UNIT 2 – Atomic StructureSection 1: History & Development of Atomic Theory

(Chapter 3)

History of the Atom Video

The Greeks

Democritus World made of empty space and tiny

particles (“atoms”)

Thought there were different types of atoms for every material

Problem: NO experimental evidence

Credited for 1st atomic theory

“Nothing exists except atoms and empty space; everything else is opinion.”

The Greeks

Aristotle Proposed that the world was continuous

and that there were no “tiny particles”

Very influential so his ideas were widely

accepted until the 18th century

Aristotle’s 4 element theory

Early Atomic Theory

JOHN DALTON Examined the work of others

Lavoisier (early 1700s):

Law of Conservation of Mass:

Mass is neither created nor destroyed during a

chemical reaction or physical change

Antoine Lavoisier

Lavoisier: The Law of Conservation of Mass

Law of Conservation of Mass

During a chemical change,

matter is neither created nor

destroyed.

The total mass of reactants =

total mass of products

Example Problems If 5.68 g of X combine with 3.66 g of Y, how

many grams of the compound XY are produced?

◦ X + Y XY

◦ 5.68 + 3.66 = XY

◦ XY = 9.34 g

If 3.7 g of element A combines with element B to

make 8.32 g of compound AB, how many grams

of B were needed?

◦ A + B AB

◦ 3.7 + B = 8.32

◦ B = 4.62 g

John Dalton continued

Dalton also examined the work of:

Proust:

Law of Definite Proportions

Specific substances always contain

elements in the same ratio by mass

(i.e. Salt (NaCl) always contains 39.34%

Na by mass & 60.66% Cl by mass)

Joseph Proust

Example Problem

Take the reaction A + B AB

◦ If 2 g of A combine with 5 g of B to form AB, how many grams of B are needed to combine with 9 g of A to form AB?

◦ 2 + 5 AB

◦ 9 + x AB

◦ 9/2 = 4.5; 4.5 * 5 = 22.5 g of B needed

◦ In the second reaction there are 4.5 times more A than in the first reaction, so multiple 4.5 * 5 (Grams of B in 1st reaction) to determine how many grams of B are needed

Early Atomic Theory

Based on others’ work, Dalton

developed:

Law of Multiple Proportions: If two

or more different compounds are

composed of the same two elements, the

mass of the second element combined

with a specific mass of the first element

can be expressed as a ratio of small

whole numbers.

In a nutshell..

when elements combine, they do

so in a ratio of small whole

numbers

carbon and oxygen react to form

CO or CO2, but not CO1.6

Law of Multiple Proportions

Carbon monoxide (CO): 12 parts by mass of

carbon combines with 16 parts by mass of

oxygen.

Carbon dioxide (CO2): 12 parts by mass of

carbon combines with 32 parts by mass of

oxygen.

Ratio of the masses of oxygen that combines

with a fixed mass of carbon (12 parts) 16: 32

or 1: 2

Continued…

Take an example of two mineral samples

iron pyrite (FeS2) and iron troilite (FeS).

Both contain iron and sulfur atoms.

However, for a given fixed amount of iron

it requires exactly twice the mass of

sulfur needed to make pyrite than that of

troilite with the same amount of iron.

John Dalton Cont…

Dalton also developed his own Atomic

Theory

See information on “Dalton vs. Modern

Atomic Theory”—you are responsible for

knowing all information on this chart!!!

Dalton’s Atomic Theory Modern Atomic Theory1. All matter is composed of small

particles called atoms.

1. All matter is composed of small

particles called atoms.

2. Atoms of a given element are

identical in size, mass and other

properties; atoms of different

elements differ in size, mass and

other properties.

2. Atoms of the same element have the

same chemical properties; while atoms

of different elements have different

chemical properties.

3. Atoms cannot be subdivided,

created or destroyed.

3. While individual atoms of a given

element may not all have the same mass,

any natural sample of an element will

have a definite average mass that is

characteristic of that element.

4. Atoms of different elements can

combine in simple, whole number

ratios to form chemical

compounds.

4. Compounds form when atoms of two

or more elements unite and each lose

their characteristic properties as a

result.

5. In chemical reactions, atoms are

combined, separated or

rearranged.

5. Atoms are not subdivided in physical or

chemical reactions.

Biggest Differences between

Theories Dalton stated all atoms of the same

element are identical; they are not

(isotopes and ions are the same element

with different numbers of neutrons or

electrons) BUT the same atoms have the

same chemical properties

Atoms cannot be subdivided/destroyed by

CHEMICAL means (Dalton said they

cannot be at all)