Molecular Orbital Theory Overview

These flashcards cover key concepts and details from the lecture notes on Molecular Orbital Theory, focusing on the formation, properties, and significance of molecular orbitals.

What does Molecular Orbital (MO) Theory predict about molecular properties?

It predicts the magnetic properties and bonding characteristics of diatomic molecules and ions.

What type of molecular orbitals are formed by side-by-side overlaps?

π molecular orbitals are formed by side-by-side overlaps.

What does the energy diagram for the molecular orbitals show?

It displays the relative energies of the bonding and antibonding molecular orbitals.

What is the bond order formula used in MO theory?

Bond Order (B.O.) = 1/2 (number of bonding electrons - number of antibonding electrons).

How many σ bonds and π bonds are in an oxygen (O2) molecule?

1 σ bond and 1 π bond.

What happens to the energy levels of σ2p and π2p orbitals in Li2 – N2?

The energy levels switch due to s-p orbital mixing.

Which atomic orbital overlaps to form a σ bond for oxygen (O2)?

The 2p orbitals overlap to form the σ bond.

What is the significance of unpaired electrons in oxygen's magnetic properties?

Unpaired electrons in O2 make it paramagnetic.

What occurs during the reaction of sodium metal with oxygen?

The reaction produces disodium oxide (Na2O) which reacts with water.

How do molecular orbital diagrams for homonuclear diatomic molecules differ from those of heteronuclear molecules?

Homonuclear diagrams often show equal sharing of electrons while heteronuclear diagrams account for differences in electronegativity.

What does Molecular Orbital (MO) Theory explain?

MO Theory describes chemical bonding by combining atomic orbitals into molecular orbitals that span the entire molecule. It predicts electronic structure, stability, magnetic properties (like paramagnetism), and bonding characteristics of molecules.

How are \text{π} (pi) molecular orbitals formed?

\text{π} molecular orbitals are formed by the side-by-side (lateral) overlap of two parallel atomic p-orbitals. This results in electron density above and below the internuclear axis, with a nodal plane along the internuclear axis.

What information is conveyed by a molecular orbital energy diagram?

An MO energy diagram displays the relative energy levels of bonding and antibonding molecular orbitals. It shows how atomic orbitals combine and how electrons fill MOs according to Aufbau, Pauli, and Hund's rules, which helps determine bond order and magnetic properties.

How is bond order calculated in MO theory, and what does it indicate?

Bond Order (B.O.) = \frac{1}{2} \text{(number of bonding electrons - number of antibonding electrons)} . A higher positive B.O. indicates a stronger, shorter, and more stable bond, while B.O. = 0 suggests an unstable molecule.

Detail the MO bonding structure and magnetic properties of an oxygen ( O_2 ) molecule.

Oxygen ( O_2 ) has a bond order of 2 (one \sigma bond, one \text{π} bond). Its MO diagram shows two unpaired electrons in the \text{π}_{2p}^* antibonding orbitals, making O_2 paramagnetic (attracted to a magnetic field).

Explain the phenomenon of s-p orbital mixing and its effect on MO energy levels in diatomic molecules from Li_2 to N_2 .

S-p orbital mixing results from the interaction of 2s and 2p_z atomic orbitals. For diatomic molecules from Li_2 to N_2 , this mixing causes the \text{π}_{2p} molecular orbitals to become lower in energy than the \sigma_{2p} molecular orbital, altering the filling order.

Which specific atomic orbitals overlap to form the \sigma_{2p} molecular orbital in the oxygen ( O_2 ) molecule?

In O_2 , the \sigma_{2p} molecular orbital is formed by the direct, head-on (end-to-end) overlap of the two 2p_z atomic orbitals, one from each oxygen atom, along the internuclear axis.

How do unpaired electrons in molecular orbitals define molecular paramagnetism, illustrating with O_2 ?

Molecules with one or more unpaired electrons in their molecular orbitals are paramagnetic (attracted to a magnetic field). O_2 is paramagnetic due to two unpaired electrons in its \text{π}_{2p}^* antibonding orbitals. If all electrons are paired, the molecule is diamagnetic (repelled).

Describe the primary reactions of sodium metal with oxygen gas under different conditions.

Sodium metal reacts vigorously with oxygen. In excess oxygen, it forms sodium peroxide ( Na_2O_2 ). With limited oxygen, it forms disodium oxide ( Na_2O ). Both oxides are reactive and can react further, for example, with water.

What are the fundamental differences in MO diagram construction and electron distribution between homonuclear and heteronuclear diatomic molecules?

Homonuclear (identical atoms) diagrams show atomic orbitals at equal energy, resulting in symmetrical molecular orbitals and equal electron sharing. Heteronuclear (different atoms) diagrams show unequal atomic orbital energies, leading to asymmetric molecular orbitals and unequal electron sharing towards the more electronegative atom.

What is the underlying premise of Molecular Orbital (MO) Theory regarding atomic orbital combination?

The fundamental premise is that atomic orbitals combine to form delocalized molecular orbitals that encompass the entire molecule, rather than remaining localized on individual atoms or in specific bonds.

What is the crucial distinction between bonding and antibonding molecular orbitals?

Bonding MOs result from constructive interference, increasing electron density between nuclei, lowering energy, and stabilizing the molecule. Antibonding MOs result from destructive interference, creating a nodal plane between nuclei, raising energy, and destabilizing the molecule.

Elaborate on the formation of sigma ( \sigma ) and pi ( \text{π} ) molecular bonds through specific atomic orbital overlaps.

Sigma ( \sigma ) bonds form from direct, head-on overlap of atomic orbitals (s-s, s-p, p-p axial), concentrating electron density along the internuclear axis and allowing free rotation. Pi ( \text{π} ) bonds form from side-by-side overlap of parallel p-orbitals, creating electron density above and below the axis, with a nodal plane on the axis, and restricting rotation.

How are paramagnetism and diamagnetism experimentally detected and explained via molecular orbital theory?

Paramagnetism (attraction to magnetic field) is detected in molecules with unpaired electrons in MOs (e.g., O2 ). Diamagnetism (weak repulsion from magnetic field) is detected in molecules where all electrons in MOs are paired (e.g., N2 ).

What are the general rules for the combination of atomic orbitals to form molecular orbitals?

Atomic orbitals combine to form molecular orbitals if they have: 1) appropriate symmetry, 2) similar energy levels, and 3) significant overlap between them.

How are electrons distributed among molecular orbitals according to established quantum mechanical principles?

Electrons fill MOs according to the Aufbau principle (lowest energy first), Pauli exclusion principle (max two electrons per orbital with opposite spins), and Hund's rule (single occupancy of degenerate orbitals before pairing).

Briefly describe the Linear Combination of Atomic Orbitals (LCAO) approximation.

The LCAO approximation mathematically describes molecular orbitals as a linear sum (weighted average) of atomic orbitals (e.g., \Psi = c_A \psi_A + c_B \psi_B ). This simplifies the calculation of MOs from atomic orbitals.

What is a "node" in the context of molecular orbitals, and how does it relate to stability?

A node is a region within a molecular orbital where the electron probability is zero ( \Psi^2 = 0 ). Bonding MOs have fewer or no nodes between nuclei (stabilizing), while antibonding MOs have at least one node between