Thomson’s Model Rutherford’s Model
Rutherford’s atomic model lacked detail on how
electrons occupied the space around nucleus
Why don’t electrons crash into nucleus?
Chemical Behavior of elements
• At high temperatures or voltages, elements in the
gaseous state emit light of different colors.
• When the light is passed through
a prism or diffraction grating a
line spectrum results.
Bohrs Atomic model
Bohr model : After Rutherford's discovery, Bohr proposed
that electrons travel in definite orbits around the nucleus.
•Explained hydrogen’s spectrum but not
for other elements.
Each element has its own
unique set of spectral emission
lines that distinguish it from
Line spectrum of hydrogen. Each line corresponds
to the wavelength of the energy emitted when the
electron of a hydrogen atom, which has absorbed
energy falls back to a lower principal energy level.
Movement of electrons is not
The atom is mostly empty space
protons and neutrons
region where you might
find an electron
Quantum mechanical model
Modern atomic theory described the
electronic structure of the atom as the
probability of finding electrons within
certain regions of space.
• Instead of being located in orbits, the
electrons are located in orbitals.
• An orbital is a region around the nucleus
where there is a high probability of
finding an electron, can hold a
maximum of 2 electrons.
Four Quantum Numbers:
Specify the “address” of each electron in an atom
Principal Quantum Number ( n )
Angular Momentum Quantum number ( l )
Magnetic Quantum Number ( ml )
Spin Quantum Number ( ms )
1. Principal Quantum Number ( n )
Indicates the number of the energy level
As n increase, size of electron cloud
Energy increases as n increases. (electrons
closer to nucleus have less energy)
2n2 = maximum # of electrons possible in
the energy level
Ex. if n=1 (energy level 1) it can only have 2
Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem
2. Angular Momentum Quantum Number ( l )
Describes the sublevel within each energy level
# of sublevels = value of principal quantum
number of that level
Ex. n=1, has 1 sublevel
n=2, has 2 sublevels
lowest sublevel : s
second sublevel : p
third sublevel : d
fourth sublevel : f
There is just one s sublevel , thus it has one orbital that
can hold only 2 electrons.
There are three p sublevels and thus it has s
three orbitals; each can hold 2 electrons.
There are five d sublevels and thus it has
five orbitals; each can hold 2 electrons.
There are seven f sublevels and thus it has
seven orbitals; each can hold 2 electrons.
Too complicated to show with drawings
P 118 # 6 and p122 # 7,8
3. Magnetic Quantum Number ( ml )
Specifies the exact orbital within each sublevel
4. Spin Quantum Number ( ms )
An orbital can hold 2 electrons that spin in opposite
Indicated by arrows (in opposite direction):
General Rules For Writing
1. Pauli Exclusion Principle
Each orbital can hold TWO electrons with opposite
In the following diagrams boxes represent orbitals.
• Electrons are indicated by arrows: ↑ or ↓.
2. Aufbau Principle
Electrons fill the
represents n, the
3. Hund’s Rule
Within a sublevel, place one e- per orbital before pairing
8e- 1s 2s 2p
1s 2 2s2 2p4
Classwork p 128 #14
Electron Dot Diagrams
The electrons in the outer energy level (greatest value
of n ) called valence electrons are the most
important electrons for chemical reactions.
Lewis electron dot diagrams are used to represent
these outer electrons around the symbol of an
Lithium Electron configuration: 1s22s1
Select electrons that are in the outer energy level (the
ones with the largest principal quantum number):
1s22s1 Largest principal
quantum number is 2
and there is 1 electron in
Li Valence electron
1. Symbol of element represents nucleus and all electrons except
those in outer level
2. Write the electron configuration of element to determine
3. Each side of symbol represents an orbital, draw dots to
represent electrons in that orbital.
Oxygen: 1s22s2 2p4
Oxygen: 1s2 2s2 2p4
Oxygen: has 6 valence electrons (2 +4)
Krypton: 1s22s2 2p6 3s23p6 4s2 3d10 4p6
Krypton: 1s22s2 2p6 3s23p6 4s2 3d10 4p6
krypton: has 8 valence electrons (2 +6)
Classwork p 130 # 15 (Z= atomic number)