Dalton’s Atomic Theory, Experiments, and Modern Atomic Model Overview

Dalton's Atomic Theory - Rule 1

All matter is made up of tiny, indivisible particles called atoms.

Dalton's Atomic Theory - Rule 2

All atoms of a given element are identical in mass and properties.

Dalton's Atomic Theory - Rule 3

Atoms of different elements are different from one another.

Dalton's Atomic Theory - Rule 4

Atoms combine in simple, whole-number ratios to form compounds.

Dalton's Atomic Theory - Rule 5

Chemical reactions involve the rearrangement of atoms; atoms are not created or destroyed.

Correction to Rule 1

Atoms are divisible — they contain subatomic particles (protons, neutrons, electrons).

Correction to Rule 2

Atoms of the same element can differ in mass — these are called isotopes.

Overall Modern Update to Dalton's Theory

Matter is composed of atoms with subatomic structure; atoms of an element share the same number of protons but may differ in neutrons and electrons.

J.J. Thomson (Experiment)

Cathode Ray Tube Experiment.

J.J. Thomson (Discovery)

Discovered the electron, a small, negatively charged particle inside atoms.

Thomson's Atomic Model

"Plum Pudding Model" — electrons embedded in a positive sphere.

Ernest Rutherford (Experiment)

Gold Foil Experiment.

Ernest Rutherford (Discovery)

Discovered the nucleus — atoms are mostly empty space with a dense, positively charged center.

Rutherford's Atomic Model

Electrons orbit around a small, dense nucleus.

Robert Millikan (Experiment)

Oil Drop Experiment.

Robert Millikan (Discovery)

Measured the exact charge and mass of an electron.

James Chadwick (Experiment)

Beryllium Radiation Experiment.

James Chadwick (Discovery)

Discovered the neutron, a neutral particle in the nucleus.

Dalton vs Modern Atomic Theory

Dalton said atoms were indivisible and identical; modern theory says they contain subatomic particles and can differ as isotopes.

Law of Conservation of Mass

Mass is neither created nor destroyed in a chemical reaction.

Law of Definite Proportions

A compound always has the same elements in the same ratio by mass.

Law of Multiple Proportions

When two elements form multiple compounds, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios.

Gold Foil Experiment - Conclusion

Atoms are mostly empty space; the nucleus is small, dense, and positively charged.

Cathode Ray Tube - Conclusion

Electrons are small, negatively charged particles found in all atoms.

Oil Drop Experiment - Conclusion

Determined the charge and mass of the electron.

Neutron Discovery - Importance

Explained isotopes and why atomic mass > number of protons.