Unit 4 Chemistry - The Atom

Unit 4 - The Evolution of the Atom

Early Greek Theories

440 B.C. - Democritus proposed that matter is made of extremely small particles that cannot be divided further. He named these particles “atoms,” from the Greek word "atomos" meaning “indivisible.”

350 B.C. - Aristotle rejected his idea and argued that matter is continuous (not made of particles). He claimed matter consisted of the 4 elements (earth, fire, water, and air). We know that he was wrong, but he was so influential that this theory remained accepted for 2,000 years.

The "Enlightenment"

Fast forward to the 1700s - Chemists believed elements were to be defined as substances that cannot be further broken down by ordinary chemical means.

• They also noticed that when elements join together to form compounds (through a process called a chemical reaction to produce one or more new substances), the new substances would have different physical and chemical properties than the elements they came from.

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​NaCl

During those days...

Chemists were aware of the formation of compounds, but at this time they weren’t sure if elements always combined in the same ratio to make a specific compound or if the combination was random.

• In the late 1700s, scientists began to analyze those chemical reactions. This led to the discovery of several basic laws to help provide an answer to their questions.

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One of the first was the law of conservation of mass, verified by Antoine Lavoisier (noted as the Father of Chemistry) and Marie-Anne (his wife) in 1789 via experimentation and mathematics. The law states that matter (or mass) is neither created nor destroyed during ordinary physical or chemical changes.

Antoine Lavoisier

In 1799, Joseph Proust, another French Chemist, gave us another law.

• Originally it was called Proust’s Law, but is now known as the Law of Definite Proportions (or principle of constant composition).

  • It states that a compound always contains the same elements in the same proportions by mass regardless of the size of the sample or source of the compound.

Joseph Proust

The Law of Definite Proportions

Proust’s discovery inspired John Dalton, an English school teacher, to think about atoms as the basic particles that make up elements.

• In 1803 Dalton noticed that some elements can combine in more than one way to form completely different compounds.

  • This led to his Law of Multiple Proportions.

John Dalton

The Law of Multiple Proportions

The law of multiple proportions states that when the same two elements make different compounds, the amounts of one element that join with the other are always in small whole numbers.

• For example, carbon and oxygen can combine to form CO and CO₂, each with a different ratio of carbon to oxygen.

In 1808, John Dalton proposed an explanation for the three laws (Atomic Theory).

1. ​​All matter is composed of extremely small particles called atoms.

   • 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 cannot be subdivided, created, or destroyed.

3. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.

4. In chemical reactions, atoms are combined, separated, or rearranged.

Atomic Theory

Proposed that

1. All matter made of tiny atoms.

2. Atoms of the same element are identical.

3. Atoms combine in whole-number ratios.

   Atoms cannot be created, divided, or destroyed.

Dalton's Atomic Theory

Proposed that matter is made of tiny, indivisible particles called “atomos.”

Democritus

RECAP - Early Atomic Ideas

• There are a few limitations:

  • Atoms CAN be divided (subatomic particles)

    • This means the idea that the atom is one solid sphere is incorrect (discrediting Democritus and Dalton's theories) .

  • Isotopes DO exist

    • This means Dalton's idea that atoms of one element are exactly the same in mass and properties is also incorrect.

      • Not all atoms are 100% identical; they can vary in atomic mass.

What's wrong with their ideas?