Chapter 3: Elements combine to form compounds Grade 9 Science: Unit 1: Atoms, Elements, and Compounds

Section 3.1: page 72 Compounds: A pure substance made of two or more kinds of elements combined in fixed proportions. Chemical bonds hold them together. Are either ionic or covalent.

Elements Combine to form... COMPOUNDS Ionic (Charged Ions) Covalent (Sharing)

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 CO 2 and water H 2 O.

Formed from non-metals ONLY. They do NOT conduct electricity. May be a solid, liquid, or gas at room temperature.

Example: Methane CH 4

Need to Know Molecular Compounds: NameChemical Formula SugarC 12 H 22 O 11 Carbon DioxideCO 2 MethaneCH 4 WaterH2OH2O

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.

SOLID All are SOLID at room temperature. High melting and boiling points. Will Will conduct electricity when melted or dissolved in water. salts Are also called “ salts ”.

Need to Know Ionic Compounds: NameChemical Formula Sodium Chloride (table salt) NaCl Calcium carbonate CaCO 3 Sodium sulfateNa 2 SO 4 Sodium hydroxideNaOH

sodium chloride Crystal lattice of NaCl showing the 1:1 ratio of positive sodium ions in the green with negative chloride ions in the blue. Each is surrounded by 6 ions of the opposite charge, as opposites attract

Section 3.2 : page 80 Names and Formulas of Simple Compounds

Chemical Compounds Every compound has a Formula: indicates the symbols and ratio of each element present in the compound. 2.Name: indicates the elements present in the compound

1. CHEMICAL FORMULAS CO 2 Symbol for Carbon Symbol for Oxygen Number of C atoms, absence of number means 1 atom present Number of Oxygen atoms, there are 2 oxygen atoms in this molecule

2. CHEMICAL NAMES Ionic Compounds NaCl sodium chloride Metal ion – appears first and name remains unchanged Non-metal ion – appears second, name looses its ending and gains suffix “ide” Even though it’s a name, it’s written in lower case letters

See page Know Names of Non-Metal Ions on Page 81 Table 3.1 Complete practice problems on pages 82

2. CHEMICAL NAMES Covalent Compounds CF 4 carbon tetrafloride first atom name remains unchanged second atom name looses its ending and gains suffix “ide” Prefixes are added to show the number of each atom No prefix is used on the first atom if there is only one atom

See page 83 Know Prefixes used in Naming Covalent Compounds on Page 83 Table 3.4 Complete practice problems on pages 83

Section 3.3 : page 86 Physical and Chemical Changes

PHYSICAL CHANGES: A change in which NO NEW Substances are formed and no new bonds have been made.

Includes: all changes in state (melting, evaporation, condensation, freezing) dissolving cutting separating 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.

Includes: Corrosion Fruit ripening Combustion Cooking Rusting Reacting Rotting

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

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.

Core Lab Activity 3-3C pg Observing Changes in Matter