LENSES TO PERIODIC TRENDS

• There are 3 major lenses through which advanced Chemistry explains the periodic trends in the groups and periods of the Periodic Table of the Elements

» Effective Nuclear Charge

» Electron Shielding

» Electron Repulsion

LENSES TO PERIODIC TRENDS

• The nucleus of every atom provides for the concentration of positive charges.

• Each successive element has one additional proton.

• Effective Nuclear Charge is the amount of positive charge available to attract the electrons that are not shielded by electrons in full energy levels (the valence electrons)

EFFECTIVE NUCLEAR CHARGE

• Hydrogen and Helium electrons are not shielded by electrons in a full energy level, so the effective nuclear charge is the number of protons of each.

• Lithium and the remainder of the 2nd period have electrons that are shielded from the nucleus by the electrons in the filled 1st energy level. The effective nuclear charge, therefore, is reduced by those electrons according to the following formula:

Zeff = # protons - # electrons in filled energy levels

• This relationship is then used throughout the periodic table to find the effective nuclear charge.

APPLICATION OF EFFECTIVE NUCLEAR CHARGE

• Effective nuclear charge increases across the period.

– This increasing effective nuclear charge pulls electrons in closer to the nucleus and this:• Reduces atomic radius• Increases ionization energy• Increases electronegativity

• Effective nuclear charge is largely unchanged down a group/family.

– This unchanging nuclear charge then is not a dominant feature in trends down a group.

PERIODIC EFFECT OF ZEFF

• Electrons in filled energy levels dramatically reduce the effective nuclear charge.

• Filled sub-orbitals within an energy level have only minimal impact on shielding, but this can be of importance in some limited circumstances.

ELECTRON SHIELDING

• Hydrogen and Helium electrons are not shielded by electrons in a full energy level.

• Lithium and the remainder of the 2nd period have electrons that are shielded from the nucleus by the electrons in the filled 1st energy level.

• This same pattern is true of elements in the 3rd energy level and beyond where the electrons in the outermost level are shielded from the nucleus by the electrons in the filled energy levels below it.

• There is a minimal impact of shielding that results from the s orbital on those in the p orbital, but this is normally negligible.

APPLICATION OF ELECTRON SHIELDING

• Electron shielding is only minimally changing across a period, and is normally considered negligible.

– This unchanging electron shielding then is not a dominant feature in trends across a period.

• Because electron shielding increases going down a group and negates the effective nuclear charge it has a major impact on trends down a group/family.

– Since the effective nuclear charge is negated, the dominant feature becomes the distance that electrons are from the nucleus with higher energy. This:• Increases atomic radius• Decreases ionization energy• Decreases electronegativity

PERIODIC EFFECT ELECTRON SHIELDING

• All electrons are negatively charged.

• A repulsive force exists between these particles of like charge.

ELECTRON REPULSION

• Each element in the Periodic Table of Elements has different numbers of electrons to correspond to the number of protons.

• Every element that has one more electron also has one more proton.

• Every electron is separately attracted to the mass of protons in the nucleus.

• Every electron is separately repelled by all of the other electrons.

APPLICATION OF ELECTRON REPULSION

• Electron repulsion to a large degree is offset by the proton attraction of the element. As a result there is very little impact of electron repulsion both across a period or down a group/family. The impact in an element is considered negligible in the trends of the elements.

• Electron repulsion is, however, very important in negatively charged ions. – While protons attract the electrons

into the nucleus, as the number of electrons increase around the nucleus, the repulsive forces cause them to be further from the nucleus.

– Results in larger negative ions than the element from which the ion is formed.

PERIODIC EFFECT OF ELECTRON REPULSION