DOWN a Group: Atomic radius INCREASES as you
go DOWN a Group because each successive Period (row) has an additional
occupied energy level. If you visualize the not 100% accurate but still
useful Bohr model of the atom, you can think of it this way: each time
you drop down a row, you add a "ring."
ACROSS a Period: Atomic radius DECREASES as
you go ACROSS a Period because the net nuclear charge increases (Huh?).
Remember, it's the protons (+) in the nucleus that pull on or attract
the electrons in the orbitals. Across a Period you are adding more and
more protons pulling on electrons occupying the same orbitals. The
overall effect is more pulling power in the same basic space. This draws
the electrons in closer, making the overall atomic radius smaller at
the right side of a Period.
Ionization Energy:
DOWN a Group: Ionization energy DECREASES as you
go DOWN a Group because the farther the valence electrons are from the
nucleus (pulling power of the protons) the less energy it costs another
atom to steal them.
ACROSS a Period: Ionization energy INCREASES as
you go ACROSS a Period because atoms are getting ever closer to that
magic "octet" rule for stability via the Noble Gas configuration. In
plain speak - your frequent buyer punch card gets one step closer to the
freebie each time you move closer to the right of the Periodic Table so
you guard those punches more carefully. The atomic radius is getting
smaller, too, so those protons do a great job of holding on tighter.
Electron Affinity:
Electron affinity is a bit outside the scope of this class so I'm not
going to get into it too deeply, other than to point out the general
trend.
DOWN a Group: Electron Affinity DECREASES (a tiny
bit) as you go DOWN a Group because elements become slightly less
attractive toward electrons. Father from the pull of those protons,
remember?Β
ACROSS a Period: Electron Affinity INCREASES as
you go ACROSS a Period because generally speaking (and remember, exclude
the Noble Gases here) elements toward the right of the Periodic Table
give off a great deal of energy when they gaining electrons to become
more stable.
Electronegativity:
DOWN a Group: Electronegativity DECREASES as
you go DOWN a Group because the valence electrons are increasingly
farther away from the attraction of the protons in the nucleus. Less
pull, less "desire" to grab other electrons.
ACROSS a Period: Electronegativity INCREASES
as you go ACROSS a Period because the number of protons (+ charges) in
the nucleus increases. More protons in the nucleus means electrons are
more strongly attracted to the nucleus.
Chemical Reactivity:
METALS
DOWN a Group: In METALS reactivity INCREASES as
you go DOWN a Group because the farther down a Group of metals you go,
the easier it is for electrons to be given or taken away, resulting in
higher reactivity.
ACROSS a Period: In METALS reactivity DECREASES
as you go ACROSS a Period because though they still want to give away
valence electrons they have more of them to get rid of, which requires
more energy. Not as easy to blow off a little steam!
I can't help it, I just love this video on reactivity (however
embellished the reactions) down the Alkali Metals. Top Gear (BBC) fans
will recognize Richard "Hamster" Hammond as the narrator here. Enjoy!