Consider Li:
How good is such an approximation?
pretty good for s orbitals - they overlap the nucleus
mediocre for p orbitals - nodal at the nucleus
poor for d or f orbitals - multiply nodal at the nucleus
The reduction of nuclear charge seen by outer electrons due to the inner electrons is called screening or shielding
quantitatively:
Z* = Z - σ
Z* = effective nuclear charge
Z = true nuclear charge
σ = shielding constant
Shielding constants are established by ad hoc rules or quantum mechanical calculations
Trends in Z*
s orbitals are good screeners, p orbitals are moderate screeners; d, f orbitals are very poor screeners.
This means changes across the Table are bigger than changes down the Table
Ionization Potential (IP) or Ionization Energy (IE)
A(g)
A+(g) + e–
The energy associated with this reaction is the ionization energy
IP or IE is thermodynamically positive
IP generally follows Z* across the Table
Electron Affinity : EA
A(g) + e–
EA reported with the wrong thermodynamic sign
Periodic trends are reverse of IP
Atomic Size : Radius of an atom - a difficult concept to define
atomic radius depends upon the bonding situation
metallic radius : ½ the internuclear distance between atoms in the metallic state
covalent radius : ½ the internuclear distance in a homonuclear covalent bond
ionic radius : size of an ion in a solid
van der Waals radius : the distance at which an atom just starts to respond to interactions from neighboring atoms
radii all of different numerical values but similar periodic trends:
r increases as go down the Periodic Table (bigger n)
r decreases as go across the Periodic Table (bigger Z*)
Electronegativity generally used as a qualitative concept
Pauling defined electronegativity as "the power of an atom to attract electrons to itself in a bond"
Pauling Scale:
where:
Δ = E(A-B) – [E(A-A) + E(B-B)]/2
E(A-A), E(B-B), E(A-B) are bond energies in kJ/mol
Allred-Rochow scale: suggested that electronegativity is an electrical force
where:
rcov is covalent radius in Angstroms
Mulliken: made electronegativity into a true atomic property; now called absolute EN
