Periodic Trends—Ionization Energy, Electron Affinity

Coulomb’s Law

Addresses the force of attraction between the positive nucleus and negative electron

F = qe qp qe = charge of electron

r2 qp = charge of nucleus

INVERSE relationship between attractive force and distance

Attractive force is DIRECTLY related to the charge on the electron and the charge in the nucleus

First Ionization Energy (IE)

If an electron is given enough energy (in the form of a photon) to overcome the effective nuclear charge holding the electron in the cloud, it can leave the atom completely.

Energy required to remove the outermost electron from an atom Amount of energy needed to remove ONE electron

from a neutral atom Energy needed to remove electrons from ground

state to gaseous state in an atom

Formation of positive ions (cations)

Ionization Energy (cont.)

Formation of positive ions (cations)

The larger the atom is, the easier its electrons are to remove.

Ionization energy and atomic radius are inversely proportional.

Ionization Energy Trend

Increases

Decreases

Element Atomic # IE (kJ/mol)

Na 11 495.8

Mg 12 737.7

Al 13 577.6

S 16 999.6

Cl 17 1251.1

Ar 18 1520.5

K 19 418.8

1st ionization energy Energy required to remove 1st electron from

atom Taken from highest energy level Easiest to remove

Energy increases as more electrons are removed

Ionization Energy (cont.)

Can we have 2nd, 3rd, 4th, etc. Ionization

Energies? Energy needed to remove additional electrons

from an atom

Energy increases as more electrons are removed. Removing electrons from lower energy levels,

close to nucleus

1st IE 2nd IE 3rd IE 4th IE

Na 495.8 4562.4 6912 9543

Ionization Energy

Example 1:

Place these elements in order of INCREASING 1st ionization energy.

1) Mg, S, Si

2) As, N, P

(p. 321 in text)

What does affinity mean?

Electron Affinity

Creating negative ions (anions)

Energy change with the addition of an electron to an atom energy change that occurs when electrons added to gaseous

atom

Energy is released when electrons are added, exothermic process (negative value)

Electron Affinity

Electron tends to enter partially filled subshell or go to the next energy level

Does not follow predictable trend like atomic radii and ionization energy

An atom’s “desire/affinity” for more electrons, wants to get more electrons ! ! !

Metals—decrease electron affinity. Nonmetals—increase electron affinity, more reactive Stable atoms—full octet

Electron Affinity Trend

Increases

Decreases

Homework

Atomic Theory III worksheet #18-21

Read pp. 319-323

p. 334 #57-58