Unit 3

History of the Atom

Democritus

about 400 b.c. stated that the “ultimate particle”

is atmos He believed that atoms were

indivisible and indestructible was not based on the scientific

method – but just philosophy

John Dalton - 1803

English school teacher the atom was a small, indivisible

particle like a “small marble”

Dalton’s Atomic TheoryDalton’s Atomic Theory (experiment based!) (experiment based!)

3) Individual atoms of an element have slightly different masses. We use average mass.

4) Different elements have different ave masses5) Atoms can’t be divided in normal chemical

reactions.

1) All matter is made of tiny indivisible particles called atoms

2) Atoms of the same element are chemically the same. Atoms of different elements are chemically different.

John Dalton(1766 – 1844)

Discovery of the ElectronDiscovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively

charged particle: the electron

Modern Modern CCathode athode RRay ay TTubesubes

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Television Computer Monitor

Thomson’s Atomic ModelThomson’s Atomic Model

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

J. J. Thomson

Ernest Rutherford’sErnest Rutherford’sGold Foil Experiment - 1911Gold Foil Experiment - 1911

Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil Particles that hit on the detecting screen (film) are recorded

Rutherford’s FindingsRutherford’s Findings

a) The nucleus is smallb) The nucleus is densec) The nucleus is positively

charged

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:

The Rutherford Atomic ModelThe Rutherford Atomic Model Based on his experimental evidence:Based on his experimental evidence:

atom - mostly empty spaceatom - mostly empty space all positive charge, and most mass is in all positive charge, and most mass is in

the center called a “the center called a “nucleusnucleus”” nucleus = protons and neutrons nucleus = protons and neutrons electrons distributed around the electrons distributed around the

nucleus, and occupy most of the volumenucleus, and occupy most of the volume His model was called a “His model was called a “nuclear modelnuclear model””

Niels Bohr - 1913

Niels Bohr wondered why horseshoes heated in a forge changed color, but did not react

Nucleus? Electrons?

Neils Bohr

All atoms contain energy – the energy has to do with the electrons

1915 Bohr said that electrons are in definite areas with definite amounts of energy

Planetary ModelElectrons are in the “ground state” – normal lowest energy state

*excited state electrons gain energy in fixed amounts (photons) and go to higher levels

E = hf E = energy h = Planck’s constant f = frequency

So, Energy = Planck’s constant x frequency

Electrons don’t stay in the excited state, but fall back to the ground state and give off energy in the form of light called a spectrum (which is unique for each element)

http://www.mhhe.com/physsci/astronomy/applets/Bohr/applet_files/Bohr.html

http://www.colorado.edu/physics/2000/quantumzone/bohr.html

Speed of light (constant) = 2.998 x 108 m/sec c = f λ

speed of light

frequency

wavelength, lambda

c = f λ frequency=speed/wavelength 2.998 x 108 m/s divided by 7.6 x 10-7 = 3.9 x 1014 1/s = 3.9 x 1014 Hz

Energy of an electron = 2.179 x 10-18 J/n2

Schrodinger - 1925

Wave Mechanical Model – electrons exist in areas of probability called “space orbitals” (Rutherford and Bohr support this)

Quantum Mechanics – each electron is identified by 4 quantum numbers: 1. Principle quantum # - level 2. Orbital quantum # - shape 3. Magnetic quantum # - orientation 4. Spin quantum # - clockwise or

counter http://www.colorado.edu/physics/20

00/quantumzone/schroedinger.html

Werner Karl Heisenberg -1927

Uncertainty Principle – you can’t locate the exact position of an electron at any given time (too small, too fast)