Page 252 - Science Course 1 (Book 1)

Mo6-L7b: What are Substances and Compounds?

A molecule is two or more atoms that are held
together by chemical bonds and act as a unit. Key Concept

What are Substances and Compounds?
In some elements, the atoms are grouped into
molecules How do elements and compounds differ?

Individual atoms Molecules

A compound is a substance made of two or more
elements that are chemically joined in a specific
combination.

Because each compound is made of atoms in a
specific combination, a compound is a substance

Molecules of a Compound

O N C
N O H H O O O O

NO H2O NO2 CO2
nitrogen water nitrogen carbon
oxide dioxide dioxide
A chemical formula includes the symbols of each
element in the compound and small numbers, called
subscripts, that show the ratio of the elements in the
Many compounds exist as molecules, but some, such compound.
as table salt, do not.
Sometimes the same elements combine to form
Sugar particles are molecules because they always different compounds.
travel together as a unit. Salt particles do not travel
together as a unit
Na2 SO4

One sulfur
atom
Sugar in water Two sodium Four oxygen
atoms atoms

Salt in water

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