Unit B

Atoms, Electrons and the Periodic TableAtoms, Electrons and the Periodic Table

B.1 A Look Inside Matter Since the days of the ancient Greeks people

have wondered about matter: Is matter capable of being broken into

infinitely small particles or is there a basic building block – a simplest particle to an element? (The word atom comes from the Greek word átomos meaning “indivisible.”)

How can the properties of matter be explained?

B.1 A Look Inside Matter The following sections will help to explain

why elements have the properties that are observed.

The Atomic Theory will show how the facts are easily understood by using various models that have been refined over the years as new observations are discovered.

B.2 Models of the Atom

What were the facts available around the early 1800s? The combined mass of all the substances

involved in a physical or chemical change remains constant—the Law of Conservation of Mass.

Each element has a unique mass different from all other elements.

Elements react in specific mass ratios—the Law of Definite Composition. Compounds have a specific composition no matter how they are prepared.

CompoundCompoundMass of Mass of

HydrogenHydrogenMass of Mass of OxygenOxygen

Mass of Mass of CompoundCompound

Mass Ratio of Mass Ratio of ElementsElements

Percent CompositionPercent Composition

Water 2 g 16 g 18 g

Hydrogen Peroxide

2 g 32 g 34 g

2 g H 1 g H =

16 g O 8 g O

2 g H 1 g H =

32 g O 16 g O

2

2 g H100 = 11% H

18 g H O

2

16 g O100 = 89% O

18 g H O

2

2 g H100 = 6% H

34 g H O

2

32 g O100 = 94% O

34 g H O

B.2 Models of the Atom

CompoundCompoundMass of Mass of

HydrogenHydrogenMass ofMass ofOxygenOxygen

Mass Ratio of Oxygen Mass Ratio of Oxygen (constant amount of hydrogen)(constant amount of hydrogen)

Mass Ratio of Hydrogen Mass Ratio of Hydrogen (constant amount of oxygen)(constant amount of oxygen)

Water 2 g 16 g

Hydrogen Peroxide

2 g 32 g

1 g 16 g

When two elements combine to form more than one kind of compound, the ratio of masses of the one element that combine with a constant mass of the other element can be expressed in small whole numbers—the Law of Multiple Proportions.

water 16 g O 1 = =

peroxide 32 g O 2

water 2 g H 2 = =

peroxide 1 g H 1

B.2 Models of the Atom

Dalton ModelDalton Model Elements are made up of indivisible,

spherical particles called atoms.

Figure B1 – “Billiard Ball” Model

B.2 Models of the Atom

Additional Assumptions (Dalton Model):

Only atoms can work together in a reaction. Compounds contain two or more atoms

joined together. Chemical reactions consist of rearranging

atoms.

Atoms of one substance are chemically alike and have the same mass.

B.2 Models of the Atom

How did Dalton’s Model satisfy the known observations? Mass stays constant because there are the

same number and kind of atoms before and after a reaction:

Atom Exchange During a Reaction

B.2 Models of the Atom

If only whole atoms participate in a reaction and atoms have a specific mass, then the mass ratio of the elements must be constant.Water 2 g H(H2O) 16 g O

Hydrogen Peroxide 2 g H(H2O2) 32 g O

B.2 Models of the Atom