Chemistry of Life. Carbon Carbon has four valence electrons which results in 4 covalent bonds.
Chemistry - Valence and Chemical Formulae
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Transcript of Chemistry - Valence and Chemical Formulae
This presentation was produced by the Science department of Temasek Secondary School. Redistribution or reproduction of this resource is prohibited by copyright regulations. This resource should be used for educational purposes.
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