Unit 2: Reactions
● Stoichiometry: The relationship between the relative quantities of substances taking part in a
chemical reaction
○ Usually expressed as a ratio
Symbols and formulae
● Every element in the Periodic Table has a unique symbol and is assigned a unique identifying
number (proton number)
● Used to save time and paper when writing
● Used to identify the makeup of the chemical compounds formed when elements are added
together
● Examples
○ Sodium (metal) has the chemical symbol Na
○ Chlorine (nonmetal) has the symbol Cl
○ When these two elements are combined, the compound sodium chloride is formed
○ Sodium chloride has the chemical formula NaCl
■ Tells us that the compound NaCl is made up of equal numbers of sodium and
chlorine atoms
○ When magnesium (Mg) and chlorine combine, the compound magnesium chloride,
𝑀𝑔𝐶𝑙2, is formed
■ For every one atom of magnesium in the compound there are two atoms of
chlorine
○ Calcium carbonate, known commonly as limestone, has the formula 𝐶𝑎𝐶𝑂3
■ Tells us that this compound is composed of one atom of calcium, one atom of
carbon and three atoms of oxygen
● Subscript numbers represent the number of atoms of that element within the compound
○ Always come after the symbol of the element
● The formula of simple compounds can be worked out from a description of the reacting elements
or a diagram or model of the compound
○ Calcium chloride contains two atoms of chlorine for every atom of calcium, so its
formula is 𝐶𝑎𝐶𝑙2
○ Methane is composed of carbon and hydrogen – the compound contains five atoms in
total but only one of these is carbon, so the formula of methane is 𝐶𝐻4
● Stoichiometry: The relationship between the relative quantities of substances taking part in a
chemical reaction
○ Usually expressed as a ratio
Symbols and formulae
● Every element in the Periodic Table has a unique symbol and is assigned a unique identifying
number (proton number)
● Used to save time and paper when writing
● Used to identify the makeup of the chemical compounds formed when elements are added
together
● Examples
○ Sodium (metal) has the chemical symbol Na
○ Chlorine (nonmetal) has the symbol Cl
○ When these two elements are combined, the compound sodium chloride is formed
○ Sodium chloride has the chemical formula NaCl
■ Tells us that the compound NaCl is made up of equal numbers of sodium and
chlorine atoms
○ When magnesium (Mg) and chlorine combine, the compound magnesium chloride,
𝑀𝑔𝐶𝑙2, is formed
■ For every one atom of magnesium in the compound there are two atoms of
chlorine
○ Calcium carbonate, known commonly as limestone, has the formula 𝐶𝑎𝐶𝑂3
■ Tells us that this compound is composed of one atom of calcium, one atom of
carbon and three atoms of oxygen
● Subscript numbers represent the number of atoms of that element within the compound
○ Always come after the symbol of the element
● The formula of simple compounds can be worked out from a description of the reacting elements
or a diagram or model of the compound
○ Calcium chloride contains two atoms of chlorine for every atom of calcium, so its
formula is 𝐶𝑎𝐶𝑙2
○ Methane is composed of carbon and hydrogen – the compound contains five atoms in
total but only one of these is carbon, so the formula of methane is 𝐶𝐻4
