Transcript 3.7 Electron Energy Levels

Chapter 3
Atoms and Elements
Bohr Model of Hydrogen



Electrons can only
"orbit" the atom's
nucleus at special
locations.
Each special location
corresponds to a
particular amount of
energy called an energy
level.
Energy levels are
discrete . We enumerate
them with integers n .
3.7 Electron Energy Levels
Energy levels
• are assigned numbers n = 1, 2, 3, 4,
and so on.
• increase in energy as the value of n
increases.
• are like the rungs of a ladder with
the lower energy levels nearer the
nucleus.
 The lowest energy state is called
the "ground state" . It corresponds
to innermost n=1 orbit on the
diagram.
 If the electron is in the n=1 energy
level, then we say that the atom is
in its ground state.
 If the electron is in a higher energy
level, n > 1, then we call the atom
"excited" .
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Visible spectrum
When atoms are excited they
emit light of certain
wavelengths which correspond
to different colors. The emitted
light can be observed
as a series of colored lines with
dark spaces in between;
this series of colored lines is
called a line or atomic spectra.
Each element produces a
unique set of spectral lines.
Since no two elements emit the
same spectral lines, elements
can be identified by their line
spectrum
Energy Levels
Energy levels have a maximum number of electrons equal
to 2n2.
Energy level
n=1
n=2
n=3
n=4
Maximum number of electrons
2(1)2 = 2(1) = 2
2(2)2 = 2(4) = 8
2(3)2 = 2(9) = 18
2(4)2 = 2(16) = 32
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Orbitals
An orbital
• is a three-dimensional space around a nucleus,
where an electron is most likely to be found.
• has a shape that represents electron density (not a
path the electron follows).
• can hold up to 2 electrons.
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Orbitals

An s orbital
• has a spherical shape around
the nucleus.
• is found in each energy level A
p orbital
• has a two-lobed shape.
• is one of three p orbitals in
each energy level from n = 2.
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Electron Level Arrangement
In the electron level arrangement for the first 36
elements
 electrons are placed in energy levels (1, 2, 3, etc.),
beginning with the lowest energy level
 there is a maximum number in each energy level.
Energy level
Number of electrons
1
2 (up to He)
2
8 (up to Ne)
3
8 (up to Ar)
4
2 (up to Kr)
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Examples
Write the electron level arrangement for each:
1. Li
2. F
3. P
4. Sc
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Examples
Identify the element with each electron level
arrangement:
1.
2, 2
2. 2, 8, 2
3. 2, 8, 3
4. 2, 8, 10, 2
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Electron Configurations of
Multielectron Atoms
Electron Configuration: A description of which orbitals are occupied by
electrons.
Degenerate Orbitals (suborbitals): Orbitals that have the same energy
level. For example, the three p orbitals in a given subshell.
Ground-State Electron Configuration: The lowest-energy
configuration.
Aufbau Principle (“building up”): A guide for determining the filling
order of orbitals.
Electron Configurations of
Multielectron Atoms
Rules of the aufbau principle:
1. Lower-energy orbitals fill before higher-energy orbitals.
2. An orbital can only hold two electrons, which must have
opposite spins (Pauli exclusion principle).
3. If two or more degenerate orbitals are available, follow
Hund’s rule.
Hund’s Rule: If two or more orbitals with the same energy
are available, one electron goes into each until all are half-full.
The electrons in the half-filled orbitals all have the same spin.
Electron Configurations of
Multielectron Atoms
Electron
Configuration
H:
He:
1s1
1s2
1 electron
s orbital (l = 0)
n=1
2 electrons
s orbital (l = 0)
n=1
Electron Configurations of
Multielectron Atoms
Electron
Configuration
Lowest energy to highest energy
Li:
1s2 2s1
1 electrons
s orbital (l = 0)
n=2
Oxygen:
Electron Configurations and the
Periodic Table
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Give expected ground-state electron configurations
for the following atoms, draw – orbital filling
diagrams and determine the valence shell
◦ F (Z = 9)
◦ Mg (Z = 12)
◦ Sr (Z = 38)
◦ Se (Z = 34)
Valence Electrons
The valence electrons
• determine the chemical properties of the elements.
• are the electrons in the highest energy level.
• are related to the group number of the element.
Example: Phosphorus has 5 valence electrons.
5 valence electrons
P in Group 5A(15)
2, 8, 5
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Groups and Valence Electrons
All the elements in a group have the same number of
valence electrons.
Example: Elements in group 2A (2) have two (2)
valence electrons.
Be
2, 2
Mg 2, 8, 2
Ca
2, 8, 8, 2
Sr
2, 8, 18, 8, 2
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Examples
State the number of valence electrons for each.
A. O
1) 4
2) 6
3) 8
B.
C.
Al
1) 13
2) 3
3) 1
Cl
1) 2
2) 5
3) 7
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