO₄

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Footnotes: reference "Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005" (Red Book)

(1) "bi" means "two"; biannual is an event that happens twice a year, biennial is an event that occurs every two years, bicentennial is an event occuring every two hundred years, bicycle has two wheels, biathlon is an athletic competition involving two events (skiing ang shooting), biceps is a muscle with two heads or attachments, bilingual refers to a person who is fluent in two languages, etc

(2) monoatomic or monotomic or monatomic? The IUPAC document cited above uses monoatomic so we will too!

(3) The formula of a salt is the same as its empirical formula.

Also refer to the practical investigation Determination of Empirical Formula: magnesium oxide.

(4) This location around the symbol of an element is actually referred to as the "right lower index".

left upper index	symbol	right upper index
left lower index	symbol	right lower index

(5) The name of the element with the symbol Cs is caesium, but IUPAC also accepts the alternative spelling of cesium.

(6) The systematic IUPAC name for ammonium is azanium. Most people still use the name ammonium for the cation NH₄⁺ and it is an acceptable IUPAC name.

(7) To be consistent with the rules for the construction of a formula (order of increasing electronegativity, that is symbol for least electronegative atom written first, symbol for most electronegative element written last), hydroxide should be given the formula HO^- NOT OH^-. However, the formula OH^- for hydroxide is well established and accepted.
Systematic IUPAC names for hydroxide are:

oxidanide (substitutive name : water, H₂O, named as oxidane, negative ion produced by dehydronation (deprotonation) is HO^- hence named as oxidanide)
hydridooxygenate(1-) (additive name)