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Chapter 3: Elements combine
to form compounds
Grade 9 Science:
Unit 1: Atoms, Elements, and Compounds
Compounds
A pure substance made of two or
more kinds of elements combined in
fixed proportions.
Represented by a chemical formula.
Chemical bonds hold them together.
Are either ionic or covalent.
Model of HCl ( hydrochloric acid)
Covalent Compounds
Atoms combine by sharing electrons to form molecules.
Molecules: a group of atoms held together by sharing one or more pairs of electrons.
Examples include carbon dioxide and water.
Formed from non-metals only.
They do not conduct electricity.
May be a solid, liquid, or gas at
room temperature.
Methane CH4
C H
C H
H
H
H
Examples of Covalent
Compounds
Table sugar C12H22O11
Carbon Dioxide CO2
Water H2O
Methane CH4
Ionic Compounds
Atoms gain or lose electrons to form ions.
All the positive ions attract all the negative ions everywhere in the same crystal.
Formed from metals and non-metals.
All are solid at room
temperature.
High melting and boiling points.
Will conduct electricity when
melted or dissolved in water.
Are also called “salts”.
Magnesium Chloride MgCl2
Examples of Ionic Compounds
NaCl Sodium Chloride
CaCO3 Calcium Carbonate
Na2SO4 Sodium Sulfite
NaOH Sodium Hydroxide
Naming Compounds
Every compound has a...
1. Name: indicates the elements present in the compound
2. Formula: indicates the symbols and ratio of each element present in the compound.
Rules for Naming Compounds
Ionic Compounds
See page 81-2
Covalent Compounds
See page 83
Complete practice problems on
pages 82 &83
Physical & Chemical Changes
Physical Changes:
the appearance of a substance may have changes but the bonds holding the atoms together in molecules and ions have not been broken and no new bonds have been made.
Includes:
all changes in state (melting,
evaporation, condensation,
freezing)
Dissolving
Cutting
Tend to be easy to reverse.
Chemical Changes:
Produce new substances with
new properties; may or may not
be noticeable.
New bonds are formed while
others are broken.
Chemical equations can be
written for all chemical changes.
For example the composition of
water.
In a chemical reaction the mass
of the reactants = the mass of the
products. The elements are
conserved but not the
compounds.
Ex. Corrosion, fruit ripening,
combustion
Evidence of a Chemical Change:
1. Color change
2. Heat, light, sound produced
or consumed
3. Gas bubbles released
4. A precipitate formed
5. Difficult to reverse
Applications of Chemical
Changes
Harnessing combustion
Combustion releases large
amounts of energy which can be
used to provide heat and light,
electrical and mechanical energy.
Solving the corrosion problem
(a process by which metals combine with oxygen; oxidation)
Using chemical change for traditional products
(example: tanning hides, making dyes and medicines from plants, and preserving food)
Core Lab Activity 3-3C pg. 92-3Observing Changes in Matter
CORE STSE:
“Plastics and Modern Life”