Periodicity.
Why is the periodic table this goofy shape?
Why this shape?
There is nothing fundamental about this arrangement of elements.
In fact, the f-block even has to get cut out just so the thing will print on a
standard piece of paper well.
Some Other Shapes
Here are some other arrangements:
In 3D!
Why this shape?
All of these try to do the same thing:
arrange all the elements in a way that makes for useful patterns.
Mendeleev’s original table was revolutionary, but not terribly useful for
many modern things.
So What is Ours Good At?
Metals vs non-Metals
Pretty good here.
A stepped dividing line is a bit non-optimal.
The placement of hydrogen is odd for this.
So What is Ours Good At?
Valence electrons
Very good here.
Helium is awkward, but that’s inevitable due to lack of p orbitals.
So What is Ours Good At?
Three new things:
1. Atomic radius
2. Ionization Energy
3. Electron Affinity
So What is Ours Good At?
Three new things:
1. Atomic radius: where you define the ‘end’ of an atom is a little
nebulous, but we can make up an arbitrary choice and get relative
sizes that way.
2. Ionization Energy
3. Electron Affinity
So What is Ours Good At?
Three new things:
1. Atomic radius: where you define the ‘end’ of an atom is a little
nebulous, but we can make up an arbitrary choice and get relative
sizes that way.
2. Ionization Energy: the amount of energy it takes to pull an electron
completely off an atom. ALWAYS endothermic.
3. Electron Affinity
So What is Ours Good At?
Three new things:
1. Atomic radius: where you define the ‘end’ of an atom is a little
nebulous, but we can make up an arbitrary choice and get relative
sizes that way.
2. Ionization Energy: the amount of energy it takes to pull an electron
completely off an atom. ALWAYS endothermic.
3. Electron Affinity: the amount of energy you get out from adding an
electron to an atom. Exothermic.
Atomic Radius
Two factors:
1. Where are the electrons (which orbitals)?
2. How many protons are there?
Atomic Radius
Two factors:
1. Where are the electrons (which orbitals)?
2. How many protons are there?
Bigger as you go down
Smaller as you go across
Radius of Ions
Can only compare size of ions if:
1. Number of protons are the same: Mg vs Mg2+
OR
2. Number of electrons are the same: F— vs Mg2+
In both cases, the question comes down to the same questions as before:
1. Where are the electrons?
2. How many protons are there (compared to how many electrons)?
Ionization Energy
First ionization energy:
A  A+
Second ionization energy: A+  A2+
Third ionization energy:
A2+  A3+
Etc
Values come from….PES!
(always more than first)
(always more than second)
Ionization Energy
First ionization energy:
A  A+
Second ionization energy: A+  A2+
Third ionization energy:
A2+  A3+
Etc
(always more than first)
(always more than second)
Values come from….PES!
Only the first ionization energy follows a trend.
The second and third and beyond depend more on where the electrons are
The second for sodium will be very high because you’re removing nonvalence electrons. The second for magnesium will be lower because it’s
also a valence electron.
Ionization Energy
Across a row: more protons holding onto the electrons
Down a column: electrons are farther away and ‘shielded’ from nucleus.
Can compare on one diagonal (Ge vs P) but not on other (Si vs As)
Electron Affinity
A + e-  ARarely see second or third electron affinities, but technically possible.
ANNOYING FACT WARNING:
Almost always exothermic, but usually given as a positive number.
Electron Affinity
This mostly comes down to the same two factors:
1. Where are the electrons
2. How many protons are there?
Electron Affinity
A lot of exceptions in this one, mostly having to do with full or half-full
orbitals.
Summary