Transcript Trends in the Periodic Table

Trends in the
Periodic Table
Atomic radius

The best measure of atomic radius is the
bond radius. Measure the distance
between the nuclei of 2 atoms bonded
together and divide by two.

Going down a group, the atomic radius
increases.
 Larger
atoms have more electrons farther
away from the nucleus.
 The inner electrons shield the outer electrons
from the full effect of the positive charge of the
nucleus.

Going across a period, the atomic radius
decreases
 Electrons
are being added to the same
principal energy level.
 For every added electron, a proton is also
being added to the nucleus, increasing the
charge, pulling the electrons tighter in.
 This change is not as noticeable with heavier
elements (inner electrons shield).
Ionization Energy
The energy required to remove an electron
from an atom in the gas phase
 There is a series of ionization energies for
each electron removed. These energies
get higher for each subsequent electron.
 The trends given are for the first electron
removed.


Going down a group, the ionization energy
decreases.
 Electrons
are further out, so the nuclear
charge is not felt as strongly.
 Shielding effect contributes.

Going across a period, the ionization energy
increases.
 For
every added electron, a proton is also being
added to the nucleus, increasing the charge.
 The same principal energy level is being filled, so the
shielding effect is a constant.

There are some exceptions to this trend,
normally in cases of full or half-full energy
sublevels.
Electron Affinity


Measures how much an atom “wants” to gain
electrons
Is the change in energy associated with gaining
an electron
electron affinity: really “wants” to gain an
electron- rE is a negative number with a high
absolute value
 High

e.g. F: rE = -328.5 kJ/mole
electron affinity: doesn’t “want” to gain an
electron
 Low

e.g. Noble gases: have positive values of rE

Going down a group, the electron affinity
decreses
 Shielding
more than offsets the increase in
nuclear charge

Going across a period, the electron affinity
increases (rE becomes more negative).
 Shielding
remains constant, the nuclear
charge increases

EXCEPTION: The noble gases have the lowest
electron affinities of all.