7.5 Periodic Trends

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Atomic Radius

Z_eff = 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.

S^2- > Cl^- > Ar > K⁺ > Ca²⁺

Ionization energy is the energy required to remove an electron from an atom in the gas phase.

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.

The 1^st 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^- → O^2-(g)	E.A.₁ = +744 kJ/mol