1. Atomic Bonding

Table of Contents

1. Bonding Forces and Energies
2. Interatomic Bond Types
3. Bond Lengths
4. Primary Bonds
   1. Ionic Bonding
   2. Metallic Bonding
5. Secondary Bonds
   1. van der Waals Bonding
   2. Hydrogen Bonding
6. Bonding Character and Properties

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Bonding Forces and Energies

The potential energy between two atoms has an equilibrium position where in the potential energy curve, it corresponds to a minimum where the atoms are at their equilibrium states. This is called equilibrium interatomic spacing.

Bond energy ($E_0$) represents the energy required to separate the two atoms to an infinite separation:

• Attraction: Depends on the particular type of chemical bond.
• Repulsion: Overlap of the negatively charged electron clouds.

Interatomic Bond Types

• Primary or Chemical Bond:
  • Ionic
  • Covalent
  • Metallic
• Secondary or Physical Bond:
  • Van der Waals Bond
  • Hydrogen Bonding ⇒ Ionic and Covalent bonds are stronger than Metallic bonds. ⇒ Primary bonds is stronger than Secondary bonds.

Bond Lengths

The average (equilibrium) distance between nuclei of two bonded atoms:

• Primary Bonds: 0.1-0.2 nm
• Secondary Bonds: 0.2-0.5 nm ⇒ The stronger bonds will have smaller distance between the two atoms and the energy well or minimum is deeper.

Primary Bonds

Ionic Bonding

Example: Sodium Chloride (NaCl) (Ionic Bonding)

• Electron transfer to form positive and negative ions.
• Attraction: Coulomb’s Force
• Non-directional

Ionic Character (IC):

Equation

% ionic character

$=\{1-[-(0.25)(X_A-X_B{)}^2]\}\cdot 100$

where $X_A$ and $X_B$ are the electronegativities for the respective elements

Metallic Bonding

E.g., Cu, Ag, Au, Al, Ni, Pt, Cr, Mo, etc

• Non-Directional Bonding
• Free electron cloud is due to delocalization.
• Found in all elemental metals.
• The free electrons serve as a “glue” to hold the ion cores together — bonding.

Secondary Bonds

van der Waals Bonding

• Much weaker than primary bonds.
• Originate from atomic/molecular dipoles: separation of positive and negative charges. How dipoles are formed:

1. Fluctuating induced dipole bonds
2. Polar molecule-induced dipole bonds
3. Permanent dipole bonds (hydrogen bonding)

Hydrogen Bonding

• Second strongest bond - a special polar molecule bond.
• Examples: $HF$, $H_{2}O$, $N{H}_3$, $H-F$, $H-O$, $H-N$, bonds have positively charged bare proton at hydrogen end.

Bonding Character and Properties

• Strongly bonded materials ⇒ high melting and boiling temperatures
• For primary bonds, covalent and ionic bonds are generally stronger than metallic bonds
• Thermal and electronic properties depends on free electrons cloud.
• Mechanical properties of solids depends on strength of bonds as well as the directional nature (i.e., crystal structure) ⇒ to be elaborated