Chemistry 401

Electronegativity

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:

r_cov is covalent radius in Angstroms

Mulliken: made electronegativity into a true atomic property; now called absolute EN

 

Covalent Bonding

Bonding between atoms occurs because electron pairs are shared between atoms.

Why does this cause a nuclear-nuclear attraction?

Accumulated electron density causes a net nuclear-nuclear attraction (increased electron-nuclear attraction is greater than the increased electron-electron repulsion)

 

Valence Bond Theory: at an elementary level, electrons occupy a volume of space and orbitals are constructed to match the electron occupation.

Molecular Orbital Theory: orbitals are constructed and then filled with electrons according to the Aufbau and Pauli Principles.

 

Lewis Dot Structures: the usual method of denoting VBT bonding

Classical description/rationalization: atoms in molecules strive to attain a rare gas electron configuration

This is probably a bad generalization for most of the Periodic Table

two electrons on each atom

eight electrons on each atom

Better: atoms share electrons in molecules to attain the minimum formal charges

Formal charge: found by splitting the electrons in each equally between the bonded atoms.

Oxidation number: found by assigning all the electrons in a bond to the more electronegative atom.

Reality : somewhere in between