Grade 9 Science: Unit 1: Atoms, Elements, and Compounds

Chapter 3: Elements combine

to form 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 oneor 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 CH 4

C H H C H H H

Examples of Compounds Table sugar Carbon Dioxide Water Methane Covalent C 12 CH 4 H CO 2 H 2 O 22 O 11

Ionic Compounds  Atoms gain or lose electrons to form ions.

 All the the the positive ions attract all negative ions everywhere in 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 MgCl 2

Examples of Ionic Compounds

NaCl Sodium Chloride

CaCO 3 Na 2 SO 4 NaOH Calcium Carbonate Sodium Sulfite Sodium Hydroxide

Naming Compounds Every compound has a...

1.

Name : indicates the elements present in the compound 2.

Formula compound.

: indicates the symbols and ratio of each element present in the

Rules for Naming Compounds  Ionic Compounds

See page 81-2

 Covalent Compounds

See page 83 Complete practice problems pages 82 &83 on

Physical & Chemical Changes

Physical Changes:  the appearance of may have changes holding the atoms molecules and ions have not been broken and no new bonds have been made.

a substance but the bonds together in

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 others are broken.

while

 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.

3.

Heat, light, sound or consumed produced Gas bubbles released 4.

5.

A precipitate formed Difficult to reverse

Applications of Chemical Changes  Harnessing combustion Combustion amounts of releases large 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. 92-3

Observing Changes in Matter

CORE STSE: “

Plastics and Modern Life”