Chapter 3
Sec. 3.4: Elements & Compounds
Objectives
• Distinguish between elements and
compounds.
• Describe the organization of elements on
the periodic table.
• Explain how all compounds obey the laws
of definite and multiple proportions.
Recall . . .
Matter that has a uniform,
unchanging composition
Anything that has mass
and takes up space
Physical combination of
2 or more substances
Matter
Substance
Element
Mixture
Compound
Mixture with a constant
composition throughout
Homogeneous
Heterogeneous
Mixture that is not blended
smoothly throughout.
Elements
• An element is a pure substance that cannot
be separated into simpler substances by
physical or chemical means.
• They are considered to be the basic building
blocks of all matter.
• 91 occur naturally; the rest of the elements are
synthetic.
• Each element has a unique symbol consisting
of 2-3 letters: the first is always uppercase &
the rest are always lowercase.
Stands for the
element carbon
Stands for the
element lead
Stands for the
element thorium
The Periodic Table
• The periodic table organizes the elements
into a grid.
• The vertical columns in the grid are called
groups or families.
• Elements in the same group have similar
chemical and physical properties.
The Periodic Table
• The horizontal rows in the periodic table are
called periods.
• The table is called periodic because the pattern of
similar properties repeats from period to period.
Compounds - 4 part Definition
 A compound is a chemical combination
of 2 or more elements. (There are
chemical bonds between the elements a reaction has occurred between them.)
Elemental symbols
make it easy to write the
formula for a compound.
Butane, pictured here,
has 4 carbon atoms and
10 hydrogen atoms. Its
formula is C4H10.
Compounds - 4 part Definition
 The properties of a compound are different
from those of its component elements.
Sodium chloride
Compounds - 4 part Definition
 The composition of a compound is fixed. (The
elemental components of a compound are
always in the same ratio. Water, for example,
always has 2 hydrogen atoms and 1 oxygen
atom.)
Compounds - 4 part Definition
 Compounds can only
be broken down into
their component
elements by
chemical means.
(The bonds between
the elements must
be broken by the
addition of external
energy.)
The electrolysis of water
Law of Definite Proportions
(also called the Law of Constant Composition)
• Recall that the elemental components of a
compound are always in the same, fixed
ratio.
• This means that the elements are in a
definite proportion by mass.
• Summarized as the Law of Definite
Proportions, scientists observe that a
compound is always composed of the
same elements in the same proportion by
mass.
Percent Composition
• When the law is applied to compounds, the
sum of the masses of the elements that
make up the compound is equal to the
mass of the compound.
• Water, H2O, contains 2 H atoms (each has a
mass of 1.008 amu) and 1 O atom (with a
mass of 15.999 amu); the mass of 1
molecule of water is, therefore, 18.015 amu.
Percent Composition
• The law also shows us that the ratio of the
mass of each element in a compound to
the total mass of the compound will never
change.
• The ratio described above is called the
percent by mass or percent composition
of a compound. It is summarized by the
formula:
percent by mass = mass of element x 100
mass of compound
Samples of water from any source will always have the
same mass proportions. A different proportion means
it’s a different compound!
Practice Problems
• If 3.5 g of X reacts with 10.5 g of Y to form XY,
what is the percent by mass of X in the
compound? Of Y?
• If 150 g of XY are obtained, how many grams
are X? How many grams are Y?
• Compound 1 contains 15.0 g of hydrogen
and 120.0 g of oxygen. Compound 2
contains 2.0 g of hydrogen and 32.0 g of
oxygen. Are the compounds the same?
Law of Multiple Proportions
• Compounds composed of the same
elements CAN be different compounds IF
they have different mass compositions.
• The law of multiple proportions says that
when different compounds are formed by
a combination of the same elements,
different masses of one element combine
with the same relative mass of the other
element in a ratio of small whole numbers.
Law of Multiple Proportions
• H2O (water) and H2O2 (hydrogen peroxide)
both contain H and O.
• In water, the ratio of hydrogen to oxygen is
2/1; in hydrogen peroxide, the ratio is 2/2.
• The mass ratio, then, of water to hydrogen
peroxide is 2/1 or 2/1. This is a small, whole
2/2
number ratio!
Another example
• Compound 1 contains 64.2% Cu and 35.8%
Cl; compound 2 contains 47.27% Cu and
52.73% Cl.
• In compound 1, the Cu/Cl ratio is 64.2 or
35.8
1.79; in compound 2, it is 0.8965.
• The mass ratio of Cu to Cl in compound 1 to
compound 2 is 1.79/0.8965 or 2.00. Again,
this is a small, whole number ratio!
Practice Problem
Complete the table and then analyze the data
to determine if
• Compounds 1 and 2 are the same
compound
• If they are not, find the small whole number
ratio that iron combines to the same relative
mass of oxygen.
Compound Total Mass
(g)
Mass Fe
(g)
Mass O (g)
1
75.0
52.5
22.5
2
56.0
43.5
12.5
Mass % Fe
Mass % O