How the Periodic Trends Relate to Atomic Structure

This is why the trends are what they are:

Ionization Energy:

What it is: the energy required to remove an electron from an neutral atom
The trend: increases up and to the right
Why across: As you move to the right, there are more protons in the nucleus, so it is more positive and pulls harder on the electrons...SO...it is harder to steal an electron--ionization energy goes up
Why down: As you move down the table the electrons are further from the nucleus and, therefore, feel less attraction...SO...it is easier to steal an electron--ionization energy is lower down the table.

Electron Affinity

What it is: the amount of energy released when an electron is added to a neutral atom
The trend: increases up and to the right
Why across: As you move to the right, there are more protons in the nucleus, so it is more positive and pulls harder on the electrons...SO...the electron is pulled in with more force, so more potential energy is given off when the electron drops into the atom.
Why down: As you move down the table the electrons are further from the nucleus and, therefore, feel less attraction...SO...the electron doesn't fall as far, so there is less potential energy change and less energy is given out.

Electronegativity

What it is: the force with which one atom pulls on the electrons of another atom in a covalent bond
The trend: increases up and to the right
Why across: As you move to the right, there are more protons in the nucleus, so it is more positive and pulls harder on the electrons...SO...the nucleus pulls on ALL electrons harder - even someone else's
Why down: As you move down the table the electrons are further from the nucleus and, therefore, feel less attraction...SO...the nucleus doesn't pull as hard on the other atom's electrons.

Atomic Radius

What it is: the radius an an atom
The Trend: increases down and to the left
Why Across: As you move to the right, there are more protons in the nucleus, so it is more positive and pulls harder on the electrons...SO...the nucleus pulls the electrons in tighter making the atom smaller
Why Down: As you move down the table the electrons are further from the nucleus...SO...it's bigger

Ionic Radius

What it is: the radius of the ion that is isoelectronic with the nearest noble gas
The Trend: increases down. Going across, it initially decreases, then increases dramatically, then decreases again.
Why Across: this is a complex idea and deserves it's own page.
Why Down: As you move down the table the electrons are further from the nucleus...SO...it's bigger