elements: a pure substance that cannot be separated into
simpler substances
 all matter can be broken down in elements
 92 naturally occurring elements
 each element has a unique chemical name & symbol
o Symbol is 1-3 letters with first one capitalized
 elements are not equally abundant
o hydrogen makes up about 75% mass of universe
o oxygen & silicon together = 75% mass Earth’s
crust
o oxygen, carbon and hydrogen = 90% of human
body
periodic table: organizes the elements
 first devised in 1869 by a Russian chemist Dmitri
Mendeleev
 horizontal rows called periods
 vertical columns called groups or families.
 elements in the same group have similar chemical
and physical properties
compound: pure substance made up of two or more
different elements that are combined chemically
 most matter in the universe exists in the form of
compounds
 can be separated into simpler substances (elements)
 properties of compounds are different than the
elements they are made up of – water
law of definite proportions: compounds always
combine in definite proportions by mass no matter how
large or small the sample
 as you learned – mass of the compounds is always
equal to the mass of the elements that make the
compound
 relative mass of the compounds – percent by mass
percent by mass: the ratio of the mass of each element
to the total mass of the compound.
(What percent there will be of that element in the
compound – no matter how big or small of a sample)
percent by mass = mass of element/mass of
compound X 100
Example with water:
Water
18.0g
Hydrogen
2.0
+ Oxygen
16.0
Percent by mass of H in pure water = 2.0/18.0 x 100
= 11.11%
Percent by mass of O in pure water = 16.0/18 x 100 =
88.89%
As long as your sample of water is pure – you will
always get the same percent by mass of H & O
percent by mass of elements in sucrose in 2 different
samples – table 3.4, page 88
Problems:
Ex: a 78.0g sample of a compound contains 12.4g H,
What is the percent by mass of H in the compound?
Ex: If 3.5g of element X reacts with 10.5g of element
Y to form the compound XY, what is the percent by
mass of element X in the compound?
The percent by mass of element Y?
law of multiple proportions: When different
compounds are formed by a combination of the same
elements, different masses of one element combine with
the relative mass of the other element
 ratio express relationship of elements in the
compound
 can be simple whole number ratios of elements or
ratios of their masses
 example: Hydrogen Peroxide=H2O2 when
compared to Water=H2O has an oxygen ratio of 2:1
 necessary to make the great number of compounds
out of a finite number of elements
Copper + Chlorine law of multiple proportions (Table
3.5 p.89)
Compound Mass Cu
(g)
Mass Cl
(g)
I
II
35.80
52.73
64.20
47.27
Mass ratio
(mass
Cu/mass
Cl)
1.793
0.8964
Find the ratio of relative masses of copper in both
compounds.
mass ratio of Compound I = 1.793 = 2
mass ratio of Compound II 0.8964 1
element: pure substance that cannot be separated into
simpler substances by physical or chemical means
 92 naturally occurring on Earth
 each has unique name and symbol
periodic table: organizes elements into a grid that
groups similar properties
 arranged by proton number
 horizontal rows = periods
 vertical columns = families or groups
compound: made up of two or more different elements
combined chemically
 written as a combination of element symbols
(example: Water = H2O)
 can be broken down into components if supplied
with external energy (like electrolysis)
 properties are different than individual elements
Law of definite proportions: compound is always
composed of the same elements in the same proportion
by mass no matter how big or small the sample
percent by mass: ratio of mass of each element to the
total mass of the compound (units = %)
percent by mass = mass of element x100
mass of compound
Example: Sucrose (table sugar) is composed of
carbon, hydrogen, and oxygen. Analysis of a 20.00 g of
sucrose showed it was made up of 8.44 g of Carbon,
1.30 g of Hydrogen, and 10.26 g of Oxygen. Find the
percent by mass of all three elements.
Answer: percent by mass = mass of element
mass of compound
Carbon: (8.44/20.00) x 100 = 42.2%
Hydrogen: (1.30/20.00) x 100 = 6.50%
Oxygen: (10.26/20.00) x 100 = 51.30%
x100