Elements, Compounds, and Mixtures – song Chapter 3 – lesson 2

Compounds - objectives Explain how elements make up compounds. Describe the properties of compounds. Explain how a compound can be broken down into its elements. Give examples of common compounds.

Introducing “Compounds” I am also a PURE SUBSTANCE. I am made up of a fixed number of two or more different elements chemically combined.

I can be represented as a “chemical formula” “Compound” Property #1 I can be represented as a “chemical formula” The different elements present in a compound are combined in a fixed ratio General Rules: - Only write the name of the metallic element first - Write the number of atoms of any element in the compound in subscript (applies to only 2 or more atoms) - “ide” will always be written at the end of the non metal element

I am formed by atoms of different elements. “Compound” Property #2 I am formed by atoms of different elements. However, I do not have the same properties as them. Examples Na + Cl2  NaCl (s) (g) (s) H2 + O2  H2O (g) (g) (l)

“Compound” Property #3 2H2O (l)  2 H2 (g) + 2 O2 (g) Electrolysis (breaking down of compound by electricity) 2H2O (l)  2 H2 (g) + 2 O2 (g) Thermal Decomposition (breaking down of compound by heat) Because of my chemical bonds, I cannot be broken down by physical means. I can only be broken down by chemical reaction or electricity.

What is the difference between a compound and a molecule? Video Clip A molecule is formed when two or more atoms join together chemically. A compound is a molecule that contains at least two different elements. All compounds are molecules, but not all molecules are compounds.

EXAMPLES Molecular hydrogen (H2), molecular oxygen (O2) and molecular nitrogen (N2) are not compounds because each is composed of a single element. Water (H2O), carbon dioxide (CO2) and methane (CH4) are compounds because each is made from more than one element.

heat, light and explosion heat, light and explosion Making compounds from their elements Example: Making water (picture) lighted splint ELEMENTS COMPOUND hydrogen (colorless gas) oxygen (colorless gas) heat, light and explosion water (colorless liquid) Example: Making water (models) mixture of hydrogen and oxygen water heat, light and explosion Water molecule Hydrogen molecule Oxygen molecule

Making compounds from their elements Example: Making iron sulfide compound FeS compound S elements Fe heat sulfur + iron  iron (II) sulfide synthesis + Yellow solid Low melting pt. Grey solid magnetic Black solid High Melting pt.

write chemical formula Rule 1 For many compounds that contain both metallic and non-metallic elements, the symbol of the metallic element is written first. • calcium oxide (CaO) • sodium chloride (NaCl) • magnesium carbonate (MgCO3)

write chemical formula Rule 2 The number of atoms is written as a subscript, to the right of the atom’s symbol. • water (H2O, not H2O or 2HO) • magnesium carbonate (MgCO3, not MgCO3 or MgC3O)

Write chemical formula Rule 3 It is not necessary to write the subscript ‘1’. • water (H2O, not H2O1) • calcium oxide (CaO, not Ca1O1)

write chemical formula Rule 4 When writing formulas for ALL nonmetal elements, the order of the elements follows the structure of the molecule (AP chem) Usually, Oxygen is written at the end of the formula • water (H2O, not OH2) • carbon dioxide (CO2, not O2C) glucose (C6 H12O6)

Summarizing A compound is made up of two or more elements chemically joined together A compound has a fixed composition, Law of Definite Ratios Every compound has a unique chemical formula (rules)

summarizing A compound has a completely different properties from its elements!!!! A chemical reaction (decomposition, heat, acid, or electrolysis) is needed to separate the elements in the compound

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