7.5 Periodic Trends
Effective Nuclear Charge
Zeff = Z – S Z = # protons S = # shielding (screening) electrons
The bond length is equal to the sum of the atomic (covalent bond) or ionic (ionic bond) radii.
S2- > Cl- > Ar > K+ > Ca2+
Ionization energy is the energy required to remove an electron from an atom in the gas phase.
Exceptions in the Ionization Energy Trend
Successive Ionization Energies (2nd, 3rd, 4th, etc.)
Core electrons have SIGNIFICANTLY higher ionization energies than valence electrons.
|Ionization Energies of Boron|
|B(g) → B+(g) + e-||I.E.1 = 801 kJ/mol|
|B+(g) → B2+(g) + e-||I.E.2 = 2,427 kJ/mol|
|B2+(g) → B3+(g) + e-||I.E.3 = 3,660 kJ/mol|
|B3+(g) → B4+(g) + e-||I.E.4 = 25,026 kJ/mol|
Electron affinity is the energy change for an atom in the gas phase gaining an electron.
Successive Electron Affinities
The 1st electron affinity for most elements is negative, but any additional successive electron affinities are always positive.
|Electron Affinities of Oxygen|
|O(g) + e- → O-(g)||E.A.1 = -141 kJ/mol|
|O-(g) + e- → O2-(g)||E.A.1 = +744 kJ/mol|