Elements vs. Compounds

ELEMENTS AND
COMPOUNDS

Lesson 2.3 Elements and Compounds

proportion: the relation of one part to another or to the whole with respect to quantity

• Distinguish between elements and compounds.
• Describe the organization of elements in the periodic

table.

• Explain how all compounds obey the laws of definite and

multiple proportions.

A compound is a combination of two or more elements.

element

periodic table

compound

law of definite proportions

percent by mass

law of multiple proportions

Lesson 2.3 Elements and Compounds

Elements

• An element is a pure substance that cannot be

separated into simpler substances by physical or
chemical means.

• 92 elements occur naturally on Earth.

• Each element has a unique name and a one, two, or

three-letter symbol.

• The periodic table organizes the elements into a

grid of horizontal rows called periods and vertical
columns called groups.

• A compound is a made up of two or more

elements combined chemically.

• Most of the matter in the universe exists as

compounds.

• Table salt, NaCl, and water, H2O, are

compounds.

Compounds

• Elements can never be separated.
• Compounds can be broken into components

by chemical means.

Compounds

Open Ended

Define element and compound.

• This figure shows

electrolysis of water to
form hydrogen and
oxygen.

Compounds

Open Ended

Explain the process of electrolysis.

• The properties

of a compound are different from its component elements.

Compounds

Open Ended

Summarize how the properties of a compound and the properties of its component elements compare.

• The law of definite proportions states

that a compound is always composed of
the same elements in the same proportion
by mass, no matter how large or small the
sample.

Law of Definite Proportions

• The relative amounts are expressed as percent

by mass, the ratio of the mass of each element
to the total mass of the compound expressed as
a percentage.

Law of Definite Proportions

• This table demonstrates that the percentages of

elements in sucrose remain the same despite
differences in sample amount.

Law of Definite Proportions

Open Ended

State the law of definite proportions in your own words.

1. A 78.0-g sample of an unknown compound contains 12.4g of

hydrogen. What is the percent by mass of hydrogen in the compound?

Practice Problems

2. 1.0 g of hydrogen reacts completely with 19.0 g of fluorine.

What is the percent by mass of hydrogen in the compound that is formed?

Practice Problems

3. If 3.5 g of element X reacts with 10.5 g 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?

Practice Problems

4. Two unknown compounds are tested. Compound I contains

15.0 g of hydrogen and 120.0 g of oxygen. Compound II
contains 2.0 g of hydrogen and 32.0 g of oxygen. Are the
compounds the same? Explain your answer.

Practice Problems

5. All you know about two unknown compounds is that they

have the same percent by mass of carbon. With only this
information, can you be sure the two compounds are the
same? Explain.

Practice Problems

•

The law of multiple proportions states 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 whole
number ratios.

H2O2 and H2O

Copper(I) chloride and copper(II) chloride

Law of Multiple Proportions

Open Ended

State the law of multiple proportions in your own words

Law of Multiple Proportions

Copper reacts with chlorine under different sets of
conditions to form two different compounds.

Multiple Choice

An element is a substance that cannot be

• broken down into simpler substances by chemical means
• transformed into different elements
• created from simpler substances
• broken down into simpler elements

Multiple Choice

What is a period on the periodic table of the elements?

• A period is a group of elements with similar properties.
• A period refers to the atomic number of an element.
• A period is a horizontal row of elements in the periodic table.
• A period is a vertical column of elements in the periodic table.

Open Ended

Compare and contrast elements and compounds.

Open Ended

Describe the basic organizational feature of the periodic table of the elements.

Lesson 2.3 Elements and Compounds

Key Concepts

• Elements cannot be broken down into simpler

substances.

• Elements are organized in the periodic table of the

elements.

• Compounds are chemical combinations of two or more

elements and their properties differ from the properties
of their component elements.

Lesson 2.3 Elements and Compounds

Key Concepts

• The law of definite proportions states that a

compound is always composed of the same
elements in the same proportions.

• The law of multiple proportions states that if elements

form more than one compound, those compounds will
have compositions that are whole-number multiples of
each other.