History of the Atom Aristotle 400 BC 4

History of the Atom

Aristotle (400 BC) 4 elements – earth, air, fire and water (Continuous Theory) – not a scientist

Democritus (400 BC) - First to use the word “atom” Could not break down the atom (Discontinuous Theory) Did not test his theories

John Dalton (1808) – “Modern Father of the Atom” – Proposed a theory of matter based on the existence of atoms Solid Sphere Model Similar to a golf ball or billiard ball

John Dalton (cont’d) 4 postulates 1) All elements are composed of atoms – indivisible and indestructible (Not True Today) 2) All atoms of the same element are exactly the same (ex: mass, color, shape) (Not True Today – isotopes)

John Dalton (Cont’d) 3) Atoms of different elements are different (ex: color, mass, boiling point) (Still True Today) 4) Compounds are formed by the joining of atoms of two or more elements in a whole -number ratio (Still True Today)

Crookes (1895) - Crookes tube (or Cathode Ray tube) led to the discovery of the electron - Beam of light travels from cathode (- end) to anode (+ end) - Applications include; TV and computer screens, RADAR screens, and X-rays

Cathode Ray Tube

JJ Thomson (1897) – discovered that a magnet distorted the ray. The ray was attracted to the + end of a magnet - Conclusion – cathode rays must be a stream of negatively charged particles - He called them – ELECTRONS

JJ Thomson

JJ Thomson Developed Plum Pudding Model “Cookie Dough Model”

Ernest Rutherford (1909) Gold Foil Experiment (Alpha Scattering Experiment)

Comparison of Two Models Thomson’s Model Rutherford’s Model

Conclusion Each atom contained a small, dense, positively charged nucleus. Proton and Nucleus were then discovered Nuclear Model

Big Problem with Rutherford’s Model What is keeping the positively charged nucleus from being attracted to the electrons and drawing them closer? ? ? ? Bohr to the rescue…….

Niels Bohr (1913) Proposed improvements to the Rutherford Model Key Idea – there are definite paths (orbits) in which an electron can travel around a nucleus without spiraling towards it.

Bohr’s Solar System Model

Ground vs. Excited Ground State – original energy level where an electron can be found - Very Stable Excited State – When electrons gain energy and move to a higher energy level - Very unstable

Ground vs. Excited State ** When an electron moves from the excited state, back to ground state, they lose their energy in the form of …. . LIGHT Quantum – bundle of energy

Light Continuous Spectrum – band of colors produced when white light passes through a prism Ex: ROY G. BIV

Light Bright Line Spectrum – band of colors produced when a gas is in the excited state Each element produces its own specific line spectrum – “like fingerprints”

Charge Cloud Model

Charge Cloud Model - Also called Quantum Mechanical Model - Does not show the path of electrons - Instead, it shows the most probable location of an electron - As you move away from the nucleus, the chance of finding an electron decreases!