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Chapter 11
The Atom
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The Atom
• The smallest particle into which an element can be broken down.
• size: 3x10-8 cm = .00000003 cm
3 hundred millionths cm
• Structure - Subatomic particles located in the nucleus and the electron cloud
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Nucleus
• small, dense, positively charged center of the atom
Contains:+ Protons – positively charged particles
• Mass: 1amu (atomic mass unit) = 1.7x10-24 g
0 Neutrons – no charge• Mass: 1 amu
• Most of the mass of an atom is found in the nucleus.
• The volume is small which is why it is dense.
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Electron Cloud
• mostly empty space.
Contains:– Electrons – negatively charged particles
• Mass: 0 amu
• Can only predict where the electrons can be found around the nucleus.
• Most atoms have an equal number of protons and electrons.
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Forces in the Atom
• Electromagnetic force – holds the electrons around the nucleus.
Negative electrons are attracted to the positive protons, opposites charges attract.
• Strong force – holds the nucleus together. Stronger than the electromagnetic force that pushes the protons apart, like charges repel.
• Weak Force – plays a key role in radioactive atoms where neutrons turn into protons and electrons
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Atomic Number
• Number of protons in the nucleus
• Determines the elements
• Found in each elements square on the periodic table.
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Isotopes
• Atoms of the same element with a different number of neutrons.
• You can tell them apart by their mass numbers
• Mass number = number of protons plus neutrons
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Naming Isotopes
• Element name – mass number
Hydrogen – 1
(1 proton + no neutrons =1)
Hydrogen – 2
(1 proton + 1 neutron = 2)
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Number of Neutrons in an Isotope
# neutrons = mass # – atomic #
p + n – p = n
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Unstable Isotopes
• Some isotopes are unstable making them radioactive (break apart over time)
Carbon – 14 used in carbon dating
Uranium – 238 used in nuclear chain reaction
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Mass
• average of all the known naturally occurring isotopes of an element.
WeightAtomic
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Mass
Copper’s atomic mass = 63.6 amu
Known Isotopes: copper – 63 (69%)
copper – 65 (31%)
Equation:mass# X percentage + mass# X percentage +.…= atomic mass
63 amu (.69) + 65 amu (.31) = 63.6 amu
WeightCalculating Atomic
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Try These
Chlorine =
Chlorine – 35 (76%)
Chlorine – 37 (24%)
Thallium =
Thallium – 203 (30%)
Thallium – 205 (70%)
Gallium =
Gallium – 69 (60%)
Gallium – 71 (40%)
Uranium =
Uranium – 238 (99.284%)
Uranium – 235 (0.711%)
Uranium – 234 (0.005%)
Titanium =
Titanium – 46 (8%)
Titanium – 47 (7.3%)
Titanium – 48 (73.8%)
Titanium – 49 (5.5%)
Titanium – 50 (5.4%)
35.5 amu
204.4 amu
69.8 amu
237.978 amu
47.9 amu
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Ions• Atoms with a charge because they gained or lost an
electron.• Have an unequal number of protons and electrons
charge = protons – electrons
Positive Ion Sodium atom has 11 protons Sodium Ion has 10 electrons +11 – 10 = +1
Negative Ion Chlorine atom has17 protons Chlorine Ion has 18 electrons +17 – 18 = – 1
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