Energy & the
Unit 1 Lecture 5
The Shielding Effect
• Recall there is an attraction between the positive protons in the nucleus and the orbiting negative electrons • According to Coulomb’s Law, the further away electrons are, the less attraction they experience
• There is also the shielding effect taking place o Electrons that are furthest from the nucleus are partly “shielded” by the inner core electrons o They experience some electrostatic repulsion from the inner core electrons also ( recall that like charges repel)
• The shielding effect reduces electrostatic attractions between the outer electrons and the nucleus.
Two factors explain the lessened attraction of outer electrons to the nucleus
1. Coulomb’s Law o more distance = less attraction
2. Shielding Effect o Layers of electrons in middle exert repulsive forces on outer electrons
This means atoms of different sizes & # of e- will require different amounts of energy to remove electrons during chemical bonding…Ionization Energy!
• The minimum amount of energy that is required to remove the outermost, least tightly held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+ (due to removal of 1 e-)
• It is the energy needed to carry out this change per mole of X. o Specify gas because there are no intermolecular forces to compete with and that is how all of these values given on tables are measured in originally.
• Ionization energies are measured in kJ mol -1 (kilojoules per mole). They vary in size from 381 (which you would consider very low) up to 2370 (which is very high).
First Ionization Energy cont’d • Values can be obtained by shining different frequencies of light on a pure sample of natural atoms in the gas phase and then recording the frequency at which electrons are removed.
• Then use E= hv to calculate the first ionization energy.
• Side Notes… o All ionization energy is positive, it requires that energy be added to remove an electrons. The process is then considered endothermic. o All elements have a first ionization energy - even atoms which don't form positive ions in test tubes normally. The reason that helium (1st I.E. = 2370 kJ mol-1) doesn't normally form a positive ion is because of the huge amount of energy that would be needed to remove one of its electrons.
• Notice the repeating pattern and when the pattern begins anew… o For example, look at the pattern from Li to Ne, and then compare it with the identical pattern from Na to Ar
• Also notice the amount of energy in general decreased each time the pattern repeats itself…
Here is another graph…try comparing to a PT and notice two things
1. Ionization Energy decreased when moving down a group of the PT
2. Generally, ionization energy increases when moving from left to right across a period of the
Summary of PT Trends
• Notice: Ionization Energy is inversely related to atomic radius…later we will discuss atom size as a factor in ionization energy.
Factors that affect Ionization
• The charge on the nucleus. o The more protons there are in the nucleus, the more positively charged the nucleus is, and the more strongly electrons are attracted to it.
• The distance of the electron from the nucleus. o Remember Coulomb’s Law... an electron close to the nucleus will be much more strongly attracted than one further away.
• The number of electrons between the outer electrons and the nucleus. o Recall the Shielding Effect
• Whether the electron is on its own in an orbital or paired with another electron. o Two electrons in the same orbital experience a bit of repulsion from each other. This offsets the attraction of the…