Summarizing the Evolution of Atomic Theory

Summary of lessons so far

Early Ideas in Atomic Theory

The ancient Greeks proposed that matter consists of extremely small particles called atoms. Dalton postulated that each element has a characteristic type of atom that differs in properties from atoms of all other elements, and that atoms of different elements can combine in fixed, small, whole-number ratios to form compounds. Samples of a particular compound all have the same elemental proportions by mass.

When two elements form different compounds, a given mass of one element will combine with masses of the other element in a small, whole-number ratio. During any chemical change, atoms are neither created nor destroyed.

Evolution of Atomic Theory

Although no one has actually seen the inside of an atom, experiments have demonstrated much about atomic structure. Thomson’s cathode ray tube showed that atoms contain small, negatively charged particles called electrons. Millikan discovered that there is a fundamental electric charge—the charge of an electron.

Rutherford’s gold foil experiment showed that atoms have a small, dense, positively charged nucleus; the positively charged particles within the nucleus are called protons. Chadwick discovered that the nucleus also contains neutral particles called neutrons. Soddy demonstrated that atoms of the same element can differ in mass; these are called isotopes.

Glossary of Words

Alpha particle (α particle)

positively charged particle consisting of two protons and two neutrons

Dalton’s atomic theory

set of postulates that established the fundamental properties of atoms

Electron

negatively charged, subatomic particle of relatively low mass located outside the nucleus

Isotopes

atoms that contain the same number of protons but different numbers of neutrons

Law of constant composition

(also, law of definite proportions) all samples of a pure compound contain the same elements in the same proportions by mass

Law of definite proportions

(also, law of constant composition) all samples of a pure compound contain the same elements in the same proportions by mass

Law of multiple proportions

when two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small whole numbers

Neutron

uncharged, subatomic particle located in the nucleus

Nucleus

massive, positively charged center of an atom made up of protons and neutrons

Proton

positively charged, subatomic particle located in the nucleus