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GHS Honors Chem
Periodic Trends
OBJECTIVES:
� Interpret group trends in atomic
radii, ionic radii, ionization energies,
and electronegativities.
� Interpret period trends in atomic
radii, ionic radii, ionization energies,
and electronegativities.
GHS Honors Chem
Trends in Atomic Size
� First problem: Where do you start
measuring from?
� The electron cloud doesn’t have a
definite edge.
� They get around this by measuring more than 1 atom at a time.
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Atomic Size
Atomic Radius = half the distance between
two nuclei of atoms
� in the solid state (by X-ray diffraction)
� or of a diatomic molecule.
Radius
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Atomic Size
Influenced by three factors:
1. Energy Level
- Higher energy level is further away.
2. Charge on nucleus
- More positive charge pulls electrons in
closer.
3. Shielding effect
- The inner electrons shield the outer
electrons from the nuclear
charge/attraction.
GHS Honors Chem
Shielding
� The electron on the outermost energy level has to look through all the other energy levels to see the nucleus.
� Second electron has same shielding, if it is in the same period
� Shielding Increases down a Group, and is Constant across a Period. � Shielding Across a Period is Constant, but the EFFECTIVE NUCLEAR CHARGE INCREASES.
GHS Honors Chem
Group Trends in Atomic Size
� As we go down a group... � each atom has another energy level, � so the atoms get bigger. � Shielding increases as well, so the nucleus has less of a hold on e -^ … distance is longer. � The Increased size of the Energy Levels down a group outweighs the increased nuclear charge
H Li
Na
K
Rb
GHS Honors Chem
Periodic Trends in Atomic Size
� As you go across a period, the radius
gets smaller.
� Electrons are in same energy level.
� More nuclear charge.
� Shielding is constant … not an issue.
� Outermost electrons are closer.
Na Mg Al Si P S Cl Ar GHS Honors Chem
Atomic Radius
Overall
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Trends in Ionization Energy
� The amount of energy required to
completely remove an electron from a gaseous atom.
� Removing one electron makes a 1+
ion.
� The energy required to remove the
first electron is called the first ionization energy.
GHS Honors Chem
Ionization Energy
� The second IE is the energy required to remove the second electron.
� Always greater than first IE.
� The third IE is the energy required to remove a third electron.
� Greater than 1st or 2nd IE.
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Symbol First Second Third
H
He Li Be B C N O F Ne
Ionization Energy Table
Where are the Group Effects?
1A 2A 3A
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H
He
Li
Be
B
C
N
O
F
Ne
Symbol First Second Third
Can we See the Effect of
Nuclear Charge in this Table?
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Trends in Electron Affinity
� It’s easiest to add an electron to
Group 7A.
� It gets them to a full energy level, or
completes the OCTET.
� Increase from left to right: atoms
become smaller, with greater nuclear
charge.
� Decrease as we go down a group.
GHS Honors Chem
Electron Affinity in the Periodic Table
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Trends in Ionic Size
� Cations form by losing electrons.
� Cations are smaller that the atom they come from.
� Metals form cations.
� Cations of representative elements have noble gas configuration.
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� Anions form by gaining electrons.
� Anions are bigger that the atom they come from.
� Nonmetals form anions.
� Anions of representative elements
have noble gas configuration.
Trends in Ionic Size
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Revisiting Configuration of Ions
� Ions always have noble gas
configuration.
� Na is: 1s^2 2s^2 2p 6 3s^1
� Forms a 1+ ion: 1s^2 2s^2 2p 6
� Same configuration as Neon.
� Metals form ions with the
configuration of the noble gas before them - they lose electrons.
GHS Honors Chem
� Non-metals form ions by gaining
electrons to achieve noble gas configuration.
� They end up with the configuration of the noble gas after them.
Revisiting Configuration of Ions
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Group Trends
� Going down a Group,
you are adding energy
levels
� Ions get bigger as you
go down.
Li 1+ Na1+ K1+
Rb 1+
Cs1+
GHS Honors Chem
Periodic Trends
� Across the period, nuclear charge increases so they get smaller.
� Energy level changes between anions and cations.
Li 1+
Be2+
B3+
C 4+
N 3- O2-^ F 1-
GHS Honors Chem
Size of Isoelectronic ions
� Iso- means the same
� Iso electronic ions have the same #
of electrons
� Al 3+^ Mg2+^ Na 1+^ Ne F 1-^ O 2-^ and N 3-^ all
have 10 electrons
� all have the configuration:
1s^2 2s^2 2p 6
GHS Honors Chem
Size of Isoelectronic ions
� Positive ions that have more protons
would be smaller.
� Increase in size from most positive to
most negative
Al 3+
Mg 2+
Na1+^
Ne F^
1- O
2- N^
GHS Honors Chem
Electronegativity
� The tendency for an atom to attract
electrons to itself when it is chemically
combined with another element.
� How fair is the sharing?
� Big electronegativity means it pulls the
electron towards it.
� Atoms with large negative electron
affinity have larger electronegativity.
GHS Honors Chem
Electronegativity Group Trend
� The further down a group, the farther
the electron is away, and the more
electrons an atom has.
� Pull/Attraction of the positive nucleus is
lessened due to increased distance and
Shielding.
� Electronegativity decreases.
� More willing to share.
