Compounds consist of molecules made by combining atoms of two or more elements in definite ratios. In order to talk
about compounds then we need a way to represent them and their molecules. Chemists do this by using chemical formulas and chemical names.
The letters are based on the Latinate forms of the element names, a throwback to the time when scientists used Latin as
a common language.
The symbol represents both the element as a substance and the atom of the element depending on context. Thus "C" can stand
for the element carbon or an atom of carbon.
For instance carbon is C, oxygen is O, and nitrogen is N. Calcium is Ca, osmium is Os, and sodium is Na (from natrium,
Latin for soda ash a sodium compound). Iron is Fe (ferrum) and copper is Cu (cuprum). The names used for the elements vary according to the language but the symbols are international.
Even languages, like Russian or Greek, which do not use the Roman alphabet, use the chemical symbols in the original Roman
characters. There are officially 109 elements given a name and symbols by the IUPAC. The elements that have been known since ancient
times, like iron, copper, and sulfur, are given their common names in the language used by the chemist. Some names of newer
elements were given based on some property, e. g.:
- oxygen = "acid former"
- phosphorus = "luminous"
- bromine = "stench"
Some are named from minerals or compounds, e. g.:
- sodium > "soda"
- potassium > "potash"
- nickel > kupfernickel = Ger "Old Nick's copper"
Others are named from places, e. g.:
- yttrium, terbium, erbium, and ytterbium > Ytterby in Sweden
- polonium > Poland
- germanium > Germany
- americium > America
- californium and berkelium > University of California, Berkeley
Some are named from celestial bodies, e. g.:
- helium > Grk helios = Sun
- uranium > Uranus
- neptunium > Neptune
- plutonium > Pluto
Finally some were named after persons or mythological figures, e. g.:
- einsteinium > Albert Einstein
- fermium > Enrico Fermi
- nobelium > Alfred Nobel
- thorium > Thor
- promethium > Prometheus
If you look at the Periodic Table you will note that each element symbol has associated with it two numbers. First there
is the atomic number which runs from 1 for H to 109. Each element also has a second number associated with it called the atomic mass. Atomic masses
are in atomic mass units, amu's. An amu is about 1.66 X 10-24 grams. The unit amu is sometimes called a Dalton.
As mentioned before atoms of a given element, i. e. with the same atomic number, can come in different
masses. Thus carbon atoms commonly have masses of about 12 amu, 13 amu, or 14 amu. A mass of 12 is most common, followed by
13 and then 14. These different massed atoms are called isotopes. The difference in the masses is due to a difference in the structure of the atoms.
The atomic mass is the average mass of all the naturally occurring isotopes of this element. Thus if we take the
weighed average of naturally occurring carbon we get an average atomic mass of 12.011 amu
A molecule of a compound consists of a combination of atoms in a definite ratio. Thus it is obvious that one can represent the molecule using the elemental symbols.
One simply lists the symbols for the elements present and uses numbers to indicate the relative numbers of atoms in the molecule.
For instance water molecules consist of two hydrogen atoms and one oxygen atom. Thus we can symbolize water as H2O.
That is we list the elements and place a subscript number after the symbol to indicate the number of atoms of each. The number
one (1) is left out as being the understood default value. Table salt is NaCl. That is it consists of one sodium atom to each
The order of listing the elements is by conventions based on the class of compound. Usually you will note that metals (from the left side
of the periodic table) come before nonmetals (from the right side). Hydrogen usually comes first (e. g. HCl), although there are
exceptions (e. g. NH3). Organic compounds usually have the order C, H, O, N, and then other elements. Generally
it is simpler to learn the common compounds as you come to them.
Along with the formula for a compound we need a name for the compound. Some names are common or trivial.
For instance H2O is water, NaCl is salt.
However chemists usually use a more systematic method of naming compounds. The most common rules of nomenclature are those
standardized by the IUPAC. Thus chemists usually call NaCl sodium chloride. CO2 is carbon dioxide, FeCl3
is iron(III) chloride, C6H12O6 is glucose, etc..
The mass of a molecule, the molecular mass, is simply the sum of the masses of its component atoms. Thus the molecular mass of water,
H2O, is (2 X 1.01) + 16.00 = 18.02 amu.
The molecular mass of CO2 is 12.01 + (2 X 16.00) = 44.01 amu.
Many compounds do not exist as discrete molecules. For instance a crystal of salt consists of a lot of sodium ions combined
with an equal amount of chloride ions. Thus the formula NaCl indicates the ratio of sodium to chlorine atoms. However molecules
of one sodium with one chlorine do not exist. In such cases we can calculate a mass from the formula, 22.99 + 35.45 = 58.44
amu, which is called the formula mass.
There are therefore two kinds of formulas.
Molecular formulas indicate the numbers and kinds of atoms in discrete
e. g. H2O, CO2, CCl4.
Empirical formulas indicate the lowest common ratio of atoms
in the compound,
e. g. NaCl, CaCl2, CaO.
All compounds have an empirical formula but only molecular compounds have a molecular formula. Some compounds
have the same empirical and molecular formula, e. g. H2O, CO2. Others have empirical formulas different
from the molecular. For instance the compound benzene has a molecule consisting of 6 carbons and six hydrogens. Therefore
its molecular formula is C6H6. However the lowest common ratio of C to H is one to one. Therefore
benzene's empirical formula is CH.
When compounds are mixed together they may react to form other compounds. Whether or not they react depends on the chemical properties of the compounds and the conditions of reaction. We can represent a reaction by writing a chemical equation. For instance, under the right conditions
carbon will burn in oxygen to form carbon dioxide:
C + O2 ----> CO2
On the left are the reactants, the compounds reacting. On the right are the products, the compounds resulting from reaction.
These are separated by an arrow that indicates the direction of the reaction, reactants to products. Each component formula
of the reaction is preceded by a coefficient that indicates the relative numbers of molecules reacting. (A 1 is left out as
2H2 + O2 ----> 2H2O
Subscript symbols after a formula indicate the state of the compound. (s = solid, l = liquid, g = gas, aq = aqueous i.
e. dissolved in water):
CaCO3(s) + 2HCl(aq) ----> CaCl2(aq) + CO2(g) + H2O(l)
The conditions of the reaction and other information can be placed above or below the arrow:
N2 + 3H2 ----------> 2NH3
To be accurate, an equation must be balanced. Balancing an equation depends on the axiom of the atomic theory: Atoms cannot be created
or destroyed. Therefore the numbers of each kind of atom on the right
of the equation must equal those on the left.
This equation is not balanced:
C2H8 + O2 ----> CO2 + H2O
There are 2 C's, 8 H's, and 2 O's on the left and 1 C, 3 O's, and 2 H's on the right. Placing in the right coefficients
gives us a balanced equation:
C2H8 + 4O2 ----> 2CO2 + 4H2O
Confirm for yourself that the atoms balance now.
Types of Reactions:
Simple reactions can fall into four main classes
In a combination or synthesisreaction two compounds combine to form one product compound:
A + B ----> C
P2O5 + 3H2O ----> 2H3PO4
NH3 + HCl ----> NH4Cl
CaO + H2O ----> Ca(OH)2
A decomposition reaction is the reverse of a combination. One compound forms two or more compounds.
A ----> B + C
H2CO3 ----> CO2 + H2O
CaCO3 ----> CaO + CO2
KClO4 ----> KCl + 2O2
In a single replacement reaction an element replaces a similar element in one of its compounds:
A + B-C ----> B + A-C
Zn + CuSO4 ----> Cu + ZnSO4
Fe + 3AgNO3 ----> 3Ag + Fe(NO3)3
Cl2 + 2NaBr ----> Br2 + 2NaCl
Br2 + 2KI ----> I2 + 2KBr<
Another name for these reactions is single displacement.
An interesting aspect of single replacement is that they only go spontaneously in one direction. Thus zinc will replace
copper as shown above but copper will not replace zinc:
Cu + ZnSO4 ----> no reaction
Which of these reactions will go as written?
2Na + CaO ----> Ca + Na2O
Yes, since Na is above Ca.
3Cu + Fe2(SO4)3 ----> 2Fe + 3CuSO4
No, since Cu is below Fe.
Zn + 2HCl ----> H2 + ZnCl2
Yes, since Zn is above H.
Cu + H2SO4 ----> H2 + CuSO4
No, since Cu is below H.
In a double replacement or double displacement reaction there are two compounds that consist each of two parts. During the reaction these parts are exchanged:
A-B + C-D ----> A-D + C-B
Double replacement is often also called an exchange or metathesis (me-TATH-a-sis) reaction. The compounds reacting are often ionic compounds in solution which
are exchanging the ions.
An important aspect of double replacement is whether the reaction will actually go forward. Consider the reaction:
NaCl(aq) + KBr(aq) ----> NaBr(aq) + KCl(aq)
In aqueous solution, (aq), all four of these compounds are dissolved to form the free ions. Thus we can write the equation as:
Na+ + Cl- + K+ + Br- ----> Na+ + Br- + K+
If you examine the equation you will see that as long as all ions remain in solution, nothing is happening!
You have sodium ions, potassium ions, chloride ions, and bromide ions, and that's all! So no real change take
Consider if one of the products is insoluble:
NaCl(aq) + AgNO3(aq) ----> NaNO3(aq) + AgCl(s)
Sodium chloride, silver nitrate and sodium nitrate are all soluble. However, silver chloride is insoluble in water and
We can write the equation showing the dissolved ions:
<NA+ + Cl- + Ag+ + NO3- ----> Na+ + NO3-
in that all dissolved salts are shown as separate ions. Note that some ions are found unchanged on both sides of the equation.
These are called spectator ions. If we remove the spectators from the equation we get the real change during reaction, the net ionic equation:
Cl- + Ag+ ----> AgCl(s)
You can see that there is a net change in that silver ions and chloride ions come together to form solid silver chloride.
Thus the reaction goes forward. We say that the precipitation of the solid AgCl drives the reaction forward.
Basically anything that causes a net change in species will drive the reaction. Without that the reaction does not go.
There are four typical ways to drive an exchange reaction
- Precipitation, e.g.:
BaCl2(aq) + (NH4)2SO4(aq) ----> 2NH4Cl(aq)
- Formation of a gas, e.g.:
H2SO4(l) + NaCl(s) ----> NaHSO4(s) + HCl(g)
- Formation of a stable product, e.g.:
NaOH(aq) + HCl(aq) ----> NaCl(aq) + H2O(l)
- Decomposition of a product, e.g.:
Na2CO3(aq) + 2HCl(aq) ----> 2NaCl(aq)
H2CO3(aq) ----> H2O(l) + CO2(g)
Other unstable compounds like H2CO3 are:
- H2SO3 ----> H2O + SO2
- NH4OH ----> NH3 + H2O
Consider the equation:
P2O5 + 3H2O ----> 2H3PO4
P2O5 weighs 141.94 amu, H2O weighs 18.02 amu, and H2PO4 weighs 96.99
amu. According to the equation then:
1 molecule of P2O5 will react with 3 molecules of water to give 2 molecules of H2PO4
141.94 amu of P2O5 will react with 3 X 18.02 = 54.06 amu water to give 2 X 96.99 = 193.98 amu H2PO4
However no balance is precise enough to weigh out those small quantities. What is needed is a way to scale up easily from
the molecular level to the macro level of substances in beakers.
The number of species in a mole is called Avogadro's number, NA and is 6.02252 X 1023.
The molar mass in g/mole is numerically equal to the molecular mass or formula mass of a compound in amu. Molar mass can be used as a conversion factor in problems that relate mass to number of moles.
How many moles are there in 45.7 g of water?
mole 45.7 g water X ------- = 2.54 moles water
The mole is a bridge between the macro world of compounds and the micro world of atoms and molecules. One
macro property of compounds is their elemental composition. The corresponding micro property is the formula,
especially the empirical formula. Thus is should not be a surprise that the two are related by the concept of the mole. Given
the elemental percentage composition it is possible to calculate the empirical formula and visa versa.
The word stoichiometry refers to problems involving amounts of reacting substances and their relationships. Questions like:
- How much of one compound will react with a given amount of another?
- How much product can be made from a given amount of a reactant?
- How much reactant is needed to make a given amount of product?
- How much of one product will be produced along with a given amount of another product? involve stoichiometry.
These questions are a type of conversion problem. They use a stoichiometric
conversion factor derived from the coefficients in the balanced equation.
Each component in a reaction is related to each other component by this stoichiometric equivalence. We say that they are equivalent. Thus in the reaction:
2NaOH + H2SO4 ----> Na2SO4 + 2H2O
- 2 moles of NaOH are equivalent to 1 mole of H2SO4.
- 1 mole of Na2SO4 is equivalent to 2 moles of water.
- 2 moles of NaOH are equivalent to 2 moles of water.
- 1 mole of H2SO4 is equivalent to 1 moles of Na2SO4.
How many moles of Na2SO4 will be produced by reacting 3.66 moles of NaOH?
1 moles Na2SO4
3.66 moles NaOH X -------------- = 1.88 moles Na2SO4
2 moles NaOH
How many moles of H2SO4 are needed to react with 5.76 moles of NaOH?
1 mole H2SO4
5.76 moles NaOH X ------------ = 2.88 moles H2SO4
2 moles NaOH
How many grams of NaOH are needed to produce 35.8 g of Na2SO4?
1 mole Na2SO4 2 moles NaOH 40.00 g NaOH
35.8 g Na2SO4 X --------------- X ------------- X ------------
142.04 g Na2SO4 1 mole Na2SO4 1 mole NaOH
= 20.2 g NaOH
Note how we used the molar mass of NaOH and Na2SO4 to convert between grams and moles!
How many grams of water would be produced along with 25.9 g of Na2SO4?
1 mole Na2SO4 2 mole H2O
18.02 g H2O
25.9 g Na2SO4 X --------------- X ------------- X ----------
142.04 g Na2SO4 1 mole Na2SO4 1 mole H2O
= 6.57 g H2O
Things to Remember
- What are the chemical symbols for the elements?
- How are chemical formulas written?
- How are compounds named?
- What is a molecular formula?
- What is an empirical formula?
- What is a chemical equation and what does it tell you?
- How do I balance an equation?
- What are the basic classes of reactions?
- What is a mole and why is it useful?
- What is a molar mass and how is it used?
- How do I convert between percent composition and empirical formula?
- What is stoichiometry?
- How do I do a stoichiometric conversion?
1. Look up the chemical symbol, atomic number, and atomic mass of these elements:
2. Calculate the molecular or formula mass of these compounds:
3. Balance these equations:
C2H4 + O2 ----> CO2 + H2O
Fe + AgNO3 ----> Fe(NO3)3 + Ag
BaCl2 + K2SO4 ----> BaSO4 + KCl
CaO + H3PO4 ----> Ca3(PO4)2 + H2O
CH3COOH + Na2CO3 ----> NaCH3COO + CO2 + H2O
5. How many moles are there in 36.5 g of sodium carbonate?
6. When magnesium metal (0.69 g) was heated in air the oxide was formed. The oxide weighed 1.14 g. What is the empirical
formula for magnesium oxide?
7. Consider this reaction:
2Fe + 3CuCl2 ----> 2FeCl3 + 3Cu
a. How many moles of iron(III) chloride can be made from 0.0366 moles of copper(II) chloride?
b. How many grams of copper(II) chloride are needed to react with 0.337 moles of iron metal?
c. How many grams of iron are needed to replace all the copper in 4.78 g of copper(II) chloride?
d. If one makes 2.67 grams of copper with this reaction, how many grams of iron(III) chloride will also be produced?