Download Periodic Trends

November 07, 2014
Periodic Trends
Remember from the "Periodic Table" Notes...
• The periodic table is a tabular display of the chemical
elements, organized by their atomic number, electron
configuration, and recurring properties.
• Periodic law: There is a periodic repetition of
chemical and physical properties of the elements when
they are arranged by increasing atomic number
November 07, 2014
Atomic Radius Graph
• What are some initial observations about the atomic
radius data/graph?
• What is atomic radius?
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Go finish the rest of the worksheet with your
group! You have 20 minutes.
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Atomic Radius Trend Discussion
• What happens to atomic radius as you go across the
period? Why?
• What happens to atomic radius as you go down the
group? Why?
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Periodic Trends Notes
Get your handout out!
November 07, 2014
Why is it called a periodic table?
• The properties of the elements in the table repeat in a
"periodic" way (specific pattern).
• Periodic law: There is a periodic repetition of chemical
and physical properties of the elements when they are
arranged by increasing atomic number
• The modern periodic table is arranged by
> atomic number = # of protons
> properties
> electron configuration
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Periodic Law
• Now lets look at some properties of elements
> We looked at some of these in "Meet My Family"!
Alkali Metals
Halogens
November 07, 2014
Periodic Trends
• Chemical properties of elements are determined by
their electron configuration.
• Properties are periodic because the number of valence
electrons is periodic.
November 07, 2014
Electron Configuration and the Periodic Table
• Remember electrons are found in atomic orbitals
> Principle energy level (n, shells) tells us the
relative size and energy of atomic orbitals.
> Each shell can hold a certain number of electrons.
> # of e- = # of p+ for a neutral atom
> Valence electrons = electrons in outermost shell
2
32
18
8
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Electron Configuration and the Periodic Table
• If we draw these orbital diagrams for H, He, Li, Be Na,
Mg,
2
32
18
8
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Electron Configuration and the Periodic Table
• Periods: Period equals the highest principle energy
level of electrons (shell of the valence electrons)
• Groups: Elements in the same group have the same
number of valence electrons
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Now lets look at two trends
1. Atomic radius
2. Ionization energy
November 07, 2014
1. Atomic Radius
The electron cloud surrounding a nucleus is based on
probability and does not have a clearly defined edge.
• Atomic radius: half the distance between nuclei of
adjacent identical atoms
> Adjacent nuclei in a crystal
> Nuclei of two atoms bonded together
http://en.wikibooks.org/wiki/High_School_Chemistry/Atomic_Size
November 07, 2014
Atomic Radius
• Atomic radius is determined by the strength of
attraction between the valence electrons and the
nucleus
> # of protons in nucleus
> # of shielding electrons that "block" protons from
valence electrons
November 07, 2014
Atomic Radius Trend
• What happens to atomic radius as you go across the
period?
• What happens to atomic radius as you go down the
group?
November 07, 2014
So why does the atomic radius...
• Decrease across a period?
• Increase down a group?
http://employees.csbsju.edu/cschaller/Principles%20Chem/atoms/atomperiodic.htm
November 07, 2014
Atomic Radius
• Moving down a group: Atomic radius increases
> Principal energy levels increase (bigger shell) =
increased orbital size, electrons are farther from
nucleus
http://employees.csbsju.edu/cschaller/Principles%20Chem/atoms/atomperiodic.htm
http://www.chem.ox.ac.uk/vrchemistry/Machinery/html/page02.htm
November 07, 2014
Atomic Radius
• Moving across a period: Atomic radius decreases
> Principal energy level remains the same
> # of protons increases: electrons pulled in more
http://chemwiki.ucdavis.edu/
http://employees.csbsju.edu/cschaller/Principles%20Chem/atoms/atomperiodic.htm
http://chemwiki.ucdavis.edu/
November 07, 2014
Example 1: Order the following atoms from smallest to
biggest atomic radius:
Ga, Se, K
I, F, Cl
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2. Ionization Energy
• Ionization energy = energy required to remove
an electron from a gaseous atom. (kJ/mol)
When you remove an
electron from an atom,
what do you get?
November 07, 2014
Ionization energy
• Ionization energy is an indication of how strongly an
atom's nucleus holds onto its valence electrons
> Greater IE = harder to ionize
• Ionization energy is always a positive value
• You can take off more than one electron!
> IE1 (1st ionization energy) = energy required to
remove 1st valence e> IE2 (2nd ionization energy) = energy required to
remove a 2nd e-
Na
Na+ + e-
H=495.8 kJ/mol
Mg
Mg+ + e-
H=737.7 kJ/mol
Mg+
Mg2+ + e-
H=1450.6 kJ/mol
November 07, 2014
Ionization Energy
• Is an atom with a high ionization energy more likely
or less likely to form a positive ion?
• What about one with low ionization energy?
What is the periodic trend for
ionization energy?
What factors affect ionization energy?
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Ionization Energy
What pattern
do you see?
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Ionization Energy
• Moving down a group: IE decreases
> There are more electrons in between the nucleus
and the valence electrons (shielding effect)
> Electrons are not as tightly bound to nucleus.
> Electrons are further away, attraction decreases.
• Moving across a period: IE increases
> Increased nuclear charge (more p+, greater
attractive force)
November 07, 2014
Ionization Energy
• What about successive IE (removing more than one e-)?
> Successive IE increases
• There is a big "jump" in some IE required. Why?
> W/in a sublevel: small increases
> Between sublevels: greater increase
> Between energy levels: greatest increase
http://www.avon-chemistry.com/p_table_lecture.html
November 07, 2014
Example 2:
In the following pairs, which atom is more
difficult to ionize/Has the higher IE?
Na or Li
Na or Mg
C or Na
Cl or Al
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Using these trends, can you explain the
alkali metal reactions?