Early Developments & Modern Atomic Theory

These flashcards cover key concepts, definitions, and significant historical figures in the development of atomic theory.

Democritus

First proposed that matter is made of indivisible particles called 'atomos'.

Aristotle's view of matter

Believed matter was continuous and made of four elements; delayed acceptance of atomic theory.

Dalton’s atomic theory postulates

1) All matter is made of tiny indivisible atoms; 2) Atoms of the same element are identical.

Incorrect part of Dalton's theory

Atoms are not indivisible; they contain subatomic particles (electrons, protons, neutrons).

Thomson’s cathode-ray experiment

Demonstrated that cathode rays are streams of negatively charged particles called electrons.

Thomson's 'plum pudding' model

Suggested that electrons are embedded in a positively charged sphere within the atom.

Rutherford’s gold-foil experiment setup

Most alpha particles passed through gold foil, but some were deflected indicating a dense nucleus.

Differences between Rutherford’s and Thomson’s models

Rutherford’s model features a central nucleus with electrons outside, unlike Thomson's mixed model.

Chadwick's contribution to atomic structure

Discovered the neutron, which explains the presence of extra mass in atoms.

Relative size and density of an atom’s nucleus

The nucleus is extremely small and dense, about 1/10,000 the diameter of the atom.

Bohr’s model of electron stability

Proposed that electrons move in fixed, quantized orbits with specific energy levels.

Experimental evidence for Bohr’s model

Hydrogen emission spectra showed discrete lines corresponding to specific energy levels.

Why Bohr’s model was replaced

Failed for multi-electron atoms; quantum mechanics introduced orbitals instead of fixed orbits.

Difference between orbit and orbital

Orbit = fixed circular path; Orbital = region of high probability for finding electrons.

Heisenberg’s uncertainty principle

States that one cannot simultaneously know both the position and momentum of an electron.

Schrödinger's role in atomic theory

Developed the wave equation explaining electron behavior through wave functions.

Probability of finding an electron in modern model

Described as a probability distribution for electron locations (electron cloud model).

Role of experimental evidence in atomic models

Each new experiment iteratively refined theories leading to improved atomic models.

Chronological order of key scientists

Dalton → Thomson → Rutherford → Bohr → Chadwick.

Accepted atomic model today

The quantum-mechanical model, treating electrons as wave-particles defined by probabilities.