History of the AtomHistory of the Atom

Aristotle (400 BC)Aristotle (400 BC)

4 elements – earth, air, fire and water 4 elements – earth, air, fire and water (Continuous Theory)(Continuous Theory)

– – not a scientist not a scientist

Democritus (400 BC)Democritus (400 BC)

- First to use the word “atom”First to use the word “atom”- Could not break down the atom Could not break down the atom

(Discontinuous Theory)(Discontinuous Theory)- Did not test his theoriesDid not test his theories

John Dalton (1808)John Dalton (1808)

John DaltonJohn Dalton (1808) – “Modern Father of the (1808) – “Modern Father of the Atom” – Proposed a theory of matter Atom” – Proposed a theory of matter based on the existence of atomsbased on the existence of atoms

Solid Sphere ModelSolid Sphere Model

Similar to a golf ball or billiard ball

John Dalton (cont’d)John Dalton (cont’d)

4 postulates4 postulates

1)1) All elements are composed of atoms – All elements are composed of atoms – indivisible and indestructibleindivisible and indestructible

(Not True Today)(Not True Today)

2) All atoms of the same element are exactly 2) All atoms of the same element are exactly the same (ex: mass, color, shape)the same (ex: mass, color, shape)

(Not True Today – isotopes)(Not True Today – isotopes)

John Dalton (Cont’d)John Dalton (Cont’d)

3) Atoms of different elements are different 3) Atoms of different elements are different (ex: color, mass, boiling point)(ex: color, mass, boiling point)

(Still True Today)(Still True Today)

4) Compounds are formed by the joining of 4) Compounds are formed by the joining of atoms of two or more elements in a whole-atoms of two or more elements in a whole-number rationumber ratio

(Still True Today)(Still True Today)

Crookes (1895)Crookes (1895)

- Crookes tube (or Cathode Ray tube) led - Crookes tube (or Cathode Ray tube) led to the discovery of the electronto the discovery of the electron

- Beam of light travels from cathode - Beam of light travels from cathode

(- end) to anode (+ end)(- end) to anode (+ end)

- Applications include; TV and computer - Applications include; TV and computer screens, RADAR screens, and X-raysscreens, RADAR screens, and X-rays

Cathode Ray TubeCathode Ray Tube

JJ Thomson (1897)JJ Thomson (1897)

– – discovered that a magnet distorted the discovered that a magnet distorted the ray. The ray was attracted to the + end of ray. The ray was attracted to the + end of a magneta magnet

- Conclusion – cathode rays must be a - Conclusion – cathode rays must be a stream of negatively charged particlesstream of negatively charged particles

- He called them – ELECTRONS- He called them – ELECTRONS

JJ ThomsonJJ Thomson

JJ ThomsonJJ Thomson

Developed Developed Plum Pudding ModelPlum Pudding Model

“Cookie Dough Model”

Ernest RutherfordErnest Rutherford(1909)(1909)

Gold Foil Experiment (Alpha Scattering Experiment)

Comparison of Two ModelsComparison of Two Models

Thomson’s Model

Rutherford’s Model

ConclusionConclusion

Each atom contained a small, dense, Each atom contained a small, dense, positively charged nucleus. positively charged nucleus.

Proton and Nucleus were then discoveredProton and Nucleus were then discovered

Nuclear ModelNuclear Model

Big Problem with Big Problem with Rutherford’s ModelRutherford’s Model

What is keeping the positively charged What is keeping the positively charged nucleus from being attracted to the nucleus from being attracted to the electrons and drawing them closer???????electrons and drawing them closer???????

Bohr to the rescue…….Bohr to the rescue…….

Niels Bohr (1913)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 ModelBohr’s Solar System Model

Ground vs. ExcitedGround vs. Excited

Ground StateGround State – original energy level – original energy level where an electron can be foundwhere an electron can be found

- - Very StableVery Stable

Excited StateExcited State – When electrons gain – When electrons gain energy and move to a higher energy energy and move to a higher energy levellevel

- - Very unstableVery unstable

Ground vs. Excited StateGround 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

LightLight

Continuous SpectrumContinuous Spectrum – band of – band of colors produced when white light colors produced when white light passes through a prismpasses through a prism

Ex: ROY G. BIVEx: ROY G. BIV

LightLight

Bright Line SpectrumBright Line Spectrum – band of colors – band of colors produced when a gas is in the excited produced when a gas is in the excited statestate

Each element produces its own specific Each element produces its own specific line spectrum – “like fingerprints”line spectrum – “like fingerprints”

Charge Cloud ModelCharge Cloud Model

Charge Cloud ModelCharge Cloud Model

- Also called Quantum Mechanical ModelAlso called Quantum Mechanical Model

- Does not show the path of electronsDoes not show the path of electrons

- Instead, it shows the most probable Instead, it shows the most probable location of an electronlocation of an electron

- As you move away from the nucleus, the As you move away from the nucleus, the chance of finding an electron decreases!chance of finding an electron decreases!