ATOMIC MODELS

A Brief History
ATOMIC
MODELS
A BRIEF HISTORY

THOMSON: PLUM PUDDING
• Negative charges
embedded in a ball of
positive charge.
• Problems
• Could not explain
scattering (Gold Foil)
• Ionization

RUTHERFORD: NUCLEAR MODEL
• Atom mostly empty space
• Small dense core of positive
charge (the nucleus)
• Electrons in random orbits
around the nucleus
• Problems
• Nuclear catastrophe: expected
orbiting e- to crash into nucleus due
to Columbic attraction
• Could not explain chemical
properties of the elements

BOHR: BOHR MODEL
• Electrons
• orbit nucleus in distinct, fixed orbits
called energy levels
• Energy level
• a region around the nucleus where the
electron is likely to be moving with a
constant energy
• Further away from the
nucleus, the higher the
energy

• Electrons may move to higher or excited energy states by
absorbing a quanta of energy
• Electrons may drop to lower energy states by releasing a
quanta of energy, usually as light
• Electrons may NOT exist between energy levels

BOHR MODEL
• Explained
• Stability of the atom
• Atomic Emission Spectra
• Periodic Chemical Behavior
• Problems
• Only explained observations for Hydrogen atoms

QUANTUM MECHANICAL MODEL
• A mathematical model
• Determines the allowed energies an
electron can have
• Defines a probability region around the
nucleus where an electron is likely to be
found

• Electron Cloud: probability region where an electron
may be found 90% of the time
• More dense closer to nucleus à higher probability

QUANTUM NUMBERS
• Four quantum numbers fully describe an electron in the
atom
• No two electrons can be described by the same four quantum
numbers

PRINCIPAL QUANTUM NUMBER
• Symbol – n
• Describes the main energy level occupied by the electron
• A positive integer
• Higher the energy level, the further from the nucleus
• Max number of e- allowed in an energy level à 2n 2
Energy Level Max Number of Electrons
1 2
2 8
3 18
4 32
5 50

ORBITAL QUANTUM NUMBER
• Describes an energy sublevel
• Shape of the probability region (s, p, d, f)
• Number of different sub-level types per energy level = n

ORBITAL QUANTUM NUMBER
• Describes an energy sublevel
• Shape of the probability region (s, p, d, f)
• Number of different sub-level types per energy level = n
Energy Level Allowed energy
sublevels
1 s 1
2 s, p 2
3 s, p, d 3
4 s, p, d, f 4
Number of
sublevel

MAGNETIC QUANTUM NUMBER
• Indicates the number and orientation of the atomic orbitals
around the nucleus
• The number of different ways each shape may position itself in
space
• Number of orbitals per energy level = n 2
Shape Number of Orbitals
s 1
p 3
d 5
f 7

SPIN QUANTUM NUMBER
• Indicates the orientation of the electron
• One orbital may hold a max of two electrons
• Electrons in same orbital must have opposite
spins
• Value of + ½ or – ½
• Keeps any two e- from have the same four
quantum numbers

QUESTIONS
1. How is the QM model of the atom different from other
models?
2. What is an atomic orbital?
3. How many orbitals in:
• 3p sublevel - 3d sublevel
• 2s sublevel - third energy level
• 4f sublevel - 4p sublevel
• How many sublevels in the following energy level
• n=1, n=2, n=3, n=4