Atomic Theory and Models of the Atom

Study Notes

Democritus hypothesized that matter could be divided into smaller and smaller pieces until a point where it could no longer be divided.
He called this indivisible piece an atom.
Dalton stated that all matter is composed of extremely small particles called atoms.
Atoms of the same element are identical in size, mass, and chemical properties.
The atoms of different elements are different.
Atoms cannot be subdivided or destroyed during chemical reactions.
Atoms are combined, separated, or rearranged in chemical reactions.
Atoms of different elements combine to form compounds that are always found in definite proportions.

Discovered negatively charged particles called electrons.
Proposed the plum pudding model, where electrons are embedded in a positively charged sphere.

Discovered a dense, positively charged nucleus at the center of the atom.
Electrons orbit the nucleus in an empty space around the positively charged nucleus.
Protons are found within the nucleus and are positively charged.
Neutrons are also found in the nucleus and are neutral.

Max Planck observed that excited gases produce a bright-line spectrum, meaning energy is emitted at specific wavelengths.
Planck proposed that energy is emitted or absorbed in discrete, specific quantities called quanta.

Einstein explained the photoelectric effect (light striking a metal and ejecting electrons).
He suggested that light behaves as a stream of particles called photons, and each photon possesses energy.
Photons with enough energy can cause electrons to jump to higher energy levels.

De Broglie proposed that electrons exhibit both wave-like and particle-like properties (wave-particle duality).

Schrodinger developed a model describing electron orbital clouds.
The probability of finding an electron at a specific location around the nucleus is calculated.
The location of an electron cannot be precisely determined; only the probability can be determined.
This is because the act of measuring the electron affects its speed and position (Heisenberg's Uncertainty Principle).