8 th Science 2012

Matter Defined
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Matter – anything that has volume or mass.
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Everything you see and don’t see.

Definitions
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Element- pure substance that cannot be separated into simpler substances by physical or chemical means.
 Represented by a symbol – Au stands for what?

Definitions
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Atom – Smallest particle into which an element can be divided and still be the same substance, retaining its properties

Evolution of the Atomic Theory
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Democritis – 440 BC
Greek philosopher who said that if you cut a substance in half again and again and again eventually you would have an “uncuttable” piece said that all atoms made of a single material formed into different shapes and they join together to make different materials
Greek word atomos
“indivisible”
, which means

John Dalton - 1803
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Studied the atmosphere and other gases and developed atomic theory.
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Dalton demonstrated that elements combine in specific
proportions to make different substances

John Dalton’s Atomic Theory
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1. All substances are made of atoms .
Dalton’s New System of Chemical Philosophy
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2. Atoms are small particles that cannot be created, divided, or destroyed.

Dalton’s Atomic Theory, continued
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3. Atoms of the same element are identical in size, mass and other properties.
4. Atoms of different elements combine to make chemical compounds.
5. In chemical reactions , atoms are combined, separated, or rearranged.

J.J. Thomson - 1897
► discovered that there are small particles inside the atom, so the atom can be divided into smaller parts

JJ Thomson, continued
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Experimented, using a cathode-ray tube to discover negative
particles he called corpuscles
► these negatively charged particles found in all atoms are now called electrons

Thomson’s Model
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Plum Pudding model
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Described electrons as scattered all about the atom

Ernest Rutherford - 1908

Rutherford, continued
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Discovered a tiny, extremely dense, positively charged region called a nucleus
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Most of the atom’s
mass was in the nucleus and electrons traveled around the nucleus

Rutherford cont’d
► said the atom was mostly
empty space

Niels Bohr – 1913
► said the electrons
travel around the nucleus in definite paths

Erwin Schrödinger and
Werner Heisenberg - 1926
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Said electrons travel in clouds, not definite paths but we still use the Bohr Model to represent atoms.

The Atom

Subatomic Particles
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Determine the identity of the matter
positively charged particles found in the nucleus
Mass =1.67262158 × 10 -27 kilograms
0.000000000000000000000000017 kg
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Each proton is 1 amu
 AMU stands for Atomic Mass Unit

Subatomic Particles
Neutrons are the glue that holds the protons together in the nucleus
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► no charge found in the nucleus mass is 1 amu

Subatomic Particles
Determine the reactivity
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Negatively charged
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Found around the nucleus within electron clouds
Mass is very small, almost zero

Atoms are NEUTRAL
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Atoms are neutral because the number of protons equals the number of electrons.

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How To Read a Square
8
O
Oxygen
15.9994
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Elements are identified by their atomic number – the number of protons. Also equals the number of electrons.
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Chemical Symbol
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Element Name
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Atomic Mass

PEN Method
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SLIDE, SLIDE, ROUND
THE BOTTOM
SUBTRACT TOP
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P = 8
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E = 8
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N = 8
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8
O
Oxygen
15.9994