Topic 1 - Atomic and Molecular Orbitals

What is an atomic orbital?

A wave function that defines a region with a high probability of finding an electron.

• It can be described by specifying size, shape and directional properties.

• A density plot can show orbitals.

What does a conventional 1s orbital look like?

Spherical, with no nodes.

What is a node?

An area where the probability of finding an electron is 0.

What does a conventional 2s orbital look like?

Spherical, with a spherical node.

• It is higher energy than 1s.

What does a conventional 2p orbital look like?

Dumbbell-shaped, not spherical and has a 2D nodal plane.

What cartesian axis coordinates do the different p-orbitals occupy?

The px orbital occupies the x-axis, the py orbital occupies the y-axis, and the pz orbital occupies the z-axis.

What is Hund’s Rule?

Electrons will fill degenerate (same energy) orbitals singly before pairing up. This minimizes electron-electron repulsion and maximizes total spin.

Why is the energy of the 1s orbital of helium lower than that of hydrogen?

• Helium has more protons, so has a greater nuclear attraction to electrons. This means that the electrons are lower in energy.

• This is the same for lithium and beryllium.

What is a molecular orbital?

Molecular orbitals are combinations of atomic orbitals.

• Atomic Orbitals (AOs) can be combined in or out of phase.

What is the subtractive combination?

It is out of phase - a destructive interaction, producing an antibonding molecular orbital (MO*).

What is the additive combination?

It is in phase - a constructive interaction, producing a bonding molecular orbital (MO).

What is the molecular orbital diagram for H₂?

What are σ bonds?

Bonds made of σ and σ* orbitals.

• σ bonding is often described as ‘end-end overlap‘.

σ bonding orbitals and σ* antibonding orbitals are symmetrical by rotation around the bond axis.

What are the prerequisites to forming molecular orbitals?

• Atomic Orbitals must be similar energy.

  → If two atomic orbitals are different in energy, an electron transfer occurs, producing an ionic salt.

• Atomic Orbitals must be similar size.

  → For the best overlap, orbitals should be the same size.

• Atomic Orbitals must have appropriate symmetry.

  → This is required for combination.

How can p-orbitals form?

End on, and edge on overlap.

What is end-on overlap in p-orbitals?

A type of orbital overlap where two p-orbitals align along the axis of the bond, allowing for effective interaction and bonding.

• It is symmetrical about the internuclear axis.

This interaction is responsible for the production of σ bonds.

What is edge-on overlap in p-orbitals?

A type of orbital overlap where two p-orbitals align perpendicular to the axis of the bond, allowing for side-on interaction.

• It is not symmetrical about the internuclear axis.

This results in the formation of π bonds.

What is the difference between σ and π bonds?

σ bonds are formed through end-on overlap, while π bonds are formed through edge-on overlap of p-orbitals.

• σ bonds have better overlap, and are lower in energy.