LESSON 2: ATOMS FROM THE EYES OF PHILOSOPHERS AND SCIENTISTS

Proposed that matter consists of four elements: earth, air, fire, and water.

Empedocles

Theorized that matter is made of indivisible atoms.

Democritus

Argued that matter can be infinitely divided.

Aristotle

The number of protons in an atom, which equals the number of electrons in neutral atoms.

Atomic Number (Z)

The total number of protons and neutrons in an atom.

Atomic Mass (A)

Atoms of the same element that have different masses due to variation in the number of neutrons.

Isotopes

Positively charged ions formed by the loss of electrons.

Cations

Negatively charged ions formed by the gain of electrons.

Anions

Discovered electrons in 1897.

J.J. Thompson

Measured the charge of an electron in 1909.

Robert Millikan

Discovered protons and observed that atoms are mostly empty space in 1911.

Ernest Rutherford

Proposed that electrons move in concentric orbits around the nucleus in 1913.

Niels Bohr

Discovered neutrons in 1932.

James Chadwick

q = # of protons - # of electrons.

Ion charge formula

Atoms are conserved in chemical reactions.

Conservation of Atoms

Substances that consist of multiple elements.

Compounds

Protons = Atomic Number (Z) and Neutrons = Atomic Mass - Atomic Number.

General Relationship of Protons and Neutrons