Molecular Orbital Theory

What does Molecular Orbital Theory (MOT) explain

A theory that explains how atomic orbitals combine to form molecular orbitals that extend over the whole molecule.

What are the two types of molecular orbitals

Bonding orbitals (σ, π) and antibonding orbitals (σ, π)

What is the formula for bond order

½(# of bonding electrons − # of antibonding electrons)

What is constructive overlap

When orbitals combine in-phase to increase electron density between nuclei, forming a bonding orbital.

What is destructive overlap

When orbitals combine out-of-phase, canceling electron density and forming an antibonding orbital.

What does a bond order of 0 mean

The molecule is unstable and does not exist.

What is the bond order of H₂

½(2 − 0) = 1 → stable

What is the bond order of Be₂

½(2 − 2) = 0 → unstable, doesn’t exist

How do MOT and VBT differ

MOT uses delocalized electrons over the molecule and explains magnetism; VBT focuses on localized bonds between atoms.

Why is O₂ paramagnetic

It has two unpaired electrons in its π* orbitals according to MOT.

Which orbitals form sigma (σ) bonds

s–s, s–p, or p–p orbitals overlapping head-on

Which orbitals form pi (π) bonds

p–p orbitals overlapping sideways

How many σ and π bonds are in a double bond

1 σ and 1 π

How many σ and π bonds are in a triple bond

1 σ and 2 π

Why is rotation possible around a single bond but not a double bond

Because π bonds (in double/triple bonds) restrict rotation; σ bonds allow free rotation.

What do bonding orbitals do to energy and stability

They lower the energy of the molecule and increase stability.

What do antibonding orbitals do to energy and stability

They raise the energy of the molecule and decrease stability.