Chemical Formulae

Chemical Formulae Christopher 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 ?

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.

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.

At the end of lesson, you should be able to… state the symbols of the elements and formulae of the compounds deduce the formulae of simple compounds from the relative numbers of atoms present and vice versa deduce the formulae of ionic compounds from the charges on the ions present and vice versa

Flowchart for today’s lesson: Symbolsof the elements Formulaeof the compounds Formulaeof simple molecules Formulaeof ionic compounds Chemical equations (with state symbol) NL

Learning Objective 1: State the symbols of the elements and formulae of the compounds Calcium K Mg Mn bromine H S Fill in the blanks (try not to refer to the periodic table)

Learning Objective 1: State the symbols of the elements and formulae of the compounds Fill in the blanks H2 O2 CO2 CO

Flowchart for today’s lesson: Symbolsof the elements Formulaeof the compounds Formulaeof simple compounds

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 cationsstay together during chemical reactions. It is good to memorize some common polyatomic cations. E.g. NH4+, H3O+

Simple covalent compounds

Learning Objective 2: Deduce the formulae of simple molecules from the relative numbers of atoms present Cl Cl Chlorine gas, Cl2 From the atoms shown, write down the formula of the compound that can be formed.

Learning Objective 2: Deduce the formulae of simple molecules from the relative numbers of atoms present Cl H Hydrogen chloride gas, HCl From the atoms shown, write down the formula of the compound.

Learning Objective 2: Deduce the formulae of simple molecules from the relative numbers of atoms present N H H H Ammonia gas, NH3 From the atoms shown, write down the formula of the compound.

Learning Objective 2: Deduce the formulae of simple molecules from the relative numbers of atoms present 1 sulfur atoms 2 oxygen atoms SO2 1 nitrogen atoms 2 oxygen atoms NO2 CH4 1 carbon atoms 4 hydrogen atoms Write the formula of the common covalent molecules

Flowchart for today’s lesson: Symbolsof the elements Formulaeof the compounds Formulaeof simple compounds Formulaeof ionic compounds

Learning Objective 3: Deduce the formulae of ionic compounds from the charges on the ions present and vice versa non-metal metal sodium chloride 1+ 1- Na Cl 1 1 NaCl Identify the metal and non-metal • Write the symbols • Write the charges • Cross-over the charges from top to bottom • Remove the charge • Simplify the numbers and remove the 1’s

Learning Objective 3: Deduce the formulae of ionic compounds from the charges on the ions present and vice versa non-metal metal Calcium Oxide 2+ 2- Ca O 2 1 1 2 CaO Identify the metal and non-metal • Write the symbols • Write the charges • Cross-over the charges from top to bottom • Remove the charge • Simplify the numbers and remove the 1’s

Learning Check KOH KCl K2O MgCl2 Mg(OH)2 MgO Fe2O3 FeCl3 Fe(OH)3 Complete the table by filling in the chemical formulae for the ionic compounds formed.

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 formulaeof common compounds How to write a formulae for simple covalent molecules. How to write a formulae for ionic compounds.

Chemical Formulae