Chem MO

Question 1

O2, tell me about its magnetic behaviour. What is Paramagnetism

Answer 1

This electronic structure adheres to all the rules governing Lewis theory. There is an O=O double bond, and each oxygen atom has eight electrons around it. However, this picture is at odds with the magnetic behavior of oxygen. By itself, O2 is not magnetic, but it is attracted to magnetic fields. Thus, when we pour liquid oxygen past a strong magnet, it collects between the poles of the magnet and defies gravity, as in [link]. Such attraction to a magnetic field is called paramagnetism, and it arises in molecules that have unpaired electrons. And yet, the Lewis structure of O2 indicates that all electrons are paired. How do we account for this discrepancy?

Question 2

What is O2 and what’s the relation with paramagnetism and diamagnetism?

Answer 2

Experiments show that each O2 molecule has two unpaired electrons. The Lewis-structure model does not predict the presence of these two unpaired electrons. Unlike oxygen, the apparent weight of most molecules decreases slightly in the presence of an inhomogeneous magnetic field. Materials in which all of the electrons are paired are diamagnetic and weakly repel a magnetic field. Paramagnetic and diamagnetic materials do not act as permanent magnets. Only in the presence of an applied magnetic field do they demonstrate attraction or repulsion.

Question 3

VALENCE BOND THEORY

Answer 3

-Considers bonds as localized between one pair of atoms
-Creates Bonds from overlap of atomic orbitals (s,p,d) and hybrid orbitals (sp, sp2,sp3….)
-forms sigma and pi bonds
-predicts molecular shape based on the number of regions of electron density
-needs multiple structures to describe resonance

Question 4

MOLECULAR ORBITAL THEORY

Answer 4

-considers electrons delocalized throughout the entire molecule
-combines atomic orbitals to form molecular orbitals (sigma, sigma, pi, pi)
-creates bonding and anti bonding interactions based on which orbitals are filled
-predicts the arrangement of electrons in molecules

Question 5

Fig #9, describe what’s happening

Answer 5

(a) When in-phase waves combine, constructive interference produces a wave with greater amplitude. (b) When out-of-phase waves combine, destructive interference produces a wave with less (or no) amplitude.

Question 6

Fig 11, Explain wave functions of 2 p atomic orbitals

Answer 6

Combining wave functions of two p atomic orbitals along the internuclear axis creates two molecular orbitals, σp and σ∗p.

Question 7

Fig #12, explain the formation of sigma* and pi* and sigma and pi

Answer 7

The side-by-side overlap of two p orbitals gives rise to a pi (π) bonding molecular orbital and a π* antibonding molecular orbital, as shown in the figure below. In valence bond theory, we describe π bonds as containing a nodal plane containing the internuclear axis and perpendicular to the lobes of the p orbitals, with electron density on either side of the node. In molecular orbital theory, we describe the π orbital by this same shape, and a π bond exists when this orbital contains electrons. Electrons in this orbital interact with both nuclei and help hold the two atoms together, making it a bonding orbital. For the out-of-phase combination, there are two nodal planes created, one along the internuclear axis and a perpendicular one between the nuclei.