The Atom
Objectives
Previous Unit
To know the three elementary particles which
compose atoms.
To understand the development of the atomic
model.
Let’s take this one step further…
The Atom Today
 Over time, the model of the atom evolved.
 Two early models we saw were:
 Thomson’s Plum Pudding Model
 Rutherford’s Nuclear Atom
The Atom Today
 The model proposed by Niels Bohr was the Planetary
Model
 The central nucleus (like the sun) surrounded by orbiting
electrons (like the planets)
 Explained that electrons don’t fall into nucleus because they
have fixed energy
 The Bohr model was an improvement, but was replaced by the…
Quantum Mechanical Model
 Modern model of the atom
 No longer are electrons treated like they travel like
“classical” particles (bowling balls)
 Focus is on electrons
 Electrons are found not in orbits but in “clouds”
 Based on probabilities
 Area where there is a 90% chance of finding an electron
 Electron clouds
 Are 3-dimensional
 Come in several shapes
 Are tied to specific energies
 Energy levels fill in special order
Organizing Electrons
Electrons in an atom are organized into
different energy levels
1. Principal energy levels have sublevels
2. Sublevels take the form of atomic orbitals
3. Orbitals “contain” electrons
Principal Energy Levels (n)
Principal energy levels (n) are numbered
Maximum number of electrons in a level is 2n2
n = 1
n = 2
n = 3
2 x 12 = 2 electrons
2 x 22 = 8 electrons
Each principal level has a number of sublevels
equal to the level number
n = 1
n = 2
1 sublevel
2 sublevels
Sublevels
Each sublevel contains a certain number
of atomic orbitals
Orbitals are regions where it is likely an electron
will be found
Each orbital can hold a maximum of 2 electrons
Letters are used to denote orbitals
Orbitals have characteristic shapes
Atomic Orbitals
Orbital
How many per Shape
sublevel?
s
1
spherical
p
3
peanut
d
5
4 are double-peanut
1 is a donut-ringed peanut
f
7
flower
Orbitals are regions in
Space
The energy level of
an electron is the
region around the
nucleus where the
electron is likely to
be moving.
S orbital is a sphere
P orbitals

Each P orbital can hold two electrons, but they
need to have opposite spins
The P sublevel holds 6 electrons
D level orbitals
Each The D sublevel can hold 10 electrons.
Each orbital holds 2 electrons with opposite
spins
The F sublevel has 7
orbitals
Each orbital can hold
2 electrons with
opposite spin
The F sublevel holds
14 electrons
The F Sublevel
Atomic Orbitals
s
p
d
f
Electrons can change orbitals
 Electrons can change orbital, by
absorbing energy. When an
electron absorbs a quantum of
energy, it moves up to a higher
orbital.
 When the electron falls from a high
orbital to a lower orbital, energy is
released, and we see light.
Wintergreen mint is an example
We will also see this in our
spectroscopy and flame test
labs!
Atomic Orbitals
 Electron configurations
A series of numbers and letters to show which orbitals
contain electrons for a given element
 Before we proceed…
Aufbau Principle
 Electrons enter orbitals of the lowest energy first
Pauli Exclusion Principle
 No two electrons can have the same “state”
• State = electron’s orbital and its spin
• Electrons in the same orbital will have opposite spin
Hund’s Rule
 When electrons occupy orbitals of equal energy, one
electron enters each orbital until all orbitals contain one
electron with spin in the same direction
Aufbau Diagram
Orbital Diagrams
A box stands for one orbital
An arrow stands for one electron
Each box (orbital) can only hold 2 arrows
(electrons)
 or to show direction of spin
*Note: There is some overlap of energy levels, so
sublevels do not fill in exact numerical order.
Orbital Diagrams
Orbital Diagram for A Nitrogen
Atom
N

1s

2s

2p


3s
Orbital Diagram for A Fluorine
Atom
F

1s

2s

2p


3s
Orbital Diagram for A
Magnesium Atom
Mg

1s

2s

2p



3s
Learning Check O1
Write the orbital diagram for the electrons
in an oxygen atom.
Solution O1
Write the orbital diagram for the electrons
in an oxygen atom.

1s

2s

2p


3s
Learning Check O2
Write the orbital diagram for the electrons
in an iron atom.
Solution O2
Write the orbital diagram for the electrons
in an iron atom.
 
1s
  
2s
2p
   
3d

3s

  
3p
Electron Configuration
 Once electrons have been placed in boxes, the electron
configuration is written.
Write the energy level and letter for every sublevel
that holds electrons
Use a superscript to indicate the number of electrons
in each sublevel
Example: for Mg – 1s2 2s2 2p6 3s2
Mg

1s

2s

2p



3s
Exceptional Electron
Configurations
Filled energy levels are the most stable
Half filled energy levels are the second
most stable
Copper, Chromium and Silver are very
important examples of how “exceptions” to
filling the Aufbaum diagram leads to
understanding the charge of transition
metals
Examples
 Write electron configurations for: P, Cr, Sn
 Now write the electron configuration without the
boxes: Mg, Ni
 What element has the following electronic
configuration:
1s2 2s2 2p6 3s2 3p1
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2
 For fun at home:
 http://www.lon-capa.org/~mmp/period/electron.htm