Section 3.2

Atoms and Compounds

Objectives

1. To understand and illustrate the Law of Constant Composition

2. To learn about Dalton’s theory of atoms

3. To learn how a formula describes a compound’s composition

Dalton Intro

Section 3.2

Atoms and Compounds

Law of Constant Composition (1799 Joseph Louis Proust)

• A given compound always contains the same proportion by mass of the elements of which it is composed. A mixture can have variable composition but the

composition of a compound is fixed Does this give us a clue about the nature of matter?

Section 3.2

Atoms and Compounds

You have 90 seconds to memorize these 5 parts of the theory!!

Dalton’s Atomic Theory (John Dalton 1766-1844)

Dalton’s Atomic Theory states:

− All elements are composed of atoms

− All atoms of a given element are identical

− Atoms of different elements are different− Compounds consist of combinations of atoms of different elements− Atoms are not created or destroyed in a chemical reaction

Section 3.2

Atoms and Compounds

Formulas of Compounds

• A compound is represented by a chemical formula in which the number and kind of atoms present is shown by using the element symbols and subscripts.

Example: the simple sugar, glucose

Section 3.2

Atoms and Compounds

Put your notes away and write a set of rules for how to write a formula of a compound. Be as complete as you can be using what you have learned throughout your course

Section 3.2

Atoms and Compounds

B. Formulas of Compounds

1. Each atom present is represented by its element symbol.

2. The number of each type of atom is indicated by a subscript written to the right of the element symbol.

3. When only one atom of a given type is present, the subscript 1 is not written.

4. Write a metallic element first if present (MgCl2)

5. Write the central atom of a molecule first (PCl5)

6. Write Carbon first and Hydrogen second for an organic molecule (C6H12O6)

Section 3.2

Atoms and Compounds

Formulas of compounds

• Write the formulas for the following compounds:– A phosphorus atom bonded to three chlorine atoms– A molecule containing two boron atoms and six

hydrogen atoms– A compound containing one calcium atom for every

two chlorine atoms– Four hydrogen atoms bonded to a single carbon atom– A compound containing two iron atoms for every three

oxygen atoms– A molecule of aspirin (acetylsalicylic acid – four

oxygen atoms, eight hydrogen, nine carbon atoms)

Section 3.2

Atoms and Compounds

Aspirin, acetylsalicylic acid, C9H8O4

Aspirin in MotionElement Hangman