7.2: Putting Atoms Together
pg. 248
Molecules
Molecule: tow or more atoms that are chemically joined.
Subscript: is the small number that follows the symbol for
an element in a chemical formula and shows how many
atoms of the element are present.
Most substances in the world are made up of molecules, a
group of atoms combined together through chemical bonds,
chemically. (O2, CO2, H2O)
Oxygen gas consists of two atoms of oxygen, the chemical
formula for oxygen gas is O2. The formula determines the
number and type of atoms that make up the molecule. The
‘O’ represents toe symbol of the atom, and the subscript 2
represents the number of atoms that make up the molecule.
Therefore oxygen gas is made up of two atoms of oxygen.
Ozone is a molecule made up of 3 atoms of oxygen with a
formula of O3.
Figure 2: a) An oxygen molecule (O2) consists of two
oxygen atoms joined together. b) An ozone molecule (O3)
consists of three oxygen atoms.
Molecular Elements
Oxygen is an element but in nature it combines with
another oxygen atom to form a molecule. There only 7
elements that form molecules made up of 2 of the same
atoms, these are called diatomic molecules. These are; H2,
N2, O2, F2, Cl2, Br2, and I2.
Compounds
Compound: is a pure substance made up of two or more
elements that are chemically joined.
Most pure substances occur when 2 or more different
elements combine. Water consists of 2 different types of
atoms, hydrogen and oxygen, H2O. Therefore water is
known as a compound. The formula for water indicates that
there are 2 hydrogen atoms (subscript 2) and 1 oxygen
atom. The subscript is not included in the formula, if there
is a symbol, it must represent at least 1 atom, therefore the
number 1 is not required.
Figure 4: a) Water (H2O) is made of two hydrogen (H)
atoms and one oxygen (O) atoms. b) Carbon dioxide (CO2)
is made of one carbon (C) atom and two oxygen (O) atoms.
c) Nitric oxide (NO) is made of one nitrogen (N) atom and
one oxygen (O) atom.
Interpreting Formulas
What does a chemical formula tells you?
a) The symbols in the formula tell you the elements in
the substance.
b) If there is only one symbol, the substance is an
element.
c) If there are more than one symbol, the substance is a
compound.
d) The subscripts tell you how many atoms of each
element are in each molecule of the substance.
Check Your Learning:
Questions 1 – 8, pg. 251
Wrap Up:
- Elements are pure substances that contain only one type
of atom. They are represented by the chemical symbols
found in the periodic table.
- Compounds are pure substances made up of two or more
elements.
- Atoms can chemically join together to form larger units
called molecules.
- Molecules may contain atoms of different elements or
atoms of the same element.
- Chemical formulas state the type and number of atoms in
each molecule of a pure substance.