Organization of the Periodic Table Based on Electronic Trends

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What are six key trends of the periodic table?

Atomic number
Valence electrons
Ionization energies
Electron affinity
Electronegativity
Atomic Radii

What is the trend of atomic number in the periodic table?

This is a gimme, but atomic number increases from left to right and from top to bottom:

Periodic table: Trend of atomic numbers.
$\includegraphics[width=2.7in]{img-i-periodic_atomic_num.eps}$

What are the valence electrons for the representative elements of the periodic table?

Valence electrons for the representative elements.

Periodic table group	Number of valence $e^{-}$

IA	1
IIA	2
IIIA	3
IVA	4
VA	5
VIA	6
VIIA	7
VIII	8 (octet) and stable!

Note: The general trend of valence $e^{-}$ number is to increase from left to right. Also, remember that the goal of atoms is to obtain a noble gas configuration, i.e. eight valence electrons. So, for Na, to lose an electron is a good thing, because in its cationic form, i.e. Na $^{+}$ , sodium assumes the noble gas configuration of Ne.

What is ionization energy?

A general term for the energy required to remove an electron from an orbital in an atom or ion. Think of it also as the energy required to make a cation.

Ionization energies are usually reported as the first ionization energy, second ionization energy, etc., with each subsequent ionization requiring more energy than the one before.

How does the ionization energy relate to the valence electron number?

The amount of energy required to remove an electron depends on if the electron removal (i.e. ionization) will bring the atom closer to or further from its nearest noble gas configuration. So, Na - and all the other group 1A elements for that matter - are one electron away from their noble gas configurations. As mentioned above, Na actually wants to lose that lone electron, so the first ionization energy is low.

What about Na's second ionization energy or Na $^{+}$ 's first ionization energy?

Intuitively, because Na $^{+}$ is at the noble gas configuration, the ionization energy would be extremely high.

In the end, look at the valence electron configuration to determine the ionization energy required to abstract an electron from the atom, with the ultimate goal being to reach the hallowed number of eight valence electrons.

What are the trends of ionization energy across the periodic table?

Again, ionization energy increases from left to right and also increases from bottom to top: The ionization energy increases across a period because it becomes successively more difficult to remove electrons from an atom or ion as it gets closer to having a full outer shell of electrons (octet).

Periodic table: Trend of ionization energies.
$\includegraphics{img-i-periodic_ioniza.eps}$

What is electron affinity?

The desire of an atom to have another electron. Basically the opposite of ionization energy. Cl, for example, would really like having another electron and therefore has very high electron affinity.

Think of it also as the energy required to make a negative ion, aka an anion.

What are the trends of electron affinity across the periodic table?

Again, electron affinity increases from left to right and also increases from bottom to top:

Periodic table: Trend of electron affinity.
$\includegraphics{img-i-periodic_elec_aff.eps}$

Why does the electron affinity trend increase as one goes from bottom rows to top rows?

If you think about it, the nucleus has a positive charge (protons) which causes the attraction and ultimately drives electron affinity. As you go down the rows of the periodic table the $Z_{eff}$ decreases, decreasing the overall effect the nucleus has on drawing in the electron.

What is electronegativity?

The attraction an atom has for electrons in a covalent bond.

What is the electronegativity trend of the periodic table?

Again, electronegativity increases from left to right and also increases from bottom to top.

Periodic table: Trend of electronegativities.
$\includegraphics{img-i-periodic_electronegativity.eps}$

What is the trend of atomic radii across the periodic table?

Atomic radii get smaller from left to right and also decrease from bottom to top. The reason for the decreased size across a period is that the electrostatic force of attraction of the protons in the nucleus with the corresponding number of electrons increases - greater force of attraction, smaller size.

Periodic table: Trend of atomic radii.
$\includegraphics{img-i-periodic_atomic_radii.eps}$

How can you remember all of these trends?

Remember the cartoon characters Mighty Mouse and Foghorn Leghorn?

Put Mighty Mouse in the top right hand corner of the periodic table and Foghorn Leghorn in the bottom left; now draw the trends and it should make sense. Mighty Mouse, aka fluorine, is strong (electronegative) and small (atomic radii), while Foghorn is bigger (larger radius) and slower (less electron affinity), etc.