Metallic bonding

What is the free electron model of metallic bonding

• Free electron model of bonding in a metal considers a lattice of metal ions surrounded by a sea of electrons.

• The ions are fixed in a crystal lattice, but the electrons can move. If a potential difference is applies the electrons move from high to low potential giving rise to an electrical current

What are the ionization energies of metals, and what does this lead to

• Metals often have low ionization enthalpies, meaning they lose electrons relatively. 

Where does the free electron model fail

Cannot explain Semiconductors

What is band theory

Band theory is based on overlap of atomic orbitals to describe metallic bonding.

Describe the banding in an insulator

There is a large energy gap between the filled valence band and empty conduction band.

Describe banding in a semiconductor

The energy gap between bands is quite small but the conduction band is still empty and the valence band is still full

Describe banding in a metal

No energy gap between the bands meaning that the electrons mix making two partially filled bands.

Use lithium as an example to describe band theory

• When two lithium atoms bond to form Li2, the two 2s orbitals combine to give a filled bonding molecular orbital and an unfilled antibonding molecular orbital 

• Li conducts electricity because it has a partially filled band 

• Compounds that have filled bands may be insulators of semiconductors 

• Valence bond is the highest energy band that is fully occupied at ok. 

• Valence bonds differ between electrons 

• Smaller orbitals overlap less and have narrower valence bonds 

Describe what the fermi level is

The energy level to and above which electrons must be promoted to for a material to conduct

What’s the fermi energy equation

Planck constant,

𝑚 is the mass of the fermion, and

𝑛 is the number density of fermions per unit volume.

Describe the graphite band structures

• Conjugated planar pi system 

• Bands instead of discrete energy levels 

• Half-full band with low density of states near fermi level 

 

What is doping

Doping is used to tune the electrical properties of semiconductors by deliberately introducing small amounts of impurities 

If this impurity contains more valence electrons then the host lattice then the doped solid has more electrons to conduct current than the pure host has