Formation of energy bands

Question 1

What happens when two atoms come close together?

Answer 1

their lowest energy bands split into a higher and lower band

Question 2

What condition must be met for a molecule to form from multiple atoms coming together?

Answer 2

if the resultant molecule’s energy level is lower than its atoms’

Question 3

What is a bonding orbital and an antibonding orbital?

Answer 3

When two atoms come close together, the bonding orbital is the lower energy band, and antibonding orbital is the higher energy band

Question 4

How do electrons fill a molecule when its atoms come close together?

Answer 4

The electrons from the two atoms will fill the new molecule from the lowest orbital up

Question 5

What does the number of new states formed depend on?

Answer 5

If n atoms come together, then n new states are formed (eg 2 hydrogens come together, therefore two new states are formed)

Question 6

Why are hydrogen molecules allowed to form based on their energy levels?

Answer 6

The two electrons from the hydrogen atoms occupy the bonding orbital, as it is the lowest energy level

Since the bonding orbital has electrons and the antibonding orbital doesn’t, the energy level of the hydrogen is at the bonding orbital

Since the energy of the bonding orbital is lower than the energy of the original hydrogens, a hydrogen molecule is formed

Question 7

Why are helium molecules not allowed to form?

Answer 7

Since an orbital can hold a maximum of two electrons, the four electrons from the two helium atoms occupy both the bonding and antibonding orbitals

The bonding and antibonding orbitals’ energy levels cancel each other out (i think), therefore the overall energy level is similar to the original heliums’

Therefore, a helium molecule cannot form, as the overall energy level is not lower than the original atoms’ energy levels

Question 8

What is an energy band?

Answer 8

the continuous range of energy which electrons are allowed to occupy, formed by the splitting of an orbital

Question 9

How are conduction and valence bands formed when atoms come close together?

Answer 9

As atoms come closer together, the orbitals split into energy levels

The valence electrons (outer electrons) of an atom then fill the energy band from the bottom

Since silicon’s electronic structure is 1S2 2S2 2P6 3S2 3P2, the 3S2 and 3P2 orbitals will split into energy bands when silicon atoms come together

The result of these two splits causes the conduction band, valence band and forbidden region