Transcript Periodicity Power Point 15-16 - OPHS-AP

Periodicity
Trends of the Periodic Table
Rules to Abide By:
Electrons are attracted to the
protons in the nucleus of an atom.
This means:

The closer an e- is to the nucleus
the more strongly it is attracted.
(core e-)

More protons = electron is held
more strongly
1.
2. Electrons repel each other.
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So the core electrons SHIELD
the valence electrons from the nucleus.
The core e- limits the attraction of the
valence e- for the nucleus.
The SHIELDING AFFECT increases down a
group. More core e- are being added.
The SHIELDING AFFECT doesn’t impact
periods b/c the e- are being added in the
valence shell.
3. Completed shells are very stable.
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Atoms prefer to add or subtract valence
e-.
Everybody wants to be a NOBLE GAS.
Metals lose e- to get a noble gas
configuration.
Nonmetals gain e- to get a noble gas
configuration.
Atomic Radius
Across a period  radius decreases
WHY????
Left to right, protons are added to the nucleus,
so the valence e- are more strongly attracted
WHY is shielding unimportant????
B/C even though e- are being added, they are
in the same shell and the same distance from
the nucleus
Atomic Radius
Down a group
radius increases
WHY??????
Shells are being added, this creates more
distance between the valence e- and
the nucleus (SHIELDING AFFECT)
Shielding cancels out the affect of the
additional protons in the nucleus.
ION SIZE
Cations
 Generally metals
 Positively charged ions
 Loss of valence e Smaller than parent atom
WHY?????????????
Outer shell is lost, electron-electron repulsions
are reduced, remaining valence e- move
closer to the nucleus
ANIONS
Generally nonmetals
 Negatively charged ions
 Gain valence electrons
 Larger than parent atom
WHY??????????
Electron-electron repulsions increase moving
the valence electrons farther apart

Ionic Size Trends
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Across a period  complicated trend
b/c of change from metal to nonmetals
Down a group
ion size increases,
same as atomic radius
Isoelectronic ions – ions containing the
same number of electrons

The ion size decreases as the atomic
number increases
Isoelectronic Example
Arrange the ions Se-2, Br -, Rb+ and Sr+2
in order of decreasing size.
Answer:
Se-2 > Br- > Rb+1 > Sr+2
Ionization Energy
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Energy required to remove an electron from
an atom
Multiple ionization energies b/c multiple
electrons can be removed
First, second, third….a slight increase until a
core electron is removed
Ionization energy increases dramatically
when trying to remove a core electron
Ionization Energy
Across a period  energy increase
WHY?????????
Across a period, protons are added to the nucleus,
increasing the ‘+’ charge which more strongly holds
the negative electrons
Exceptions to the trend:
 The shielding affect caused by filling the s sublevel
cause a dip from group 2 to 3
 Slight dip when electrons start to pair in the p
sublevel
Ionization Energy
Down a Group energy decreases
WHY??????????
Shielding affect increases, so energy
decreases. There are more electrons
between the nucleus and the valence e, so the increased distance, decreases
the energy needed to remove a valence
e-.
Electronegativity
How strongly the nucleus attracts
electrons shared in a bond between two
atoms
 Across a period  Increases
 Down a group
Decreases
WHY???
Across a period the nuclear charge
increases, while down a group the
shielding affect increases

Number of Protons Increases
Atomic Radius decreases
Ionization energy increases
Electronegativity / Electron Affinity
decreases
Ionization energy decreases
Atomic Radius increases
Shielding Affect Increases
Electronegativity /Electron Affinity Increases
Most Chemically
Reactive Metals
Most Chemically
Reactive Nonmetals
Periodic
Table