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The Atom, in a nutshell…

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Democritus (c. 460—370 BC)

Greek philosopher

The name atom was his idea

Proposed matter is made up of small particles

Atoms comes from atomos=indivisible

Atoms differ in size and shape and combine in different ways, & constantly moving

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John Dalton (1766—1844) English chemist & physicist, self educated

published Atomic Theory:

1. Each element made of really small particles called atoms2. All atoms of a given element are identical3. Atoms of different elements have different properties, like mass

and chemical reactivity4. Atoms aren’t changed by chemical reactions, just rearranged

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J.J. Thomson (1856—1940)

Identified the electron using cathode ray tube, determined negative charge

Atom is neutral, with a positive sphere and negative electrons embedded called the “plum pudding” model. Yum.

The television and computer monitors use this technology even today.

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Ernest Rutherford (1871—1937)

Gold Foil Experiment—bombarded gold foil with radioactive particles, most passed through, some bounced back!

Conclusions: Some parts of atom must be more dense. New picture of the atom: dense at the

center, electrons on the outside Atom mostly empty space, most of mass

found in the center—nucleus with positively charged protons

Hyperlink to demo

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Niels Bohr (1885—1962)

Used light and observed the way light interacts only with specific energies: determined the electrons are in specific levels

Electrons moving around nucleus in fixed orbits w/fixed energy

Sometimes called the planetary model Not completely accurate with how electrons

are truly arranged

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Other Scientists Louis de Broglie (1892—1987): mathematical

description of electrons in atoms

James Chadwick (1891—1974): discovered the neutron around 1932, another massive particle in the nucleus

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Basics on the atom:

Three kinds of subatomic particles are electrons, protons, and neutrons.

The nucleus is the center area of an atom and contains most of its mass

Protons and neutrons are found in the nucleus. Each has a mass of 1 amu

Electrons are found in levels outside of the nucleus

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Summary of particles

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The number of protons are equal to the number of electrons in an atom so…

Put a number where there is a letter.

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Answers:

19

All of the numbers have to be the SAME!

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Answers:

B 5 5

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16 1623 23

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Finding the amount of neutrons is a bit more work…

Li73

This is called the isotope notation.

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The notation means:

Li73

Mass number, A

Protons and neutrons added together

Atomic number, Z

Number of protons only

73

4This particular atom has

4 neutrons.

Lithium - 7

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The notation means:

Li63

Mass number

Protons and neutrons added together

Atomic number

Number of protons only

63

3This particular atom has

3 neutrons.

Lithium - 6

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Li63Li7

3

Both of these atoms would have the same chemical properties, but one is slightly heavier than the other. They are Isotopes.

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Hint: always find the atomic number FIRST then make protons and electrons equal to that!

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9 9

Neutrons = mass – atomic

[ ] = ? – [ ]

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9 9

Neutrons = mass – atomic

[ 10 ] = ? - [ 9 ] What does the “?” equal?

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9 9 F19

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9 9 F192914 14 Si22 22 25 Ti

Mn253025

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Other demos -

Link:Fun facts about atomic particles

Modern view of atom:electron

neutron

proton