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Atomic Structure
Chapter 1
Dalton’s Atomic Theory
• John Dalton: first person to use the term atom (1800s)
• Atomic Theory:
• 1. All matter is made of atoms
• 2. Atoms cannot be created or destroyed
• 3. Atoms of different elements are different
• 4. Atoms of elements combine to make new compounds
• 5. Atoms are changed in chemical reactions
Dalton’s Atomic Theory, cont’d
• Cannonball model of the atom  atom is hard, solid sphere
Figure 1.1: The cannonball model of the atom represents the atom as a solid sphere.
The Discovery of the Electron
• J. J. Thomson discovered the electron
• Showed the cannonball model was wrong
• Electron = negatively charged subatomic
particle
• Plum pudding model  atom is a
positively-charged sphere with randomly
embedded negative charges
+
+ –
– +–
– + +
+ –
–
Figure 1.2: The plum pudding model of the atom represents the atom as a positive sphere with randomly‐embedded negative charges.
The Gold Foil Experiment
• Done by Ernest Rutherford in early 1900s
• Wanted to show the plum pudding model was correct
• Set up:
• Thin sheet of gold foil surrounded by a phosphor screen
• Beam of tiny positive alpha particles
The Gold Foil Experiment, cont’d
• What did Rutherford expect to happen?
• Alpha particles should pass straight through the gold foil
• See flashes of light on the screen behind the foil
• What really happened?
• Most alpha particles passed straight through, but some were
deflected
• Saw a few flashes of light in front of the foil
The Gold Foil Experiment, cont’d
• Two conclusions:
• 1. The atom has a small, dense,
positive nucleus
• 2. The rest of the atom is mostly
empty space
• Plum pudding model is wrong
The Bohr Planetary Model
• Described by Neils Bohr
• Protons and neutrons are in the
nucleus
• Electrons travel around the nucleus
on fixed orbits or shells
• Looks like the solar system
Figure 1.5: The planetary model shows electrons moving on fixed orbits around the nucleus.
The Wave‐Mechanical Model
• Current accepted model
• Nucleus is still in the center
• Electrons are in orbitals
• Orbital: an area where an electron is
most likely to be found
• Also called electron clouds
• Electrons are free to move in their orbitals
Figure 1.6: Electrons in the wave‐
mechanical model are found in orbitals. Dark rings around the nucleus indicate where an electron is most likely to be found.
Subatomic Particles
Name
Proton
Neutron
Electron
Symbol
p+
n0
e–
Charge
+1
0
–1
Size
1.0 amu
1.0 amu
1/1836 amu
Location
Nucleus
Nucleus
Orbitals
Atomic Number & Mass Number
• Atomic number = number of protons
• Same for all atoms of the same element
• Elements are neutral  same number of protons and electrons
• Mass number = number of protons + number of neutrons
12
6
mass #
C
atomic #
14
6
C
C-12 C-14
Atomic Number & Mass Number, cont’d
• Finding the number of neutrons:
• Subtract the atomic number from the mass number
How many neutrons are in an atom of Al-27?
19
How many neutrons are in an atom of F ?
9
Atomic Number & Mass Number, cont’d
• Fill in the following chart:
Element Atomic # Mass # Protons Neutrons Electrons
9
4 Be
4
9
4
5
4
Cl-35
17
35
17
18
17
Atomic Number & Mass Number, cont’d
• Fill in the following chart:
Element Atomic # Mass # Protons Neutrons Electrons
P
15
K
19
31
39
15
16
15
19
20
19
Isotopes
• Isotopes: atoms of the same element that have different mass
numbers
• Different # of neutrons, same # of protons
12
6
C
14
6
C
Same atomic #,
different mass #
Isotopes, cont’d
• Label the following as “isotopes” or “not isotopes”
14
7
19
9
N
14
&
6
F
19
&
9
C
F
64
29
64
Cu & 27 Co
29
14
28
Si & 14 Si
Average Atomic Mass
• No mass numbers on the Periodic Table
• Average atomic mass: the weighted average of the masses of
all the naturally-occurring isotopes of an element
Average Atomic Mass, cont’d
• An element has two naturally-occurring isotopes. The first
one has a natural abundance of 92.72% and a mass of 27.91
amu. The second has a natural abundance of 7.28% and a
mass of 26.59 amu. Calculate the average atomic mass.
• Element X has three naturally-occurring isotopes. X-54 has
an abundance of 70.2%, X-55 has an abundance of 22.7%,
and X-56 has an abundance of 7.1%. Calculate the average
atomic mass.
Ions
• Ions are atoms that have either lost or gained electrons
• Cations: positive ions made by losing electrons
• Anions: negative ions made by gaining electrons
+ + + + + ++
_ _ _ _ _ _ _
This is a neutral atom.
+ + + + + ++
_ _ _ _ _ _
This is a positive ion.
+ ++ + ++
_ _ _ _ _ _ _
This is a negative ion.
Ions, cont’d
• The number and sign in the charge of an ion tell you how
many electrons were lost or gained
Na+ has lost 1 electron
F– has gained 1 electron
Ca2+ has lost 2 electrons
O2– has gained 2 electrons
Al3+ has lost 3 electrons
N3– has gained3 electrons