Periodicity. Why is the periodic table this goofy shape?
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Transcript of Periodicity. Why is the periodic table this goofy shape?
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 ShapesHere 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 RadiusTwo factors:1.Where are the electrons (which orbitals)?2.How many protons are there?
Atomic RadiusTwo factors:1.Where are the electrons (which orbitals)? Bigger as you go down2.How many protons are there? Smaller as you go across
Radius of IonsCan 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 EnergyFirst ionization energy: A A+
Second ionization energy: A+ A2+ (always more than first)Third ionization energy: A2+ A3+ (always more than second)Etc
Values come from….PES!
Ionization EnergyFirst ionization energy: A A+
Second ionization energy: A+ A2+ (always more than first)Third ionization energy: A2+ A3+ (always more than second)Etc
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 non-valence 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 electronsDown 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 AffinityA + e- A-
Rarely see second or third electron affinities, but technically possible.
ANNOYING FACT WARNING:
Almost always exothermic, but usually given as a positive number.
Electron AffinityThis mostly comes down to the same two factors:
1. Where are the electrons2. How many protons are there?
Electron AffinityA lot of exceptions in this one, mostly having to do with full or half-full orbitals.
Summary