The Development of The Development of Atomic TheoryAtomic Theory

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The The AtomAtom• The term atom is derived from the

Greek word (atomos) meaning indivisible

• Democritius (470-370 BC ) suggested that all matter was made up of indivisible particles called atoms

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Law of Constant Law of Constant CompositionComposition

A compound always contains atoms of two or More elements combined in definite proportions by mass

Example:Water H22O always contains 8 grams of oxygen to 1 gram of hydrogen

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Law of Multiple Law of Multiple ProportionsProportions

Atoms of two or more elements may combine in different ratios to produce more than one compound.

Examples:

NO NONO NO22 N N22O NO N22OO55

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Dalton’s Atomic Dalton’s Atomic TheoryTheory1. All elements are

composed of indivisible and indestructible particles called atoms.

2. Atoms of the same element are exactly alike, They have the same masses. 3. Atoms of different

elements have different masses.

4. Atoms combine to form compounds in small whole number ratios..5

SomeObjections to SomeObjections to Dalton’s Atomic Dalton’s Atomic

TheoryTheory1. Atoms are not indivisible. They

are composed of subatomic particles.

2. Not all atoms of a particular element have exactly the same mass.

3. Some nuclear transformations alter (destroy) atoms

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Crookes ExperimentCrookes Experiment

Crookes found that passing an electrical current Crookes found that passing an electrical current through a gas at very low pressure caused the gas to through a gas at very low pressure caused the gas to glow. Putting a magnet next to the beam caused it to glow. Putting a magnet next to the beam caused it to be deflected.be deflected. 7

The ElectronThe Electron1. The electron was the first subatomic

particle to be identified.2. In 1897 J.J Thomson used a cathode ray

tube to establish the presence of a charged particle known as the electron

3. Thomson established the charge to mass ratio

E/m = 1.76 x 108 coulombs/gram

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A Cathode Ray TubeA Cathode Ray Tube

Thomson found that an electrical field would Thomson found that an electrical field would also deflect an electron beam. He surmised also deflect an electron beam. He surmised that the that the ratio of charge to mass is constantratio of charge to mass is constant..

Thomsen’s Charge to Thomsen’s Charge to Mass RatioMass Ratio

Thomson proposed that the cathode rays were in fact charged particles coming from the traces gases in the Cathode Ray Tube.He then determined that the ratio of charge to mass is constant E/m = 1.76 x 108 coulombs/gram

Thomsen’s Plum Thomsen’s Plum Pudding ModelPudding Model

Thompson proposed that an atom was made up of electrons scattered unevenly through out an elastic sphere. These charges were surrounded by a sea of positive charge to balance the electron's charge like plums surrounded by pudding.

This early model of the atom was called The Plum Pudding Model. A more contemporary American label might be the “chocolate chip cookie” model 11

Millikan’s ExperimentMillikan’s Experiment

By varying the charge on the plates, Millikan found By varying the charge on the plates, Millikan found that he could suspend the oil drops or make them that he could suspend the oil drops or make them levitate.levitate. 12

Millikan’s ExperimentMillikan’s ExperimentMillikan used his data to measure the charge of an electron and then to calculate the mass of the electron from Thomson’s charge to mass ratio.Given the charge = 1.60 x 10-19 coulomb and the ratio of E/m = 1.76 x 108 coulombs/gram it is possible to calculate the mass

Mass

= 9.11 x 10-28 gram

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ProtonsProtonsFirst observed by E. Goldstein in 1896J.J. Thomson established the presence of positive charges.

The mass of the proton is1.673 x 10-24 grams

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Rutherford’s ExperimentRutherford’s Experiment1910

Ernest Rutherford

Rutherford oversaw Geiger and Marsden carrying out his famous experiment.

They fired high speed alpha particles (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 and were deflected

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Rutherford’s ExperimentRutherford’s Experiment

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Rutherford’s Rutherford’s ExperimentExperiment

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Rutherford’s Rutherford’s ExperimentExperiment

By studying this By studying this pattern, Rutherford pattern, Rutherford concluded that concluded that atoms have a very atoms have a very dense nucleus, but dense nucleus, but there are mostly there are mostly empty space.empty space.

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Subatomic ParticlesSubatomic ParticlesThe diameter of a single atom rangesFrom 0.1 to 0.5 nm. (1 nm = 10-9 m).

Within the atom are smaller particles:ElectronsProtonsNeutrons

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NeutronsNeutrons

Discovered by James Chadwick in 1932

Slightly heavier than a proton

Mass of a neutron = 1.675 x 10-24 grams

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The Bohr ModelThe Bohr ModelNiels Bohr proposed the Planetary Model in 1913. Electrons move in definite orbits around the nucleus like planets moving around the nucleus. Bohr proposed that each electron moves in a specific energy level.

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Aspects of the Bohr Aspects of the Bohr ModelModel

Bohr put together Balmer’s and Plank’s discoveries to form a new atomic model

In Bohr’s model: 1. Electrons can orbit only at certain

allowed distances from the nucleus.

2. Electrons that are further away from the nucleus have higher energy levels (explaining the faults with Rutherford’s model). 22

The Electromagnetic SpectrumThe Electromagnetic Spectrum

Emission SpectraEmission Spectra

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Flame TestsFlame Tests

According to BohrAccording to Bohr Atoms radiate energy

whenever an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit.

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Problems with the Bohr Problems with the Bohr ModelModel

The Bohr model provided a model that gave The Bohr model provided a model that gave precise results for simple atoms like hydrogen.precise results for simple atoms like hydrogen.

Using the Bohr model precise energies could Using the Bohr model precise energies could be calculated for energy level transitions in be calculated for energy level transitions in hydrogen.hydrogen.

Unfortunately these calculations did not work Unfortunately these calculations did not work for atoms with more than 1 electron.for atoms with more than 1 electron.

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Weakness of the Bohr Weakness of the Bohr ModelModel

• According to the Bohr model electrons could be found in orbitals with distinct energies.

• When the data for energies measured using spectral methods where compared to the values predicted by the Rydberg equation, they were accurate only for hydrogen.

• By the 1920s, further experiments showed that Bohr's model of the atom had some difficulties. Bohr's atom seemed too simple to describe the heavier elements.

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Modern View of the Modern View of the AtomAtom

The wave mechanical model for the atom was developed to answer some of the objections that were raised about the Bohr model. It is based on the work of a number of scientists and evolved over a period of time

The quantum theorists such as Maxwell Planck suggested that energy consists of small particles known as photons. These photons can have only discreet energies

Maxwell Planck

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Modern View of the Modern View of the AtomAtom

Albert Einstein demonstrated the equivalence of matter and energy. Hence matter and energy in Einstein’s theory were not different entities but different expressions of the same thing

Einstein then proposed the equivalence of Matter and Energy given by his famous equation

E = mc2

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Modern View of the Modern View of the AtomAtom

Louis de Broglie suggested that if energy could be thought of as having particle properties, perhaps matter could be thought of as having wave like characteristics

Louis de Broglie

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Modern View of the Modern View of the AtomAtom

Louis de Broglie proposed that an electron is not just a particle but it also has wave characteristics.

mc2 = h

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Modern View of the Modern View of the AtomAtom

Heisenberg proposed that it was impossible to know the location and the momentum of a high speed particle such as an electron.

The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa.

--Heisenberg, Uncertainty paper, 1927

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Modern View of the Modern View of the AtomAtom

The atom cannot be defined as a solar system with discreet orbits for the electrons. The best that we could do was define the probability of finding an electron in a particular location.

The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa. --Werner Heisenberg,

Uncertainty paper, 1927

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Modern View of the Modern View of the AtomAtom

Edwin Schroedinger proposed that the electron is really a wave. It only exists when we identify its location. Therefore the electrons are best thought of probability distributions rather than discreet particles.

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Modern View of the AtomModern View of the Atom The modern view of the atom suggests The modern view of the atom suggests

that the atom is more like a cloud.that the atom is more like a cloud. Atomic orbitals around the nucleus Atomic orbitals around the nucleus

define the places where electrons are define the places where electrons are most likely to be found. most likely to be found.

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Wave Mechanical ModelWave Mechanical ModelThe location of the The location of the electron in a hydrogen electron in a hydrogen atom is a probability atom is a probability distribution.distribution.

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Progression of Atomic ModelsProgression of Atomic Models

Our view of the atom has changed over timeOur view of the atom has changed over time 38

ATOMIC STRUCTUREATOMIC STRUCTURE

Particle

proton

neutron

electron

Charge

+ charge

- charge

No charge

1

1

0

Mass

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ATOMIC NUMBER AND MASS NUMBERATOMIC NUMBER AND MASS NUMBER

the number of protons in an atom

the number of protons and neutrons in an atomHH

ee22

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Atomic Number

Mass Number

Number of electrons = Number of protons in a neutral atom 40

Atomic MassAtomic MassThe atomic mass of an atom is a relative number that is used to compare the mass of atoms.

An atomic mass unit is defined as 1/12 of the mass of an atom of carbon 12.

The atomic masses of all other atoms are a ratio to carbon 12

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IsotopesIsotopesMany elements have atoms that have multiple forms

Different forms of the same element having different numbers of neutrons are called isotopes.

For example: Carbon exists as both Carbon 12 and Carbon 14

Carbon 12 Carbon 146 electrons 6 electrons

6 protons 6 protons

6 neutrons 8 neutrons 42

Isotopes and Atomic Isotopes and Atomic MassMass

Many elements have atoms that have multiple isotopes.

Isotopes vary in abundance. Some are quite common while others are very rare.

The atomic mass that appears in the periodic table is a weighted average taking into account the relative abundance of each isotope.

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Isotope:Isotope: one of two or more atoms having the one of two or more atoms having the same number of protons but different same number of protons but different numbers of neutronsnumbers of neutrons

or Na-23 or Na-24

Measuring Atomic MassMeasuring Atomic Mass--the Mass Spectrometer--the Mass Spectrometer

The mass The mass spectrometespectrometer can be r can be used to used to determine determine the atomic the atomic mass of mass of isotopes.isotopes.