Electrons in Atoms
Bohr Model of the Atom
energy level
e- are arranged in orbits
around the nucleus
e- have a fixed energy
level and cannot exist
between energy levels
n=4
n=3
n=2
Energy Levels
 a “quantum” of energy is required to move an e- to a higher level
 Like the rungs of the strange ladder, the energy levels in an atom
are not equally spaced
The Quantum Theory
1. A quantum of energy is the amount of energy required to move
an electron from its present energy level to the next higher one.
2. The higher an electron is on the energy ladder, the further it is
from the nucleus.
3. Energy levels in an atom are not spaced equally.
4. The further the energy levels are from the nucleus, the more
closely spaced they become.
5. The higher the energy level occupied by an electron, the easier
the electron escapes from the atom.
Atomic Orbitals
 Different atomic orbitals are identified by letters.
 s orbitals are spherical
 p orbitals are dumbbell-shaped
 Four of the five d orbitals have the same shape but
different orientations in space.
• Orbits (or energy levels) contain a
limited number of e– 1st orbit = 2 e- (s orbital)
– 2nd orbit = 8 e- (s and p orbitals)
– 3rd orbit = 18 e- (s, p and d orbitals)
– 4th orbit = 32 e- (s, p, d and f orbitals)
e
Configurations

Arrangement of e- around the nucleus of an atom…
1) e- enter orbitals of the lowest energy 1st
(Aufbau principle)
2) An atomic orbital may contain only 2 e –
with opposite spins
(Pauli exclusion principle)
3) Split up e- in orbitals before they pair
up (Hund’s rule)
Aufbau Table
Aufbau Diagram
 Orbitals
s
p
d
f
 Maximum # of
2
6
10
14
e- allowed
H
____

1s

He ____
1s
Energy level
1s1 number of electrons in orbital
orbital
1s2 first level filled  stable configuration
(noble gas)
Writing electron configurations:
C ___ ___ ___ ___ ___
1s
2s
2p
O ___ ___ ___ ___ ___
1s
2s
2s
1s22s22p4
2p
Ne ___ ___ ___ ___ ___
1s
1s22s22p2
2p
1s22s22p6
Writing electron configurations:
Na = 11 e-
K = 19 e-
1s2 2s2 2p6 3s1
1s2 2s2 2p63s2 3p3
1s2 2s2 2p6 3s23p6 4s1
Pb = 82 e-
1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p2
P = 15 e-
[Xe] 6s24f145d106p2
***Exceptions
Cr Family and Cu Family:
“rob” one electron from an s orbital to fill or half-fill a d orbital
Cr [Ar]4s23d4 
[Ar]4s13d5
Mo [Kr]5s14d5
Cu [Ar]4s23d 9  [Ar]4s13d10
Ag
[Kr]5s14d10
Electrons
 Dual properties (wave and particle)
 Ground state - lowest energy level
 Energy is needed to raise the e- from the ground state to
an excited state
 When the e- drops back down to the ground state, light energy is
emitted