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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 decreasesElectrons 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 High electron affinity: really “wants” to gain an
electron- E is a negative number with a high absolute value
e.g. F: E = -328.5 kJ/mole
Low electron affinity: doesn’t “want” to gain an electron
e.g. Noble gases: have positive values of E
Going down a group, the electron affinity decresesShielding more than offsets the increase in
nuclear charge Going across a period, the electron affinity
increases (E becomes more negative).Shielding remains constant, the nuclear
charge increases EXCEPTION: The noble gases have the lowest
electron affinities of all.