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Chapter 11
The Atom
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
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.
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.
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
Atomic Number
• Number of protons in the nucleus
• Determines the elements
• Found in each elements square on the
periodic table.
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
Naming Isotopes
• Element name – mass number
Hydrogen – 1
(1 proton + no neutrons =1)
Hydrogen – 2
(1 proton + 1 neutron = 2)
Number of Neutrons in an Isotope
# neutrons = mass # – atomic #
p+n–p=n
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
Atomic Mass
Weight
• average of all the known naturally occurring
isotopes of an element.
Calculating Atomic Mass
Weight
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
Try These
Chlorine = 35.5 amu
Chlorine – 35 (76%)
Chlorine – 37 (24%)
Thallium = 204.4 amu
Thallium – 203 (30%)
Thallium – 205 (70%)
Gallium = 69.8 amu
Gallium – 69 (60%)
Gallium – 71 (40%)
Uranium = 237.978 amu
Uranium – 238 (99.284%)
Uranium – 235 (0.711%)
Uranium – 234 (0.005%)
Titanium = 47.9 amu
Titanium – 46 (8%)
Titanium – 47 (7.3%)
Titanium – 48 (73.8%)
Titanium – 49 (5.5%)
Titanium – 50 (5.4%)
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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