Electron Configurations and the Periodic Table

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Electron Configurations and the Periodic Table

• Chapter 3 Section 2

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Electron Configurations

• Electron configuration the arrangement of
  electrons in an atom
• The most important of these electrons are the
  valence electrons or outermost electrons.

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Valence Electrons

• The number of valence electrons for all
  representative elements is determined by the
  number of the group in which the atom is found.
• Examples
• Hydrogen - Group IA or 1
• 1 valence electron.
• Fluorine Group VIIA or 17
• 7 valence electrons

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NOW YOU TRY!

• How many valence electrons do the following
  elements contain?
• A. Lithium
• B. Carbon
• C. Aluminum
• D. Sulfur
• E. Krypton

1
4
3
6
8
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So, who cares?

• Valence electrons are important because they
  determine how atoms interact with each other to
  form compounds!

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Arrangement of Electrons in Atoms according to
the Quantum Mechanical Model

• Electrons in atoms are arranged as
• ENERGY LEVELS
• SUBLEVELS
• ORBITALS

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Energy Levels

• Each energy level has a number called the
  PRINCIPAL QUANTUM NUMBER, n
• Currently n is 1 thru 7 because of the elements
  known, but there is a possibility of infinite
  levels.

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Relative sizes of the spherical 1s, 2s, and 3s
orbitals of hydrogen.
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Sublevels AKA Subshells

• The sublevels are named s, p, d, and f and are
  listed in increasing energy.
• We specify both the energy level and sublevel
  when describing an electron, i.e. 1s, 2s, 2p.

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Determining the number of sublevels

• The number of sublevels in any given level is
  equal to n.
• The first energy level has 1 sublevel 1s.
• The second energy level has 2 sublevels 2s and
  2p.
• How many sublevels exist on the 3rd energy level?
• What would they be called?

3
3s, 3p, 3d
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Orbitals
Orbital a specific region of a sublevel
containing a maximum of 2 electrons.

• The p sublevel has 3 orbitals.
• They are called px, py, and pz.

The s sublevel has 1 orbital.
The d sublevel has 5 orbitals.
The f sublevel has 7 orbitals.
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Shapes of Orbitals
Typical s orbital (sphere)
(peanut)
(double peanut)
Typical f orbital (flower)
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How many electrons can be in a sublevel?
Remember A maximum of two electrons can be
placed in an orbital.
s sublevel
d sublevel
p sublevel
f sublevel
Number of orbitals
1
3
5
7
Number of electrons
6
10
14
2
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Aufbau Principle

• An electron occupies the lowest-energy orbital
  that can receive it.
• This guides how electron configurations are
  written.

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Rules to Remember when Writing Electron
Configurations

• Obtain the number of total electrons from the
  periodic table.
• Electrons occupy the lowest energy orbitals
  first.
• Each energy level (n) only contains n sublevels.
• The s sublevel holds 2 e-, the p 6 e-, the d 10
  e-, and the f 14 e-.
• Follow the filling pattern only moving once each
  sublevel is full.

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Diagonal Rule

• The diagonal rule is a memory device that helps
  you remember the order of the filling of the
  orbitals from lowest energy to highest energy.

• Steps
• Write the energy levels top to bottom.
• Write the sublevels in s, p, d, f order. Write
  the same number of sublevels as the energy level.
• Draw diagonal lines from the top right to the
  bottom left.
• To get the correct order, follow the arrows!

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Diagonal Rule
1 2 3 4 5 6 7
s
By this point, we are past the current periodic
table so we can stop.
s 2p
s 3p 3d
s 4p 4d 4f
s 5p 5d 5f
s 6p 6d 6f
s 7p 7d 7f
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You may have learned it this way!
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Periodic Table Method Steps

• Find the element for which you are writing the
  configuration.
• Starting with Hydrogen, write down the energy
  level and sublevel.
• Count the boxes in the sublevel and add it as a
  superscript.
• Continue moving through the sublevels until you
  reach your destination.

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Electron Configurations

• 3d7

Number of electrons in the sublevel
Energy Level
Sublevel
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2
4f14 etc.
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OVERLAPSWhy are d and f orbitals always in lower
energy levels?

• d and f orbitals require LARGE amounts of energy
  to create.
• According to the Aufbau principle we must skip a
  sublevel that requires a large amount of energy
  (d and f orbtials) for one in a higher level but
  lower energy
• OVERLAPS are the reason for the diagonal rule!
• FOLLOW THE ARROWS IN ORDER!

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Lets Try It!

• Write the electron configuration for the
  following elements
• H
• Li
• N
• Ne
• K
• Zn
• Pb

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Lets Try It!

• Write the electron configuration for the
  following elements
• H 1s1
• Li 1s2 2s1
• N 1s2 2s2 2p3
• Ne 1s2 2s2 2p6
• K 1s2 2s2 2p6 3s2 3p6 4s1
• Zn 1s2 2s2 2p6 3s2 3p6 4s2 3d10
• Pb 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6
  6s2 4f14 5d10 6p2

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Noble Gas Notation

• A way of abbreviating long electron
  configurations
• Since we are only concerned about the outermost
  electrons, we can skip to places we know are
  completely full (noble gases), and then finish
  the configuration

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Noble Gas Notation

• Step 1 Find the closest noble gas to the atom,
  WITHOUT GOING OVER the number of electrons in the
  atom. Write the noble gas in brackets .
• Step 2 Find where to resume by finding the next
  energy level (row in periodic table).
• Step 3 Resume the configuration starting with
  ns2 where n is the next level

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Noble Gas Notation

• Chlorine
• Longhand is 1s2 2s2 2p6 3s2 3p5
• You can abbreviate the first 10 electrons with a
  noble gas, Neon. Ne replaces 1s2 2s2 2p6
• The next energy level after Neon is 3
• So you start at level 3 on the diagonal rule (all
  levels start with s) and finish the configuration
  by adding 7 more electrons to bring the total to
  17
• Ne 3s2 3p5

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Practice Noble Gas Notation

• Write the noble gas notation for each of the
  following atoms
• Cl
• K
• Ca
• I
• Bi

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Practice Noble Gas Notation

• Write the noble gas notation for each of the
  following atoms
• Cl Ne3s2 3p5
• K Ar4s1
• Ca Ar4s2
• I Kr5s2 4d10 5p5
• Bi Xe6s2 4f14 5d10 6p3