Chemistry - Valence and Chemical Formulae

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Acids and AlkalisValency & Chemical

Formulae

Today’s Lesson

The Meaning of Valency

Naming Compounds

Writing Chemical Formula

Valency Elements react in order to achieve noble gas

electronic configuration.

E.g. chlorine with an electronic configuration of 2.8.7 gains one electron to achieve an electronic configuration of 2.8.8.

Valency To write the chemical formula of a compound,

we must know the valency of the elements or ions involved.

Valency is the number of electrons that an atom needs to gain or lose in order to achieve noble gas electronic configuration.

E.g. chlorine has a valency of 1, as it needs to gain 1 electron to achieve noble gas electronic configuration.

Group number

Element

Electronic

structure

No of electrons to gain or lose to achieve the noble gas electronic

configuration

Ion formed

Valency

I 

Li 

2.1 Lose 1 electron Li+ 1

I 

Na 

2.8.1 Lose 1 electron Na+ 1

I K 

2.8.8.1 Lose 1 electron K+ 1

II 

Be 

2.2 Lose 2 electrons Be2+ 2

II 

Mg 

2.8.2 Lose 2 electrons Mg2+ 2

Group number

Element

Electronic

structure

No of electrons to gain or lose to achieve the noble gas electronic

configuration

Ion formed

Valency

II 

Ca 

2.8.8.2 Lose 2 electrons Ca2+ 2

III B 

2.3 Lose 3 electrons B3+ 3

III 

Al 

2.8.3 

Lose 3 electrons 

Al3+ 3

 V 

N 2.5 Gain 3 electrons N3- 3

 V 

P 2.8.5 Gain 3 electrons P3- 3

Group numbe

rElement

Electronic

structure

No of electrons to gain or lose to achieve the noble gas electronic

configuration

Ion formed

Valency

VI O 

 2.6

 Gain 2 electrons

O2- 2 

 VI 

S 2.8.6 Gain 2 electrons S2- 2

VII F 

 2.7

 Gain 1 electron

F- 1 

VII 

Cl 

2.8.7 Gain 1 electron Cl- 1

Polyatomic Ions There are some ions that are made up of more

than one atom. They are called polyatomic ions.

Some common polyatomic ions

Positive Ion Valency Negative Ion Valency

Ammonium ion, NH4+ 1 Nitrate ion, NO3

- 1

    Hydroxide ion, OH- 1

    Carbonate ion, CO32- 2

    Sulfate ion, SO42- 2

Chemical Formula The chemical formula of a substance shows the

number of atoms and the elements that make up the compound.Compound

Chemical formula

Number of atoms and elements

Sodium hydroxide NaOH 1 Na, 1 O and 1 H atoms

Copper sulfate CuSO4 1 Cu, 1 S and 4 O atoms

Ammonia gas NH3 1 N and 3 H atoms

Calcium carbonate CaCO3 1 Ca, 1 C and 3 O atoms

Aluminium fluoride

AlF3 1 Al and 3 F atoms

Naming Compounds

When naming an ionic compound that contains only two elements:

Metallic element comes before non-metallic element

Non-metallic elements end with ‘ide’

Naming Compounds

Elements in an ionic compound Chemical name

Calcium, chlorine

 Calcium chloride

(metal) (non-metal) 

Lithium, fluorine Lithium fluoride

Magnesium, oxygen Magnesium oxide

Potassium, sulfur Potassium sulfide

Naming Compounds

A polyatomic ion will not change its name after combining with other elements.

Examples:

Sodium carbonate

Potassium sulfate

Calcium hydroxide

Ammonium chloride

Your turn to practise!

ActivityChemical formula of compounds containing the following symbols/ groups are called:

Ion Name Example F-

 fluoride LiF, lithium fluoride

 Cl-

 chloride NaCl, sodium chloride

 Br-

 bromide KBr, potassium bromide

 I-

 iodide MgI2, magnesium iodide

ActivityChemical formula of compounds containing the following symbols/ groups are called:

Ion Name Example 

O2-

 oxide MgO, magnesium oxide

N3- nitride Na3N, sodium nitride

 P3-

 phosphide

 K3P, potassium phosphide

 NH4

+

ammonium

NH4Cl, ammonium chloride

ActivityChemical formula of compounds containing the following symbols/ groups are called:Ion Name Example

 OH-

 hydroxide KOH, potassium hydroxide

 NO3

-

 nitrate NaNO3, sodium nitrate

 CO3

2-

 carbonate MgCO3, magnesium carbonate

 SO4

2-

 sulfate K2SO4, potassium sulfate

 PO4

3-

 phosphate Na3PO4, sodium phosphate

Writing Chemical Formula

Example 1: Sodium chloride

Steps Sodium chloride

1

2

3

Write the symbols of the elements, their respective valencies below the symbols, and criss-cross.

Na Cl

1 1

Attach the valency as a subscript. Na1Cl1

Simplify the formula. NaCl

Writing Chemical Formula

Example 2: Zinc chloride

Steps Zinc chloride

1

2

3

Write the symbols of the elements, their respective valencies below the symbols, and criss-cross.

Zn Cl

2 1

Attach the valency as a subscript. Zn1Cl2

Simplify the formula. ZnCl2

Writing Chemical Formula

Example 3: Aluminium oxide

Steps Aluminium oxide

1

2

3

Write the symbols of the elements, their respective valencies below the symbols, and criss-cross.

Al O

3 2

Attach the valency as a subscript. Al2O3

Simplify the formula. Al2O3

Writing Chemical Formula

Example 4: Magnesium oxide

Steps Magnesium oxide

1

2

3

Write the symbols of the elements, their respective valencies below the symbols, and criss-cross.

Mg

O

2 2

Attach the valency as a subscript. Mg2O2

Simplify the formula. MgO

Writing Chemical Formula

TAKE NOTE when simplifying chemical formulae:

The subscript ‘1’ must be omitted

Reduce the ratio by a common factor if possible

Complete Exercise 1

Writing Chemical Formula

For compounds that contain polyatomic ions, a bracket must be used if a number is required.

Writing Chemical Formula

Example 5: Calcium nitrate

Steps Calcium nitrate

1

2

3

Write the symbols of the elements, their respective valencies below the symbols, and criss-cross.

Ca NO3

2 1

Attach the valency as a subscript. Ca1(NO3)

2Simplify the formula. Ca(NO3)

2

Writing Chemical Formula

Example 6: Sodium carbonate

Steps Sodium carbonate

1

2

3

Write the symbols of the elements, their respective valencies below the symbols, and criss-cross.

Na CO3

1 2

Attach the valency as a subscript. Na2(CO3)

1Simplify the formula. Na2CO3

Writing Chemical Formula

Example 7: Aluminium sulfate

Steps Aluminium sulfate

1

2

3

Write the symbols of the elements, their respective valencies below the symbols, and criss-cross.

Al SO4

3 2

Attach the valency as a subscript. Al2(SO4)3

Simplify the formula. Al2(SO4)3

Writing Chemical Formula

Example 8: Ammonium chloride

Steps Ammonium chloride

1

2

3

Write the symbols of the elements, their respective valencies below the symbols, and criss-cross.

NH4 Cl

1 1

Attach the valency as a subscript. (NH4)1Cl

1Simplify the formula. NH4Cl

Complete Exercise 2