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 theory Theory states that atoms are the smallest chemical building block of nature His four postulates: Each element is composed of extremely small particles called atoms All atoms of a given element are identical (including different masses) atoms by chemical reactions destroyed in chemical reactions element combine; a ratio and kind of atoms changed!) ; the atoms of http://en.wikipedia.org/w iki/John_Dalton#Atomic _theory different elements are different and have different properties Atoms of an element are not changed into different types of ; atoms are neither created nor Compounds are formed when atoms of more than one given compound always has the same ATOMS ARE SMALL AND CANNOT BE DIVIDED (later Mullis 2

Discovery of Atomic Structure

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

Electrostatic attraction: Like charges repel Opposites attract Mullis 3

J.J. Thomson

Experimented with cathode rays and found that the properties didn’t change Distinguished charges within atoms, positive and negative charges Plum-pudding model Found electron charge ratio Mullis 4

Cathode Rays and Electrons

C.R. 1 (CRTs) st discovered in mid-1880s from studies of electrical discharge thru partially evacuated tubes 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 10 8 C/g C: Coulomb, SI unit of electric charge

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Robert Millikan

Millikan 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 mass Concluded the charge on the electron must be 1.60 x 10 -19 C Mass of electron = 1.60 x 10 -19 C = 9.10 x 10 -28 g 1.76 x 10 8 C/g Mullis 8

Radioactivity (Spontaneous emission of radiation)

Type Alpha particle Beta particle Positron Symbol Charge He 2+ β or 0 -1 e 1 β or 0 +1 e 1+ Mass (amu) 4.002 60 0.000 548 6 0.000 548 6 Gamma ray γ 0 0 Mullis 9

-Conducted an experiment using alpha particles to bombard gold foil to conclude: 1. Nucleus of an atom contains positive particles that we now call protons.

2.

The nucleus is a small dense area in the atom.

-Studied three types of radioactive emissions: alpha, beta & gamma -Concluded that alpha particles were 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 through Recall “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!

neutron

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Niels Bohr

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

orbital Mullis Chemistry Holt Ch.4

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Scientist Contributions

Thomson: “Discovered” electron (1897) Cathode ray experiments “Plum pudding” atomic model Millikan: Mass of electron Oil-drop experiment (1909) Rutherford: Positively charged nucleus (1911) Gold foil experiments Discovered proton (1919) Chadwick: Discovered neutron (1932) Mullis 15

Small Numbers

Electronic Charge: 1.609 x 10 -19 C Charge on an electron: -1.609 x 10 -19 Charge on a proton: +1.609 x 10 -19 C C Atomic Mass Unit (amu): 1.66054 x 10 -24 Proton mass: 1.0073 amu Neutron mass: 1.0087 amu Electron mass: 5.486 x 10 -4 amu g Unit of length used to note atomic dimensions: 1 Angstrom (Å) = 1x10 -10 m Mullis 16

Atomic Number

Number of protons or electrons in an element Identifies the element

Atomic Mass

Nucleus 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 Number

The sum of the number of protons and number of neutrons in the nucleus Is approximately equal to the average atomic mass shown on periodic table.

Number of neutrons = mass number – atomic number Mullis 17

Isotopes Atoms of the same element with different numbers of neutrons Have the same number of protons Example: Carbon-12 and Carbon-14 Radioactive Isotopes Unstable in nature Can be used to date fossils and rocks The 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

A Z

X

Isotopes 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: 1 H = hydrogen (protium) 2 H = deuterium (heavy hydrogen) 3 H = tritium ( 3 H is radioactive.) Mullis 19

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