chapter 4: atoms and elements. recognize that all matter is composed of atoms. explain the early...
TRANSCRIPT
Section 1: Early Ideas about Matter
Chapter 4: Atoms and Elements
Learning GoalsRecognize that all matter is composed of atoms.
Explain the early ideas that led to the current understanding of the atom.
Atoms and Elements Atoms compose matter.
The properties of atoms determine the properties of matter.
An atom is the smallest identifiable unit of an element.
An element is a substance that cannot be broken down into simpler substances.
Atoms and Elements There are about 91 different elements found in nature,
and consequently about 91 different kinds of atoms.
Scientists have succeeded in making about 20 synthetic elements (not found in nature).
The exact number of naturally occurring elements is controversial because some elements previously considered only synthetic may actually occur in nature in very small quantities.
Early Ideas about MatterMany ancient scholars believed matter was composed of such things as earth, water, air, and fire.
Many believed matter could be endlessly divided into smaller and smaller pieces.
Early Ideas about MatterDemocritus (460–370 B.C.E.)
suggested that if you divide matter into smaller and smaller pieces, you end up with tiny, indestructible particles. Everything else is empty
space.
Early Ideas about MatterDemocritus called these particles “atomos” meaning “indivisible”
Democritus is the first person on record to have postulated that matter is composed of atoms.
Early Ideas about MatterAristotle (384–322 B.C.) disagreed with Democritus because he did not believe empty space could exist. Believed everything was composed of fire, air, earth, and water.
Laws Leading to Modern Atomic Theory
Law of Conservation of Mass: Antoine Lavoisier (1743-1794) In a chemical reaction, matter is neither created nor destroyed.
Laws Leading to Modern Atomic Theory
Joseph Proust (1754-1826): a French chemist who made observations on the composition of compounds.
In 1797, found that the elements composing a compound always occurred in fixed proportions.
Laws Leading to Modern Atomic Theory
Law of Definite Proportions: All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements.
2 H : 1 O
Laws Leading to Modern Atomic Theory
John Dalton (1766-1844): an English chemist who offered convincing evidence in support of Democritus. In 1804, published his law
of multiple proportions.
Laws Leading to Modern Atomic Theory
Law of Multiple Proportions: When two elements (A and B) form two different compounds, the masses of element B that combine with 1 g of element A can be expressed as a ratio of small whole numbers.
1 g Carbon 1 g Carbon
2.66 g Oxygen
1.33 g Oxygen
1.33 g Oxygen
2.66 g Oxygen = 1.0
0
2.00
Consistently getting small whole number ratios supports the idea that there is are indivisible
atoms that make up the compounds.
Dalton’s Atomic TheoryIn 1808 (over 2000 years after Democritus) John Dalton formalized a theory of atoms that gained broad acceptance.
Dalton’s Atomic TheoryDalton’s atomic theory has five parts:1. Each element is composed of tiny,
indestructible particles called atoms.
2. All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements.
Dalton’s Atomic Theory3. Atoms combine in simple, whole-
number ratios to form compounds.
4. Atoms cannot be subdivided, created, or destroyed
5. In chemical reactions, atoms are combined, separated, or rearranged.
Check-in:Compare and contrast the atomic theories of Democritus and Dalton. Mark an X under each name if a statement in the table applies to that person’s theory.
Statement Democritus Dalton
All matter is made of tiny particles.
Matter is made of empty space through which atoms move.
Atoms cannot be divided.
Atoms cannot be created.
Atoms cannot be destroyed.
Different atoms combine in whole-number ratios to form compounds.
Matter is composed of fire, air, water, and earth.