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Chapter 6
Introduction to Atoms
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Ch 6 Sec 1
Development of Atomic Theory
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Vocabulary
• Atom• Electron• Nucleus• Electron cloud
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Vocabulary
• Atom– Smallest unit of an element that still maintains the
properties of that element• Electron– Subatomic particle– Negative charge
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Vocabulary
• Nucleus– Atom’s central region– has protons and neutrons
• Electron Cloud– Region around the nucleus where electrons are
found
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Beginning of Atomic Theory
• Democritus – Greek philosopher 440 BCE– Thought you cut any thing till at some point
you could cut it down into anything simplier• Atomos
– Not able to be divided
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Dalton’s Atomic Theory• By 1700s science was in full swing• John Dalton– Wanted to know why compounds always
form in set ratios– Set forth 3 key theories • All substances made of atoms. Atoms cannot be
created or destroyed• Atoms of same elements are identical• Atoms join with other atoms to make new
substances
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Thomson’s Discovery of Electrons
• In 1837 JJ Thomson discovered the electron– Proved that the atom isn’t the smallest
particle• Found negatively charged particle– Electron
• Thought electrons scattered throughout the atom– Plum pudding model
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Rutherford’s Atomic “Shooting Gallery”
• In 1909 Ernest Rutherford wanted to tested Thomson’s subatomic particle idea
• Aimed positive particles at a thin sheet of gold foil– Most positive particles passed through but
some bounced off to side and some bounced back
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Nucleus and the Electrons
• Rutherford’s experiment proved that there’s a nucleus– Dense center of the atom– Positive charge
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Copy these diagrams
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Nucleus & Electrons
• Bohr’s electrons levels– Electron’s move in definite energy levels
outside the nucleus.– Electrons only in those energy levels, never
in between
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Nucleus & Electrons
• Modern Atomic Theory– Electrons still in energy levels– But can move from level to level
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Ch 6 Sec 2: The Atom
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Vocabulary
• Proton• Atomic mass unit• Neutron• Atomic number• Isotope• Mass number• Atomic mass
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Vocabulary
Define on your own!
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Parts of the Atom
• There are 3 main subatomic particles– Proton • Positive charge
– Neutron• No charge, neutral
– Electron• Negative charge
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Parts of an Atom
• The number of subatomic particles determine how the atoms behaves– Change the # of protons = change the
element– Change the # of neutrons = change the
weight of the atom– Change the # of electrons = change how the
atom may form chemical bonds
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Parts of an Atom
• Only protons and neutrons have measurable mass
• Mass measured in “amu” units – Atomic mass unit
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Parts of an Atom
• Electrons– Compared to protons & neutrons, electrons
have extremely small mass– Negative charge of one electron cancels the
positive charge of one proton– Ion • An atom with unequal # of electrons vs protons• Very important to chemical bonding!
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Atoms and Elements
• Role of Neutrons– Act as a buffer to the positive charges of the
protons• Otherwise the nucleus would tear apart
– Nucleus does not have to equal number of neutrons and protons• Generally more neutrons than protons inside an
atom’s nucleus
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Atoms and Elements
• Atomic number of an element = number of protons– Every oxygen atom has 8 protons– If you change the number of protons, you
change the element!
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TEST ALERT…..
# of protons = Atomic Number
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Isotopes
• Can vary the number of neutrons• Isotopes– Atoms of the same elements, but with
different number of protons
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Isotopes
• Radioactive atoms – Have unstable nucleus and decay at fast
rates• Nearly all isotopes of the same elements
have similar chemical and physical properties
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Isotopes
• Difference in isotopes– Mass number = sum of protons and neutrons•MN = P + N
–Carbon isotope• 6 protons• 10 neutrons• mass number = 6 + 10 = 16
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Isotopes
• Naming Isotopes– Write the name of the element, followed by
a hyphen and the mass numberoxygen-18Mass = protons + neutrons18 = 8 protons + ____ neutrons18 = 8 protons + 10 neutrons
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Isotopes
• Atomic mass– Weighted average of all the isotope mass
numbers naturally occurring– Most elements found in nature contain a
mix of 2 or more isotopes
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Forces in Atoms
• Gravitational force– Pulls particles together
• Electromagnetic Force– Particles with same charge repeal each other– Reason for neutrons in a nucleus
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Forces in Atoms
• Strong Force– Helps to keep protons near each other
• Weak Force– In radioactive isotopes• Plays role when neutrons change to protons or
electrons– Changing is nuclear reaction