9. Bonding of Materials Flashcards
Why does the forming of bonds cause a reduction in the overall energy of the material?
When electron orbitals overlap, energy levels split and broaden due to Pauli’s exclusion principle. This means that the electrons populate new lower energy levels
Why do antibonding orbitals form?
As the number of orbitals must be conserved, so when two atomic orbitals come together to form molecular orbitals, they will form one bonding orbital and one antibonding orbital.
Which has a higher energy level, bonding or antibonding orbitals?
Antibonding orbitals, as bonding orbitals see a reduction in energy due to electron energy levels splitting due to the Pauli exclusion principle
When might an electron enter the antibonding orbital?
If the electron was to become excited it could jump up to the antibonding orbital
What is orbital hybridisation?
Orbital hybridisation is the concept of mixing atomic orbitals into new hybrid orbitals (with different energies, shapes, etc. than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory. This was first invented to explain how carbon doesn’t do expected and form 2 bonds (CH2) but 4 bonds (CH4). The only way CH4 it can be explained is is, the 2s and the 3 2p orbitals fused together to make four, equal energy sp3 hybrid, molecular orbitals.
How does carbon form hybrid orbitals to make CH4?
Carbon’s electron structure is 1s2 2s2 2p2. To make four bonds, it takes it’s 2s orbital and all 3 of the 2p orbitals to make 4 sp3 hybridised molecular orbitals. To hybridise the orbitals, it must promote one of the 2s electrons to the vacant 2p orbital, then it can hybridise them, leaving 4 molecular orbitals that are identical in energy. Each sp3 orbital has 1 electron, leaving one space free to come from the other atom in the covalent bond.
What is an orbital diagram?
Electron orbital diagrams and written configurations tell you which orbitals are filled and which are partially filled for any atom.
They are drawn with each atom on the side with its respective atomic orbital, and the molecular orbitals go in between. (each orbital type eg 1s, 2s, 2p is shown as a separate diagram)
How does hybridisation minimise the total energy?
By arranging as many electrons as possible in bonding orbitals
What is the equation for the optimum number of covelent bonds?
X = 8 - N (for N>=4)
X = N (for N<4)
X- number of covalent bonds
N- number of valence electrons
How does silicon hybridise?
Si has 2 e- in its 3p orbitals, and these are close in energy to its 2 3s e-. It is energetically favorable for the 3s and 3p orbitals to hybridise into 4 sp3 orbitals. this is what allows it to form 4 bonds and give it its tetragonal bonding geometry.
What is electronic band structure?
Electronic band structure (or simply band structure) of a solid describes the range of energies that an electron within the solid may have (called energy bands, allowed bands, or simply bands) and ranges of energy that it may not have (called band gaps or forbidden bands).
What is the Fermi level
The energy of the highest occupied state
What does it mean if the Fermi energy is in the middle of a band?
It means that the material is a metal
What is the tight-binding approximation?
The tight-binding model is an approach to the calculation of electronic band structure using an approximate set of wave functions based upon superposition of wave functions for isolated atoms located at each atomic site.
Why does an energy band form in materials?
Each atom has its own ‘potential well’ with discrete energy levels. For a material with N atoms, these energy levels will form bands of N finely separate energy levels, forming an energy band (Pauli exclusion principle, no two electrons can occupy the same quantum state)
