TOPIC: Electrons in Atoms
Ground/Excited States
Do Now: Draw Bohr’s Model for Argon
Cathode Ray Tubes & Discovery of Electrons
Movie of cathode ray tube
Thomson’s Plum-Pudding Model
The positive
charge is
evenly
smeared out.
The negative
charge is in
bits – like
chips.
source
Rutherford’s Experiment - 1911
source
Problems with the Rutherford
Model
• Why don’t electrons crash into nucleus?
• How are electrons arranged?
• Why do different elements exhibit
different chemical behavior?
Bohr Model
• Bohr - electrons in atom can have only specific
amounts of energy
NEW idea!
• Each specific energy is associated with specific
orbit
– --electrons are restricted to these orbits
Bohr assigned quantum number (n) to each
orbit
the smallest orbit (n= 1)
is closest to nucleus
has lowest energy
larger the orbit, more energy it has
Highest energy
(least stable)
n=3
n=2
n=1
Lowest energy
(most stable)
Max Capacity of Bohr Orbits
Orbit
Max # of Electrons
1
2
2
8
3
18
4
32
n
2n2
energy levels get closer
together the farther away
they are from nucleus
Larger orbits can hold more electrons
Electron Transitions
• EXCITED STATE:
• If electron gains (absorbs) specific
amount of energy it can be excited
to higher energy level (farther from
nucleus – less stable)
Endothermic process
GROUND STATE:
Energy released when electron:
drops to lower orbit (closer to
nucleus – more stable)
Exothermic process
Ground State vs. Excited State
• Ground state:
– Lowest energy state of atom
– electrons in lowest possible energy levels
• Configurations in Reference Tables are ground state
• Excited state:
– Many possible excited states for each atom
– One or more electrons excited to higher energy
level
Ground State comes from the reference table
Ar = 2-8-8
Total number of electrons =
18
Total number of protons =
18
Atomic Number =
18
Excited State comes from the reference table
2-8-7-1
Total number of electrons =
18
Total number of protons =
18
Atomic Number =
18
Look on the periodic table for the atomic number
18, what is the ground state electron configuration
Which principal energy level of an
atom contains electron with the
lowest energy?
a)
b)
c)
d)
A) n=1
B) n=2
C) n=3
D) n=4
What is total # of occupied
principal energy levels in atom
of neon in ground state?
a)
b)
c)
d)
A) 1
B) 2
C) 3
D) 4
Which atom in ground state has
five electrons in its outer level
and 10 electrons in its kernel?
a)
b)
c)
d)
A) C
B) Cl
C) Si
D) P
Kernel electrons = inner electrons
Which electron configuration
represents atom in excited state?
A) 2-8-2
a) B) 2-8-1
b) C) 2-8
c) D) 2-7-1
Which electron configuration
represents atom of Li in an
excited state?
a)
b)
c)
d)
A) 1-1
B) 2-0-1
C) 2-1
D) 2-2