11 Atomic Theory. 2 A HISTORY OF THE STRUCTURE OF THE ATOM.

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Atomic Theory

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A HISTORY OF THE A HISTORY OF THE STRUCTURE OF THE ATOMSTRUCTURE OF THE ATOM

HISTORY OF THE ATOMHISTORY OF THE ATOM

460 BC Democritus develops the idea of atoms

he pounded up materials in his pestle and

mortar until he had reduced them to

smaller and smaller particles which he

called

ATOMAATOMA

(greek for indivisible)

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HistoryHistory

Greek Philosopher Democritus (460-370 B.C.): all matter composed of small atoms

atomos = indivisible

1803, John Dalton (brit.): atoms are the fundamental building blocks of matter

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Dalton's PostulatesDalton's Postulates

Each element is composed of extremely small particles called atoms.

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Dalton's PostulatesDalton's Postulates

All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

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Dalton's PostulatesDalton's Postulates

Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

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Dalton’s PostulatesDalton’s Postulates

Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

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John Dalton’s Atomic TheoryJohn Dalton’s Atomic Theory(ca 1803)(ca 1803)

1. Each element is composed of extremely small particles called atoms.

2. All atoms of a given element are identical. The atoms of different elements are different and have different properties (including different masses).

3. Atoms of an element are not changed into different types of atoms by chemical reactions. Atoms are neither created nor destroyed in chemical reactions. This is the Law of Conservation of Mass.

4. Compounds are formed when atoms of more than one element combine. A given compound always has the same relative number and kind of atoms. This is the Law of Constant Composition.

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John Dalton’s Atomic Theory

Led him to deduce the Law of Multiple Proportions:

When two or more elements combine to form more than one compound, the relative masses of the elements which combine will be in in the ratio of small whole numbers.

In carbon monoxide, CO, 12 g carbon combine with

16 g oxygen. C:O ratio is 12:16 or 3:4.

In carbon dioxide, CO2, 12 g carbon combine with

32 g oxygen. C:O ratio is 12:32 or 3:8.

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John Dalton’s Atomic John Dalton’s Atomic TheoryTheory

Almost right. A good start.

Structure of the atom after Dalton (ca. 1810)

very small

1898 Joseph John Thompson

found that atoms could sometimes eject a

far smaller negative particle which he

called an

ELECTRONELECTRON

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J.J. Thomson (1897): Cathode Rays

Atoms subjected to high voltages give off cathode

rays.

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J.J. Thomson: Cathode Rays

Cathode rays can be deflected by a magnetic field.Cathode rays are negatively charged particles

(electrons).Electrons are in atoms.

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J.J. Thomson – The Electron

Structure of the atom after Thomson (ca. 1900)

“Plum pudding” model: Negative

electrons are embedded in a

positively charged mass.

Positively charged mass

Electrons (-)

Unlike electrical charges attract, and that is what holds the atom

together.

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RadioactivityRadioactivity

Radioactivity is the spontaneous emission of radiation by an atom.

First observed by Henri Becquerel (1852-1908).

Marie and Pierre Curie also studied it. Nobel Prize in 1903 (physics).

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Studies of Natural Radioactivity

Structure of the atom after Becquerel (early 1900s)

Positively charged mass

Electrons (-)

Some atoms naturally emit one or more of the following types of radiation:

alpha (α) radiation (later found to be He2+ - helium nucleus)

beta (β) radiation (later found to be electrons)

gamma (γ) radiation (high energy light)

γ

α

α

γ

Alpha particles

Somehow gamma radiation is in there, too.

1910 Ernest Rutherford

oversaw Geiger and Marsden carrying out

his famous experiment.

they fired Helium nuclei at a piece of gold foil which was only a few atoms thick.

they found that although most of them passed through. About 1 in 10,000 hit

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RadioactivityRadioactivity Three types of radiation were

discovered by Ernest Rutherford: α particles (positive, charge 2+, mass 7400

times of e-) β particles (negative, charge 1-) γ rays (high energy light)

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Ernest Rutherford (1910) Scattering experiment: firing alpha particles at a

gold foil

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The Nuclear AtomThe Nuclear Atom

Some alpha particles bounce off the gold foil. This means the mass of the atom must be concentrated in the center and is positively charged! Thompson’s model could not be correct.

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Ernest Rutherford The Nucleus and the Proton

Structure of the atom after Rutherford (1910)

The mass is not spread evenly throughout the atom, but is concentrated in the center, the nucleus.

Electrons (-) are now outside the nucleus.

The positively charged particles in the nucleus are

protons.

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James Chadwick – The Neutron

Structure of the atom after Chadwick (1932)

In the nucleus with the protons are particles of similar mass but no electrical charge called

neutrons.

Electrons (-) are now outside the nucleus in quantized energy states called orbitals. (From Niels Bohr and quantum mechanics)

The positively charged particles in the nucleus are

protons.

nn+

1913 Niels Bohr

studied under Rutherford at the Victoria

University in Manchester.

Bohr refined Rutherford's idea by

adding that the electrons were in orbits.

Rather like planets orbiting the sun.

With each orbit only able to contain a

set number of electrons.

Bohr’s Atom

electrons in orbits

nucleus

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Structure of the Atom

proton (+)

neutron

nucleus - responsible for the mass of the atom, positively charged

electrons - responsible for the volume and size of the atom, negatively charged

10-14 m

10-10 m