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.