n ---> shell 1, 2, 3, 4, ...
l ---> sub-shell 0, 1, 2, ... n - 1
ml ---> orbital +l ... 0 ... -l
ms ---> electron spin +1/2 and -1/2
• Orbitals for atoms other than hydrogen The resulting net positive charge
have the same shapes as those in the
hydrogen atom; However, the presence of is called the effective nuclear
more than one electron greatly changes charge (Zeff)
the energies of those orbitals
• In many-electron atoms, electrons are both
attracted to the nucleus and repelled by
Zeff = Z - S
other electrons. Where:
• Any electron density between the nucleus Z = number of protons
and the electron of interest will reduce the
S = number of electrons
nuclear charge acting on that electron. between electron of
interest and the nucleus.
The nuclear charge experienced by outer Electron Configurations
electrons is always less than that experienced by
= The arrangement of electrons distributed
“The Screening Effect” among the orbitals of an atom.
The extent to which electrons will be
affected by the screening effect determines the Three Guiding Principles in
order in which orbitals will be filled by electrons.
That is why the 4s 1. Aufbau’s Principle
orbital fills before the 3d
orbitals. 2. Pauli-exclusion Principle
3. Hund’s Rule
Bohr’s Aufbau Principle
States that electrons will
occupy the lowest possible energy
• e- are added to atoms level possible.
into the lowest This gives rise to the Order-of-Fill:
energy level & sub-
• Gives rise to the
Following the arrows on the diagram
gives rise to the following Order-of-Fill:
Pauli Exclusion Principle
• An atomic orbital may describe
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s at most two electrons, each
behaving as if they spin on an
4f 5d 6p 7s 5f 6d 7p 8s axis.
• According to the Pauli
•Notice, 4s is exclusion principle, only
electrons spinning in opposite
lower in directions can occupy the same
energy than 3d orbital within a sublevel.
and fills first.
Pauli Exclusion Principle
n l ml s
1 0 0 +1/2
1 0 0 -1/2 The lowest energy is attained when the
2 0 0 +1/2 number of electrons filling a multiple
2 0 0 -1/2 orbital sublevel having the same spin is
2 1 1 +1/2
2 1 0 +1/2
maximized. In other words, one electron
will fill each orbital of a sublevel all with
2 1 -1 +1/2
the same spin before a second will fill.
Assigning Electrons to Atoms Electron Configuration
• Electrons generally assigned to
orbitals of successively higher • Electron Notation
energy. • Orbital Notation
• For H atoms, E = - k(1/n2). E • Noble Gas Notation
depends only on n.
• For many-electron atoms, energy At this point you should learn to read
depends on both n and l. the order-of-fill from the periodic
Electron Configurations Electron Configurations
• Distribution of
• Distribution of all electrons in
all electrons in an atom
Therefore, 4th an atom • Consist of
main energy level • Consist of Number
sublevel in the 4th
or n = 1 Number denoting the
main energy level
denoting the energy level
energy level Letter
type of orbital
Electron Configurations Given that an s orbital only holds 2
electrons, p orbital holds 6 electrons, d
• Distribution of all
electrons in an atom. holds 10 and f holds 14, the electron order
• Consist of of fill also designates the number of
Number denoting the electrons in each sublevel:
Therefore, 5th energy level.
electron of the Letter denoting the 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6
possible 6 filling type of orbital.
the 3 orbitals of Superscript denoting 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6 8s2
the p sublevel in the number of
the 4th main electrons in those
energy level orbitals.
Using the order of fill, we can designate the A second method of writing electron
electron configuration for any element. Observe: configurations, know as orbital fill notation,
•Chlorine has 17 electrons in its electron cloud shows not only the main energy level, sublevels
and electrons in each sublevel, but also includes
•From the order of fill: the individual orbitals.
2 + 2 + 6 + 2 + 5 = 17
• Each box represents
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 one orbital.
• Half-arrows represent
6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6 8s2 the electrons.
• The direction of the
•Therefore: arrow represents the
spin of the electron.
17Cl 1s2 2s2 2p6 3s2 3p5
So, where electron configurations look like:
9F 1s2 2s2 2p5 3Li
Orbital fill looks like: 6C
9F ↑↓ ↑↓ ↑↓ ↑↓ ↑
1s 2s 2px 2py 2pz
s And p elements make up the
and the Periodic Table
representative, or “main
d elements are the transition
f elements are the lanthanide
Some Anomalies Some Anomalies
Some For instance,
irregularities the electron
occur when configuration
there are for copper is
enough [Ar] 4s1 3d10
electrons to rather than the
half-fill s and d expected
orbitals on a
given row. [Ar] 4s2 3d9.
Some Anomalies The result of electron
configuration anomalies is the
• This occurs
existence of multiple ionic charges
because the 4s
and 3d orbitals
are very close in Consider: Zn2+, Ag+, Cu+, Fe3+ Sn2+
• These Ions with full or half-full sub-levels
are particularly stable
atoms, as well.
Remember, ions are formed as atoms
gain or loose electrons to obtain an 1.Write an isoelectronic
electron configuration equal to that of the
closest noble gas, or an octet. series involving five
Consider: atoms, including Kr.
Mg2+,Na+, Ne, F-,O2-
They all have the same electron
Chemical species with the same e-
configuration are ISOELECTRONIC
Essentially two 2.Write the correct electron
configurations, using all
classes of electrons three notations, for the
following elements, then
core e- circle all valence level
Li, Na, Be, Mg,B, Al, O, S, F, Br, Hg, Am.