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”