Chem Quiz #1

What is a bonding molecular orbital?

A bonding molecular orbital is lower in energy than the atomic orbitals from which it is formed. There is an increased electron density in the internuclear region.

Explain the difference between a paramagnetic species and a diamagnetic one.

A paramagnetic species has unpaired electrons in one or more molecular orbitals. A paramagnetic species is attracted to a magnetic field. The magnetic property is a direct result of unpaired electrons. The spin and angular momentum of the electrons generate tiny magnetic fields. A diamagnetic species has all of its electrons paired. The magnetic fields caused by the electron spin and the orbital angular momentum tend to cancel each. A diamagnetic species is not attracted to a magnetic field and is in fact slightly repelled.

Name the hybridization scheme that corresponds to each electron geometry: A. linear , B. trigonal planar, C. tetrahedral, D. trigonal bipyramidal, E. octahedral

A. linear = sp B. trigonal planar = sp^2 C. tetrahedral = sp^3 D. trigonal bipyramidal = sp^3d E. octahedral = sp^3d^2

What is a chemical bond according to valence bond theory?

According to valence bond theory, a chemical bond results from the overlap of two half filled orbitals with spin pairing of the two valence electrons

In valence bond theory, what determines the geometry of a molecule?

According to valence bond theory, the shape of the molecules is determined by the geometry of the overlapping orbitals

What is an anti bonding molecular orbital?

An anti bonding molecular orbital is higher in energy than the atomic orbitals from which it is formed. There is less electron density in the internuclear region which results in a node.

Describe the relationship between the state of a substance, its temperature , and the strength of its intermolecular forces.

Because the most molecular motion occurs in the gas phase and the least molecular motion occurs in the solid phase, a substance will be converted from a solid to a liquid and then a gas as the temp. increases. The strength of the intermolecular interactions is the least in the gas phase because there are large distances between particles and they are moving very fast. Intermolecular forces are stronger in liquids and solids, where molecules are "touching" one another. The strength of the interactions in the condensed phases determines what at temps. the substance will melt or boil.

How does hybridization of the atomic orbitals in the central atom of a molecule help lower the overall energy of the molecule?

Hybrid orbitals minimize the energy of the molecules by maximizing the orbital overlap in a bond.

What is hybridization? Why is hybridization necessary in valence bond theory?

Hybridization is a mathematical procedure in which the standard atomic orbitals are combined to form new atomic orbitals called hybrid orbitals. Hybrid orbitals are still localized on individual atoms, but they have different shapes and energies from those of standard atomic orbitals. They are necessary in valence bond theory because they correspond more closely to the actual distribution of electrons in chemically bonded atoms.

What is a chemical bond according to molecular orbital theory?

In molecular orbital theory, atoms will bond when the electrons in the atoms can lower their energy by occupying molecular orbitals of the resultant molecule.

Explain the difference between hybrid atomic orbitals in valence bond theory and LCAO molecular orbitals in molecular orbital theory?

In valence bond theory, hybrid orbitals are weighted linear sums of valence atomic orbitals of a particular atom, and the hybrid orbitals remain localized on that atom. In molecular orbit theory, the molecular orbitals are weighted linear sums of valence atomic orbitals of all the atoms in a molecule, and many of the molecular orbitals are delocalized over the entire molecule.

Why are intermolecular forces important?

Intermolecular forces are important because they hold many liquids and solids, water and ice for example, together. These intermolecular forces determine many of the physical properties of a substance. All living organisms depend on intermolecular forces for many physiological processes. Intermolecular forces are responsible for the very existence of the condensed phases.

From what kinds of interactions do intermolecular forces originate?

Intermolecular forces originate from the interactions between charges, partial charges, and temporary charges on molecules, much like bonding forces originate from interactions between charged particles in atoms.

Why are intermolecular forces generally much weaker than bonding forces?

Intermolecular forces, even the strongest ones, are generally much weaker than bonding forces. The reason for the relative weakness of intermolecular forces compared to bonding forces is related to Coloumb's law. Bonding forces are the result of large charge interacting in very close distances. Intermolecular forces are the result of smaller charges interacting at greater distances.

How is the number of molecular orbitals approximated by a linear combination of atomic orbitals related to the number of atomic orbitals used in the approximation?

Molecular orbitals can be approximated by a linear combination of atomic orbitals (AOs.) The total number of MOs formed from a particular set of AOs will always equal the number of AOs used.

In molecular orbital theory, what is a non bonding orbital?

Non bonding orbitals are atomic orbitals not involved in a bond that remain localized on the atom.

What factors cause transitions between the solid and liquid state? The liquid and gas state?

One phase of matter can be transformed to another by changing the temp., pressure or both. A liquid can be converted to a gas by heating, and a gas can be condensed into a liquid by cooling. In general, increases in pressure favor the denser phase; so increasing the pressure of a gas sample can result in a transition to the liquid phase. A solid can be converted to a liquid by heating and liquid can be converted to a solid by cooling. In general, increases in pressure favor the denser phase; so increasing the pressure of most liquids can result in a transition to the solid phase.

What is the fundamental difference between an amorphous solid and crystalline solid?

Solids may be crystalline, in which case the atoms or molecules that compose them are arranged in a well ordered three dimensional array, or amorphous, in which case the atoms or molecules that compose them have no long range order.

In molecular orbital theory, what is bond order? Why is it important?

The bond order in a diatomic molecule is the number of electrons in bonding molecular orbitals (MOs) minus the number of electrons in anti bonding MOs divided by two. The higher the bond order, the stronger the bond. A negative or zero bond order indicates that a bond will not form between the atoms.

Why does energy ordering of the molecular orbitals of the period 2 diatomic molecules change in going from N2 to O2?

The degree of mixing between two orbitals decreases with increasing energy difference between them. Mixing of the 2s and 2px orbitals is greater in B2, C2, and N2 than in O2, F2, and Ne2 because in B, C, and N, the energy levels of the atomic orbitals are more closely spaced than in O, F, and Ne. This mixing produces a change in energy ordering for the pi2p and sigma2p molecular orbitals.

In the Lewis model, the two bonds in a double bond look identical. However, valence bond theory shows that they are not. Describe a double bond according to valence bond theory. Explain why rotation is restricted about a double bond, but not about a single bond.

The double bond in Lewis theory is simply two pairs of electrons that are shared between the same two atoms. However, in valence bond theory, we see that the double bond is made up of two different kinds of bonds. The double bond in valence bond theory consists of one sigma bond and one pi bond. Rotation about a double bond is severely restricted because of the side by side overlap of the p orbitals, the pi bond must essentially break for rotation to occur. The sigma bond consists of end to end overlap. Because the overlap is linear, rotation is not restricted.

What is the role of wave interference in determining whether a molecular orbital is bonding or anti bonding?

The electrons in orbitals behave like waves. The bonding molecular orbital arises from the constructive interference between the atomic orbitals and is lower in energy than the atomic orbitals. The anti bonding molecular orbital arises from the destructive interference between the atomic orbitals and is higher in energy between than the atomic orbitals.

Explain how a gecko is able to walk on a polished glass surface.

The key to gecko's sticky feet, lies in the millions of microhairs, called setae, that line its toes.Each seta is between 30 and 130 long, and branches out to the end in several hundred flatten tips called spatulae. This unique structure allows the gecko's toes to have unusually close contact with the surface it climbs. This close contact allows the intermolecular forces - which are significant only at short distances- to hold the gecko to the wall.

What are the main properties of liquids ( in contrast to gases and solids)?

The main properties of liquids are that liquids have much higher densities than comparison to gases and generally have lower densities in comparison to solids, liquids have an indefinite shape and assume the shape of their container, liquids have a definite volume and are not easily compressed

What are the main properties in solids (in contrast to liquids and gases)?

The main properties of solids are that solids have much higher densities than comparison to gases (and usually higher densities than liquids) solids have a definite shape, they do not assume the shape of their container. Solids have a definite volume, they are not easily compressed. And they must be crystalline (ordered) or amorphous (disordered).

How is the number of hybrid orbitals related to the number of standard atomic orbitals that are hybridized?

The number of standard atomic orbitals added together always equals the number of hybrid orbitals formed. The total number of orbitals is conserved.

In valence bond theory, the interaction energy between the electrons and nucleus of one atom with the electrons and nucleus of another atom is usually negative (stabilizing) when ________.

When the interacting atomic orbitals contain a total of two electrons that can spin - pair.

When applying molecular orbital theory to heteronuclear diatomic molecules, the atomic orbitals of different energies make two molecular orbitals, how are the energies of the molecular orbitals related to the energies of the atomic orbitals? How is the shape of the resultant molecular orbitals related to the shapes of the atomic orbitals?

When two atomic orbitals are different, the weighting of each orbital in forming a molecular orbital may be different. When a molecular orbital is approximated as a linear combination of atomic orbitals of different energies, the lower energy atomic orbital makes a greater contribution to bonding orbital and the higher energy orbital makes a greater contribution to the anti bonding molecular orbital. The shape of the molecular orbital shows a greater electron density at the atom that has a lower atomic orbital energy.