Compounds and their formulas
Molecular Compounds
A molecular compound is a substance made up of discreet entities called
molecules. A molecule consists of a small number of non-metal atoms held
together by covalent bonds, or sharing electrons. We can represent the molecule
by its chemical formula. The chemical formula is a comprised of the symbols of
the elements present and the relative number of each element.
An empirical formula is a representation that shows only the lowest whole number
ratio of the elements present. The molecular formula is based on the actual
molecule. For example the empirical formula of a sugar is CH2O. The molecular
formula is some whole number multiple of this (i.e., C5H10O5 or C6H12O6). A
structural formula is a formula that indicates how the atoms are attached to each
other. This formula uses lines to denote the bonds between atoms. A condensed
structural formula is a simplified version of the structural formula. For example,
an alcohol is represented as CH3CH2OH. In this formula, the hydrogens that
follow the carbons are attached to those carbons, and the oxygen is attached to the
second carbon. A structural formula for this looks like:
H
H
O
C
H
H
C
H
H
We need to remember that all of these molecules, tiny as they may be, take up
space. They also have a three-dimensional shape which these formulas do not
indicate. We can represent these aspects of molecules by building models of them.
There are several different kinds of models such as ball-and-stick and space filling
models. We will look at these later in this course.
Ionic Compounds
Compounds of metals and nonmetals are usually ionic compounds. These
compounds are comprised of positive and negative ions. Positive ions are called
cations and negative ions are anions. There is not an actual bond between the
ions. The attraction of positive and negative charges holds these compounds
together. The chemical formulas do not show the actual number of atoms in each
molecule because there are no molecules. The formulas show the ratio of the ions
in the compound. For example, in Sodium Chloride, NaCl, there is one Sodium
ion for every Chloride ion. This is called the formula unit of the compound.
Molecular mass: The sum of the atomic masses of all the atoms in a molecule of
a substance.
Formula mass: The sum of the atomic masses of all the atoms in a formula unit
of a substance.
For molecules: Molecular weight and formula weight are the same.
For ionic compounds: Only the term formula weight applies.
Molar Mass: The mass of one mole of substance.
The molar mass in g/mol is numerically equivalent to the molecular or formula
mass in amu.
Examples:
1. How many formula units are in 254.2 g of Lead(II) Oxide?
The formula for Lead(II) Oxide is PbO which results in a molar mass of
223.19 g (207.19 g + 15.9994 g).
254.2 g PbO ×
1 mol PbO
6.022 × 10 23 formula units PbO
×
= 6.859 × 10 23 formula units PbO
223.19 g PbO
1 mol PbO
2. 3.42×1022 molecules of water has what mass in grams?
Water has the formula H2O so it has a molar mass of 18.0153 g
(2×1.0079 g + 15.9994 g).
3.42 × 1022 molecules H 2O ×
1 mole H 2O
18.0153 g H 2O
×
= 1.02 g H 2O
23
6.022 × 10 molecules H 2O
1 mole H 2O
Remember that one mole of the elements of Hydrogen, Nitrogen, Oxygen,
Fluorine, Chlorine, Bromine, Iodine, Phosphorus and Sulfur are not in the form of
atoms. The elements exist as molecules:
H2
N2
O2
F2
Cl2
Br2
I2
P4
S8
Mass percentages: The percentage by mass of all the elements in a compound.
mass % ofA =
mass of A in the sample
× 100%
mass of the sample
The sum of the percentages for a compound must equal 100%.
Determining chemical formulas
Empirical formula: a formula for a compound that shows the lowest
whole number ratio of the elements in the compound.
Empirical formula for water: H2O
Empirical formula for hydrogen peroxide: HO (H2O2)
Empirical formula for glucose: CH2O (C6H12O6)
Empirical formula for ribose: CH2O (C5H10O5)
Determining the empirical formula from mass percentages…
1. Assume 100.00 g of compound. This converts the percentages directly to
masses of the elements.
2. Convert the mass of each element to moles of that element.
3. Divide each of the moles by the smallest.
4. If you do not get a series of integers from step 3. Multiply each of the
values by a factor to get everything to integers (±0.1). This factor will usually be
about 2 or 3, but it may be larger.
Determining molecular formulas:
1. Find the empirical formula.
2. Find the empirical mass (the mass of one empirical formula unit).
3. Divide the molar mass by the empirical mass. This should be an integer.
4. Multiply this integer by all the subscripts in the formula.
For compounds containing the elements C, H, and O, the percentages can be
determined by combustion. Combustion itself gives carbon dioxide, which gives
the percentage of C, and water, which gives the percentage of H. Percentage of O
is determined by difference.
Example:
Therefore the empirical formula is C3H6O.
If the molar mass of this compound is 116.16 g mol-1, what is the molecular
formula?
Molar mass
116.16 g mol-1
=
=2
Empirical mass 58.08 g mol-1
So the molecular formula is twice the empirical formula, C6H12O2.
Oxidation States
A useful concept in describing compounds and their changes is the concept of the
oxidation state. The oxidation state is just an indication of the number of electrons
lost (oxidized) or gained (reduced) by an element in a compound. This is a
hypothetical charge that the element would have if it were in a purely ionic
compound. This means that if we have a purely ionic compound comprised of
monatomic ions, the oxidation state is the same as the charge on the ion. If the
compound contains polyatomic ions or is a molecular compound, the oxidation
state is the charge the element would have if it were actually an ionic compound.
We have some rules to help us to determine the oxidation state of atoms. These
are :
1. The oxidation number of an atom in an element is zero.
2. The oxidation number of an atom in an ionic compound is equal to its
charge.
3. The oxidation number of oxygen is -2. Exceptions: In peroxides it is -1
and in compounds with fluorine it is +1.
4. The oxidation number of hydrogen is +1 unless it is combined with at
metal then it is -1.
5. The oxidation number of fluorine in its compounds is -1. The oxidation
number of other halogens is -1 unless it is combined with oxygen or
fluorine.
6. The sum of the oxidation numbers in a compound must equal zero. In
polyatomic ions it must equal the charge on the ion.
Because oxidation states are not really charges, we can have fractional values for
the oxidation state.
Nomenclature – Naming of compounds
Ionic Compounds
To name ionic compounds you just name the ions that are present. The
name of the positive ion (cation) is first followed by the name of the negative ion
(anion)
Naming monatomic ions.
1. Metal ions are named the same as the metal, if there is only one ion.
2. If there is more than one charge on the ion, the ion is named with the metal
name with the charge written after it in Roman numerals in parenthesis.
An older system of naming metals that form more than one kind of ion
involves the use of the prefixes –ous and -ic. The stem is derived from the
name of the metal if the symbol is not from the Latin name. If the symbol
comes from the Latin name the stem is from the Latin name of the metal.
Fe2+
Fe3+
3.
Iron(II) or Ferrous
Iron(III) or Ferric
Cu+ Copper(I) or Cuprous
Cu2+ Copper(II) or Cupric
Au+ Gold(I) or Aurous
Au3+ Gold(III) or Auric
Sn2+
Sn4+
Pb2+
Pb4+
Hg22+ Mercury(I) or Mercurous
Hg2+ Mercury(II) or Mercuric
Lead(II) or Plumbous
Lead(IV) or Plumbic
Tin(II) or Stannous
Tin(IV) or Stannic
Non-metals: stem name + -ide.
Ionic compounds can also contain polyatomic ions. These are groups of atoms
that are bonded together that have a charge. A list of these ions is provided in your
text and in class. This list must be memorized. You need to know the name,
formula and charge of all of the polyatomic ions in the list.
Among the polyatomic ions there are a couple of patterns. If there are two ions
with similar names (e.g., sulfate and sulfite), the one whose name ends in –ate has
1 more oxygen than the one whose name ends in –ite. This is can be extended to
other ions. If the ion has 1 more oxygen than the –ate ion, it is given the prefix
per- (e.g., perchlorate, ClO4 , chlorate, ClO3 ). If the ion has one less oxygen than
the –ite ion, it is given the prefix hypo- (e.g., hypochlorite, ClO , chlorite, ClO2 ).
Also, if an oxygen atom is replaces with a sulfur atom the prefix thio- is added to
the name (e.g., CNO-, cyanate, CNS-, thiocyanate).
The periodic law can also be applied to these patterns. For example ClO3- is the
chlorate ion and BrO3- is the bromate ion.
To get the formula for an ionic compound between any two elements we need to
know the charges on the ions.
What is the charge on the ions?
1. Main-group metals: charge = group number.
2. Main-group metal with high atomic number can have more than one
charge. Usually the other charge is the group number minus 2.
3. Most transition metals have more than one charge. Most have an ion
with a charge of +2. These charges have to be determined from the
formula.
4. Non-metals have a charge of the group number minus 8.
The total positive charge must equal the total negative charge in the compound.
This can be done by “crossing over” the charges.
Rb+ and O2- cross over Rb2O
Ca2+ and PO43- cross over Ca3(PO4)2
Naming binary molecular compounds
Order of elements in the formula
B Si C Sb As P N H Te Se S I Br Cl O F (by convention)
Rules for naming
1. Name elements in order given above.
2. First element named is given the name of the element with the
appropriate prefix see number 4.
3. Name of second element: stem name + -ide.
4. Prefixes used for compounds containing more than one of the same type
of element. Mono- not used unless it is needed to distinguish two
compounds of the same two elements.
Prefixes
1
2
3
4
5
6
7
8
9
10
monoditritetrapentahexaheptaoctanonadeca-
Exceptions to the above rule for binary molecular compounds
Some compounds that have been known for hundreds or thousands of years do not
follow this naming system. Others, such as organic compounds, follow a different
system altogether. The compounds you are required to know the common names
of are:
H2 O
N2 H4
AsH3
Water
Hydrazine
Arsine
NH3
PH3
Ammonia
Phosphine
Naming acids
Binary acids.
Hydro- stem name of anion -ic acid.
HCl
HBr
Hydrochloric acid
Hydrobromic acid
Oxoacids are acids that contain oxygen.
Anion suffix
-ite
-ate
acid suffix
-ous
-ic
NO HYDRO- PREFIX!!!!
Hydrates
Hydrates are ionic compounds with molecules of water associated with them. The
prefixes used with molecular compounds are used to denote the number of water
molecules present.
CuSO4 ½ 5 H2O (5 molecules of water for every Copper(II) Sulfate)
This is named as the name of the ionic compound + prefix-hydrate: Copper(II)
Sulfate Pentahydrate.