Bonding Energies

Bonding forces & Energies

Why study bonding? • Because the properties of materials (strength, hardness, conductivity, etc..) are determined by the manner in which atoms are connected. • Also by how the atoms are arranged in space -Crystal Structure

What determines the nature of the chemical bond between atoms? •Electronic structure (distribution of electrons in atomic orbitals) • Number of electrons and electronegativity (tendency for an atom to attract an electron)

Atomic Bonding Cation

FA

FR

Energy Anion

r

 As two ions approach each other, each atom exerts

forces on the other.  Magnitude of Forces is a function of interatomic distance Interatomic Forces - Attractive Force, FA - Repulsive Force, FR

Bonding forces

• This is typical potential well for two interacting atoms. • The repulsion between atoms, when they are brought close to each other, is related to the Pauli principle: • when the electronic clouds surrounding the atoms starts to overlap, the energy of the system increases suddenly. • The origin of the attractive part, dominating at large distances, depends on the particular type of bonding.

•Principles of atomic bonding are best illustrated by considering the interaction between two isolated atoms as they are brought into close proximity from an infinite separation. •At large distances, the interactions are negligible, but as the atoms approach, each exerts forces on the other. •These forces are of two types, attractive FA and repulsive FR •The net force FN between the two atoms, FN = FA+FR When FA and FR balance, or become equal, there is no net force; that is, FA+FR= 0 Then a state of equilibrium exists. The centers of the two atoms will remain separated by the equilibrium spacing r0,

The bonding energy for these two atoms, E0, corresponds to the energy at this minimum point , it represents the energy that would be required to separate these two atoms to an infinite separation.

Types of Bonding Primary bonding Ionic: Strong Coulomb interaction among negative atoms (have an extra electron each) and positive atoms (lost an electron). Example - Na+Cl-

Covalent: electrons are shared between the molecules, to saturate the valency Example - H2

Metallic: the atoms are ionized, loosing some electrons from the valence band. Those electrons form a electron sea, which binds the charged nuclei in place

Secondary Bonding van der Waals Fluctuating Induced Dipole (inert gases, H2, Cl2…) Permanent dipole bonds (polar molecules - H2O, HCl...) Polar molecule-induced dipole bonds (a polar molecule like induce a dipole in a nearby nonpolar atom/molecule

Ionic Bonding

Diamond

CH4

Bonding with induced dipoles