Evolution of the Atomic Model

Theories of Knowledge Empirical knowledge – knowledge or statements based on observations “The liquid boils at 100°C” “Mixing these two chemicals produces a white solid” Theoretical knowledge – knowledge or statements based on theories “Gas particles move in a random motion” “Hydrogen has 1 valence electron”

Greek Philosophers Democritus (5th century BC) All matter can be divided into smaller particles. He called this particle the atom. Many different kinds of atoms, each distinct in shape and size and that all atoms move around in space.

Dalton’s Atomic Theory John Dalton (1803) postulated that: All matter is made up of particles called atoms Atoms of each element were different from each other Atoms of two or more elements join together in fixed ratios to form new compounds (law of constant composition – compounds have a fixed, definite proportion; eg. H2O, CO2) Atoms are not destroyed in chemical reactions. They are rearranged. (law of conservation of mass – matter is not created or destroyed)

J.J. Thomson (1897) To find the make-up of an atom, he used a ray tube. He discovered there were negatively charged sub-particles  electrons. Raisin Bun Model – the atom is a (+) charged bun (mostly empty space) that contains (-) charged raisins (electrons) inside it.

Ernest Rutherford (1911) Gold foil experiment – shot (+) α particles through gold foil. Expected most will pass right through, but some particles were deflected at large angles. Determined that there was something in the middle of the atom An atom has a (+) core (nucleus) surrounded by empty space that also contains (-) electrons. In 1914, gave name “proton” for (+) sub-particle within atom

James Chadwick (1932) Built upon Rutherford’s work Discovered there were neutrally-charged sub-particles alongside with protons in atom  called them “neutrons”

Niels Bohr – “Planetary” Model If protons are (+) and electrons are (-), we expect them to be attracted to each other. But this is not really the case… why? Bohr discovered that the electrons of an atom exist in circular orbits (energy levels). Each energy level has an allowable # of electrons.

Each orbit has a certain amount of energy. When an electron: Moves to a higher orbit, it GAINS energy Moves to a lower orbit, it LOSES energy The outer-most electrons in an atom’s orbit for an element is called the valence electron. The group numbers in the periodic table refers to how many VEs an element has! When the outermost orbit of an element is full, it is stable and generally non-reactive. This explains the properties of noble gases.