Chemical FormulaeChristopher chong

Prior knowledge What are symbols used for?Answer: A symbol is use to represent one atom of an element.Can you complete the following table ?

Element SymbolOxygen OCarbon CSodium NaLead PbIron Fe

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Who is this ?• Dmitri Mendeleev • Creator of the first

version of the periodic table of elements.

• Predicted the properties of elements yet to be discovered.

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Introducing Chemical Equations

• To do calculations, a chemist must use chemical formulae and chemical equations.

• To write chemical equations, we need to first know how to write chemical formulae.

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At the end of lesson, you should be able to…

1. state the symbols of the elements and formulae of the compounds

2. deduce the formulae of simple compounds from the relative numbers of atoms present and vice versa

3. deduce the formulae of ionic compounds from the charges on the ions present and vice versa

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Symbols of the

elements

Formulae of the

compounds

Formulae of simple

molecules

Formulae of ionic

compounds

Chemical equations (with state

symbol) NL

Flowchart for today’s lesson:

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Learning Objective 1: State the symbols of the elements and formulae of the

compounds Fill in the blanks (try not to refer to the periodic table)

Name of element SymbolCa

PotassiumMagnesiumManganese

Brhydrogensulfur

KMgMn

Calcium

HS

bromine

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Learning Objective 1: State the symbols of the elements and formulae of the compounds

Name of compound FormulaeHydrogen gasOxygen gasCarbon dioxideCarbon monoxide

Fill in the blanks

H2

O2

CO2

CO

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Symbols of the

elements

Formulae of the

compounds

Formulae of simple

compounds

Flowchart for today’s lesson:

Simple rules for formulae

The valency of an element must be known in order to write the formulae of a compound.

The valency of an element is a number that shows its combining power.

In ionic compounds, the valency of an ion is the same as its charge.

Total charges in ionic compounds must be equal to zero.

In ionic compounds, metal ions are usually written on the left followed by non-metal ions (e.g. NaCl).

Naming ionic compounds

Ionic Compounds Contain a metal and a nonmetal.

The Metal loses e- and becomes a cation (+). The Nonmetal gains e- and becomes an anion(-). Metal is listed first, followed by nonmetal.

Change the name of the nonmetal to -ide. Examples: nitride, sulfide, fluoride, oxide, bromide, iodide, chloride, telluride, phosphide.

Polyatomic Anions

More than one atom covalently bond. Polyatomic anions gained electrons and thus

they are negatively charged. Polyatomic anions stay together during

chemical reactions. It is good to memorize some common

polyatomic anions. E.g. OH-, NO3-

Polyatomic cations More than one atom covalently bond. Polyatomic cations lose electrons and thus they

are positively charged. Polyatomic cations stay together during

chemical reactions. It is good to memorize some common

polyatomic cations. E.g. NH4+, H3O+

Simple covalent compounds

Compound: WaterFormula of element: H OValency of element: 1 2Simplest ratio of combining atoms:

2 1

Formula of compound: H2O

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Learning Objective 2: Deduce the formulae of simple molecules from the

relative numbers of atoms present From the atoms shown, write down the formula of

the compound that can be formed.

Chlorine gas, Cl2

Cl Cl

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Learning Objective 2: Deduce the formulae of simple molecules from the

relative numbers of atoms present

From the atoms shown, write down the formula of the compound.

Hydrogen chloride gas, HCl

Cl H

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Learning Objective 2: Deduce the formulae of simple molecules from the

relative numbers of atoms present

From the atoms shown, write down the formula of the compound.

Ammonia gas, NH3

N

H HH

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Learning Objective 2: Deduce the formulae of simple molecules from the

relative numbers of atoms present

Write the formula of the common covalent molecules

Name of compound FormulaeSulfur dioxide gasNitrogen dioxide gasMethane gas

SO2

NO2

CH4

1 sulfur atoms2 oxygen atoms1 nitrogen atoms2 oxygen atoms

1 carbon atoms4 hydrogen atoms

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Symbols of the

elements

Formulae of the

compounds

Formulae of simple

compounds

Formulae of ionic

compounds

Flowchart for today’s lesson:

Learning Objective 3: Deduce the formulae of ionic compounds from the

charges on the ions present and vice versa

Identify the metal and non-metal

i. Write the symbolsii. Write the chargesiii. Cross-over the charges

from top to bottomiv. Remove the chargev. Simplify the numbers

and remove the 1’s21

sodium chloridemetal non-metal

Na Cl1+ 1-1 1

NaCl

Learning Objective 3: Deduce the formulae of ionic compounds from the

charges on the ions present and vice versa

Identify the metal and non-metal

i. Write the symbolsii. Write the chargesiii. Cross-over the charges

from top to bottomiv. Remove the chargev. Simplify the numbers

and remove the 1’s22

Calcium Oxidemetal non-metal

Ca O2+ 2-2 2

CaO1 1

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Learning Check Complete the table by filling in the

chemical formulae for the ionic compounds formed.Chloride

Cl -Hydroxide

OH-Oxide

O2-

PotassiumK+

Magnesium

Mg2+

Iron (III)Fe3+

KCl

MgCl2

K2OKOH

FeCl3

Mg(OH)2

Fe(OH)3

MgO

Fe2O3

Class activity Students are given a tag with the symbol of an ion and

its oxidation number. Positive ions are green and the negative ions are blue.

The students are to "bond" with other ions and keep a record of their bonds.

Students are to work with their bonding partner to agree on and write a formula for the compound they formed. After which, students are to break the bond and find a different ion with which to bond.

After 5 bonds, students switch tags with another student and start bonding again.

What have we learnt… The symbols of the elements and formulae

of common compounds How to write a formulae for simple covalent

molecules. How to write a formulae for ionic compounds.