Development of the Atom Chapter 4.1-4.2 JOHN DALTON All matter is composed of atoms All atoms of a...

Development of the Atom

• Chapter 4.1-4.2

JOHN DALTON

• All matter is composed of atoms• All atoms of a given element are identical• Atoms of specific elements are different from any

other atoms• Atoms cannot be created, divided, or destroyed• Different atoms combine in simple whole number

rations to form compounds• In chemical reactions atoms are separated,

combined, or rearranged.

• Dalton’s atomic theory proves the Law of Conservation of mass

Development of the modern atoms

• Thomson– He proved that atoms of

any element are composed of different particles.

– ALL atoms must contain these negative particles.(electrons)

– atoms were not negative so there must be a positive charge as well

– Cathode ray tube experiment

• Rutherford– Learned physics in J.J.

Thomson’ lab.– Noticed that ‘alpha’

particles were sometime deflected by something in the air.

– Gold-foil experiment

Thomson

• Used cathode ray tubes to determine there were negative particles called ELECTRONS in atoms

Cathode Rays

•Cathode ray = electron

•Electrons have a negative charge

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, pages 117-118

Highvoltage

cathode

source ofhigh voltage

yellow-greenfluorescence

shadow

(A) The effect of an obstruction on cathode rays

(B) The effect of an electric field on cathode rays

Highvoltage

cathode

source ofhigh voltage

positiveplate

negative plate

anode

source oflow voltage

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William Thomson (Lord Kelvin)

• From his experiments with the cathode ray tube…

• In 1910 proposed the Plum Pudding model– Negative electrons

were embedded into a positively charged spherical cloud.

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

Spherical cloud ofPositive charge

Electrons

Milliken

• Expanded on Thomson and determined the charge of an electron.– Used his oil drop experiment to determine that an

electron has a mass of 1/1840 of a hydrogen atom– Verified that the electron is negative

Ernest Rutherford (1871-1937)

• Learned physics in J.J. Thomson’ lab.

• Noticed that ‘alpha’ particles were sometime deflected by something in the air.

• Gold-foil experiment

Rutherford ‘Scattering’

• In 1909 Rutherford undertook a series of experiments• He fired a (alpha) particles at a very thin sample of gold foil• According to the Thomson model the a particles would only

be slightly deflected• Rutherford discovered that they were deflected through large angles

and could even be reflected straight back to the source

particlesource

Lead collimator Gold foil

a

q

What he expected…

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Because, he thought the mass was evenly distributed in the atom

What he got…

Density and the Atom

• Since most of the particles went through, the atom was mostly empty.

• Because the alpha rays were deflected so much, the positive pieces it was striking were heavy.

• Small volume and big mass = big density• This small dense positive area is the nucleus

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Explanation of Alpha-Scattering Results

Plum-pudding atom

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Alpha particles

Nuclear atom

Nucleus

Thomson’s model Rutherford’s model

Rutherford’sGold-Leaf Experiment

Conclusions:

Atom is mostly empty space

Nucleus has (+) charge

Electrons float around nucleus

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

Discovery of the 3 sub atomic particles

• Rutherford – Proton• Chadwick- Neutron• Thomson/Milliken- Electron

Particle Symbol Location Charge Relative Mass

Proton p+ Nucleus +1 1 amu

Neutron no nucleus +0 1 amu

Electron e- Electron Cloud

-1 1/1840 amu

Bohr Model of Atom

The Bohr model of the atom, like many ideas in the history of science, was at first prompted by and later partially disproved by experimentation.

http://en.wikipedia.org/wiki/Category:Chemistry

Increasing energyof orbits

n = 1

n = 2

n = 3

A photon is emittedwith energy E = hf

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Models of the Atom

Dalton’s model (1803)

Thomson’s plum-pudding model (1897)

Rutherford’s model (1909)

Bohr’s model (1913)

Charge-cloud model (present)

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

Greek model(400 B.C.)

1800 1805 ..................... 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945

1803 John Dalton pictures atoms astiny, indestructible particles, with no internal structure.

1897 J.J. Thomson, a Britishscientist, discovers the electron,leading to his "plum-pudding" model. He pictures electronsembedded in a sphere ofpositive electric charge.

1904 Hantaro Nagaoka, aJapanese physicist, suggests that an atom has a centralnucleus. Electrons move in orbits like the rings around Saturn.

1911 New Zealander Ernest Rutherford statesthat an atom has a dense,positively charged nucleus. Electrons move randomly in the space around the nucleus.

1913 In Niels Bohr'smodel, the electrons move in spherical orbits at fixed distances from the nucleus.

1924 Frenchman Louis de Broglie proposes thatmoving particles like electronshave some properties of waves. Within a few years evidence is collected to support his idea.

1926 Erwin Schrodinger develops mathematicalequations to describe the motion of electrons in atoms. His work leads to the electron cloud model.

1932 James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons.

Quantum Mechanical Model

Modern atomic theory describes the electronic structure of the atom as the probability of finding electrons within certain regions of space (orbitals).

Niels Bohr &Albert Einstein