CHEM 241 Chapter 2 Atoms molecules and ions.pdf - Chapter 2...

Chapter 2
Atoms, Molecules and
ions

2.1 The Early History of Chemistry
• Greeks were the first to attempt to explain why
chemical changes occur.
• Alchemy dominated for 2000 years.
 Several elements discovered.
 Mineral acids prepared.
• Robert Boyle was the first “chemist”.
 Performed quantitative experiments.
 Provided evidence for the atoms and defined
the nature of an element.

2.2 Fundamental Chemical Laws
• Law of conservation of mass (Lavoisier):
 Mass is neither created nor destroyed.
Three Important Laws
• Total mass of reactants = Total mass of products
mercury + oxygen  mercury (II) oxide
2.53 g ( ? ) 2.73 g

• Law of definite proportion (Proust):
 A given compound always contains exactly the
same proportion of elements by mass.
Three Important Laws (continued)
 1.000 g NaCl : 0.3934 g Na + 0.6066 g Cl
 100 g H
2
O: 11 g H + 89 g O

• Law of multiple proportions (Dalton):
 When two elements form more than one
compound, the masses of one element in theses
compounds for a given mass of other element
are in ratio of small whole numbers.
C and O in
CO (1.000 g C and 1.332 g O)
CO
2
(1.000 g C and 2.664 g O)
The mass of O in CO
2
is exactly twice of that in CO
Three Important Laws (continued)

2.3 Dalton’s Atomic Theory
• All mater is composed of atoms (H, O, Cl,….)
Dalton’s Atomic Theory ( 1808)
• An element : is a type of matter composed of only
one kind of atom (He, H
2
, O
2
, Ag,….)
• A compound : is a type of matter composed of atoms
of two or more elements chemically combined in
fixed proportions (H
2
O, CO
2
, SiO
2
, CaCO
2
,….)

• A chemical reactions: consists of the rearrangement
of atoms present in the reacting substances to give
new chemical combinations in the products
C
3
H
8
+ 5O
2
 3CO
2
+ 4H
2
O
(atoms are not created or destroyed by chemical reaction)
Dalton’s Atomic Theory (continued)

• Gay — Lussac
– Measured (under same conditions of T and P) the
volumes of gases that reacted with each other.