Periodicity AP Chemistry By Diane Paskowski

Valence electrons The valence electrons (outer e-) in atoms are those shared/transferred to form bonds between atoms. The number of valence electrons will affect the type(s) of bonds an atom can form with a specific element. The number of valence electrons in an atom can be determined from the electron configuration or just by looking at its location on the periodic table.

Valence electrons cont’d Valence electrons can be predicted for the main or representative elements (s and p blocks) very easily.

Effective Nuclear Charge The determining factor in most periodic properties The "pull" that the specific electron "feels" from the nucleus

Ionization energy The energy required (added to an atom) to remove 1 valence electron. This value is always positive. First ionization energy is the energy required to remove the first electron Second ionization energy is the energy required to remove the second electron. It is “always” more energy than the first ionization. Third ionization energy is . . .

Ionization Energy cont’d Students should be able to predict which metal ion would most likely be formed from the ionization energies of an element Trend on the periodic table Increases across because the effective nuclear charge increases. Decreases down a column because the effective nuclear charge decreases

Ionization Energy cont’d

Atomic Radii The size of an atom. Measured by bonding the atom with another, measuring the distance between them, and then subtracting to find the unknown radius.

Atomic Radii cont’d Trends Decreases across a period (greater ENC) Increases down a column (more energy levels)

Main Group Elements Only Atomic Radii in 3-D Main Group Elements Only

Electron Affinity The energy released (or needed) when an atom takes in an extra electron.

Electron Affinity Elements that have high electron affinities release a lot of energy when an electron is taken in Some elements that have low electron affinities require energy to be added to the atom in order to take in an electron General Trends increase across a row decrease down a column

Electron Affinity There are many exceptions to this general trend due to the lower energy of half-filled sublevels and orbitals. This is a “bumpy” trend.

Electron Affinity Graph

Electron Affinity “Bumps” The difference between Na and Mg is due to an additional of an electron in Na completes the 3s sublevel which is a more favorable state than a half-filled 3s sublevel. Carbon will achieve a lower energy state by taking an electron and half-filling the 2p sublevel than nitrogen which on additional electron will disturb a half-filled sublevel.

Ionic Radii Cations are smaller than the corresponding atom.

Anions are larger than the corresponding atoms. Ionic Radii cont’d Anions are larger than the corresponding atoms.

Applications of Periodic Trends Predict chemical properties of elements based on their location on the periodic table Metals: lower electron affinity and lower ionization energy. Tend to contribute electrons to bonds Nonmetals: higher electron affinity and higher ionization energy. Tend to share or take in electrons in bonds.

Student Constructed Periodic Tables Ionization Energy Density

Different Design