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Democritus (460-370 B.C.)Democritus was one of a few Greek philosophers who believed that all matter in the world was made of of indivisible parts which he called “atomos,” which means “indivisible.”

Although this theory was to be discovered to later be the truth,Democritus’ ideas faded until the seventeenth century in Europe.

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John Dalton (1766-1844)Dalton is known for his atomic theoryTheory states that atoms are the smallest chemical building block of natureHis four postulates:

Each element is composed of extremely small particles called atomsAll atoms of a given element are identical; the atoms of different elements are different and have different properties (including different masses)Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created nor destroyed in chemical reactionsCompounds are formed when atoms of more than one element combine; a given compound always has the same ratio and kind of atomsATOMS ARE SMALL AND CANNOT BE DIVIDED (later changed!)

http://en.wikipedia.org/wiki/John_Dalton#Atomic_theory

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Discovery of Atomic Structure

By 1850 scientists knew that atoms were composed of charged particles.Electrostatic attraction:

Like charges repelOpposites attract

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J.J. ThomsonJ.J. ThomsonExperimented with cathode rays and found that the properties didn’t changeDistinguished charges within atoms, positive and negative charges Plum-pudding modelFound electron charge ratio

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Cathode Rays and Electrons

C.R. 1st discovered in mid-1880s from studies of electrical discharge thru partially evacuated tubes (CRTs)Cathode rays = radiation produced when high voltage is applied across the tube.The voltage causes negative particles to move from the negative electrode (cathode) to the positive electrode (anode).The path of electrons can be altered by the presence of a magnetic field.

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Consider cathode rays leaving the positive electrode through a small hole….

If they interact with a magnetic field perpendicular to an applied electric field, then the cathode rays can be deflected by different amounts.Amount of deflection depends on applied magnetic and electric fields.Deflection also depends on the charge-to-mass ratio of an electron.Thomson determined the charge-to-mass ratio of an electron in 1897.

Charge-to-mass ratio = 1.76 x 108 C/gC: Coulomb, SI unit of electric charge

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Robert MillikanMillikan built on J.J. Thomson’s work on electrons by measuring their charge with his famous oil-drop experiment. Using Thomson’s ratio, Millikan calculated electron’s mass of

9.10 x 10-28 g which proved how much smaller electrons are than nucleus particles.

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Millikan Oil-Drop Experiment

Sprayed oil drops over the hole in a positively charged plate and measured the electrostatic force of attraction.Found the charge on the electron to determine its massConcluded the charge on the electron must be 1.60 x 10-19 CMass of electron = 1.60 x 10-19 C = 9.10 x 10 -

28 g 1.76 x 108 C/g

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Radioactivity(Spontaneous emission of radiation)

Type Symbol Charge Mass (amu)

Alpha particle

He 2+ 4.002 60

Beta particle

β or 0-

1e1- 0.000 548 6

Positron β or 0

+1e1+ 0.000 548 6

Gamma ray

γ 0 0

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-Conducted an experiment using alpha -Conducted an experiment using alpha particles to bombard gold foil to particles to bombard gold foil to conclude:conclude:

1. Nucleus of an atom contains positive 1. Nucleus of an atom contains positive particles that we now call particles that we now call protonsprotons..

2.2. The The nucleusnucleus is a small dense area in is a small dense area in the atom.the atom.

-Studied three types of radioactive emissions: -Studied three types of radioactive emissions: alpha, beta & gammaalpha, beta & gamma-Concluded that -Concluded that alpha alpha particles were particles were He nuclei He nuclei

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A Positively Charged Nucleus

Rutherford shot alpha particles though a thin piece of gold foil.Some of these particles were deflected instead of passing straight throughRecall “like repels like.”When a + alpha particle encountered a nucleus of a gold atom, it was deflected by the dense positively charged nucleus.

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James Chadwick suspected there were neutral particles when he saw in experiments like Rutherford’s that some of the particles were deflected backwards, meaning that they had no charge. Chadwick had discovered the neutron!

neutronneutron

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Niels BohrBohr proposed that an electron circles the nucleus in allowed orbits at specific energy levels.

orbital

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Scientist ContributionsThomson:

“Discovered” electron (1897)Cathode ray experiments“Plum pudding” atomic model

Millikan:Mass of electronOil-drop experiment (1909)

Rutherford:Positively charged nucleus (1911)Gold foil experimentsDiscovered proton (1919)

Chadwick: Discovered neutron (1932)

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Small NumbersElectronic Charge: 1.609 x 10-19 C

Charge on an electron: -1.609 x 10-19 CCharge on a proton: +1.609 x 10-19 C

Atomic Mass Unit (amu): 1.66054 x 10-24 gProton mass: 1.0073 amuNeutron mass: 1.0087 amuElectron mass: 5.486 x 10-4 amu

Unit of length used to note atomic dimensions: 1 Angstrom(Å) = 1x10-10 m

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Atomic Number Number of protons or electrons in an elementIdentifies the element

Atomic MassNucleus contains most of the mass of an atom.Protons and neutrons are each ~ 1.67 x 10-24 g.Electrons are each ~ 9.11 x 10-28 g.Use atomic mass unit (amu) instead of gram.The mass of one proton is ~ 1 amu.

Mass NumberThe sum of the number of protons and number of neutrons in the nucleusIs approximately equal to the average atomic mass shown on periodic table.Number of neutrons = mass number – atomic number

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IsotopesAtoms of the same element with different numbers of neutronsHave the same number of protonsExample: Carbon-12 and Carbon-14

Radioactive IsotopesUnstable in nature Can be used to date fossils and rocksThe time it takes for half of the radioactive atoms in a piece of the fossil to change to another element is its half-life.

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Isotopes AXIsotopes have the same Z, but different A.Isotopes have different numbers of neutrons.An atom of a specific isotope is called a nuclide.Nuclides of hydrogen include:

1H = hydrogen (protium)2H = deuterium (heavy hydrogen)3H = tritium (3H is radioactive.)

Z

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