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Compounds
What is a compound?
Compounds
• A compound is a substance
made of two or more elements.
Water is made of hydrogen
and oxygen.
• A compound always has the
same ratio (a fixed ratio) of
elements. Water always has
two atoms of hydrogen for
each atom of oxygen.
• The elements are not just
mixed together—they are
combined through a chemical
reaction.
Writing Compounds
• Some compounds, like water and sugar, have
there own names. Most do not.
• All compounds have names that tell what they
are made of. Salt is made of a sodium ion and a
chloride ion, so we call it sodium chloride.
• Often, we will just write the symbols. NaCl
• If a compound needs more than one of an atom,
sometimes its name can tell us. Carbon dioxide
has one carbon atom and two oxygen atoms.
Writing Compounds
• When we write the
symbols the number
of atoms needed
goes after the symbol
as a subscript (below
the line).
• Carbon dioxide is
written CO2.
• The way we write
down the symbols for
a compound tells us
how it is made, so we
call it the chemical
formula for that
compound.
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Carbon dioxide: CO2
Water: H2O
Sodium chloride (salt): NaCl
Hydroxide: OHGlucose: (sugar) C6H12O6
Ammonia: NH3
Iron oxide (rust): Fe2O3
Hydrochloric acid: HCl
Octane (in petrol) C8H18
Ethanol (alcohol) C2H6O
Methane (natural gas) CH4
Propane (cooking gas) C3H8
Molecules
• Molecules are groups
of atoms.
• Since compounds are
made of more than
one element, the
smallest piece of a
compound is a
molecule.
• Some non-metals
form molecules made
of only that element.
Chemical Reactions
• Chemical reaction happen when two or more atoms
come together to form a new substance.
• Oxidation is one of the most common chemical
reactions.
• A burning piece of wood, a piece of apple turning brown,
and a piece of iron rusting are all examples of oxidation.
Writing Chemical Reactions
• When we write down a chemical reaction, we
write the substances we start with (the
reactants) on the left, with “+” between each.
Usually we write only the chemical symbols.
• The substances we finish with (the products) go
on the right, with plusses in between.
• The reactants and products are separated by a
“→”. We say “yields” when we read the reaction.
Na+ + Cl- → NaCl
Sodium plus chloride yields sodium chloride (salt).
Writing Chemical Reactions
• Anytime we write a chemical formula, the
number of atoms of an element is written
as a subscript after the symbol for that
element.
NaOH + HCl → NaCl + H2O
Sodium hydroxide plus hydrochloric acid
yields sodium chloride (salt) and water.
Writing Chemical Reactions
• When we write a chemical reaction, it is like
writing the recipe for a cake.
• We must tell how much of each thing we need.
• We write the number of atoms or molecules of
each substance in front of the symbol, unless we
only need one of that atom or molecule.
Ca2+ + 2F- → CaF2
Calcium plus two fluoride ions yields calcium
fluoride.
Writing Chemical Reactions
• Sometimes we want to keep track of
certain molecules even after the chemical
reaction.
• We do this by putting parentheses “( )”
around the formula for that molecule.
Ca2+ + 2(OH-) → Ca(OH)2
Calcium plus two hydroxide ions yields
calcium hydroxide.
Conservation of Mass
• Matter can not be create or destroyed in a
chemical reaction.
• When a chemical reaction happens there is no
change in mass.
• The mass of all the reactants equals the mass of
all the products.
2g H2 + 16g O2 → 18g H2O
40g NaOH + 36g HCl → 58g NaCl + 18g H2O