Transcript Chapter 3: Atoms and Compounds

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