U4: Ch. 10 SB

Oxidation number and formal charge differ in several important ways.

*Oxidation number -All shared bonding electrons are assigned to the more electronegative atom -Does not change for a particular atom from one resonance form to another. *formal charge -Half the shared bonding electrons are assigned to each atom of the bond in order to calculate -may change for a particular atom from one resonance form to another.

Which of the following statements correctly describe how formal charge is assigned to an atom in a Lewis structure?

- Formal charge = (valence e-) - (unshared e- + 1/2shared e-) - The number of shared electrons equals twice the number of bonds in the structure. - The atom is considered to "own" half the shared electrons.

Place the steps for determining the shape of a covalently bonded species in the correct order.

- Use the molecular formula to draw a Lewis Structure - Count all electron groups and assign an electron-group arrangement. - Determine the molecular shape by counting bonding groups and nonbonding groups separately.

Which of the following statements correctly describe the effect of molecular polarity on the behavior of a molecular compound?

-The physical properties of a molecular compound are directly related to the molecular polarity -a polar compound will experience stronger intermolecular forces than nonpolar compound

The Lewis structure for the AsF5 is shown. The approximate value of the bond angle marked "a" is equal to _____, whereas the approximate value of the bond angle marked "b" is equal to _____. (Remember that this species is NOT flat, as indicated by the wedged and dashed bonds.)

90°, 120° Recall that the equatorial bonds form a single plane, with 120° angles between them, while the axial bonds are at right angles to the equatorial plane.

Which of the following statements correctly describe the relative stabilities of resonance forms?

A resonance structure is more stable if a negative charge resides on a more electronegative atom. A resonance structure with formal charges closer to zero on individual atoms is preferred.

Which of the following statements correctly describe how to draw a Lewis structure?

A single bond contains two electrons and is represented by a solid line. A Lewis structure includes all the valence electrons in the species. An electron is added to the total count for each negative charge on the species.

The Lewis structure for one of the resonance forms of the sulfate ion, SO42-, is shown. What is the approximate value of the marked O-S-O bond angle?

Although this structure appears to be flat with right angles between the atoms, the true shape is tetrahedral because there are four electron groups. The bond angles are 109.5°.

Which of the following statements correctly describe resonance structures?

An individual resonance structure does not accurately represent the structure of the species. Resonance structures differ only in the arrangement of electrons.

A bond angle is the angle formed by the nuclei of _____ surrounding atoms with the nucleus of a central atom. The bond angle predicted from the molecular shape is a(n) ____ bond angle. If the bonding electron groups around the central atom are not _____, the real bond angle will deviate from the predicted value.

Blank 1: 2 or two Blank 2: ideal or theoretical Blank 3: identical, the same, equivalent, or equal

The arrangement of 5 electron groups around a central atom is trigonal _____________. This system has two different bond angles. Three groups lie in a trigonal plane around the central atom with bond angles of ____________________o while the other two groups lie above and below this plane, at an angle of o to the trigonal plane.

Blank 1: bipyramidal Blank 2: 120 Blank 3: 90

The molecular shape of a covalent species is determined not only by the number of ______ electron groups that join the atoms, but by the number of ______ electron groups as well, since these electrons also occupy space. shared; unshared

Blank 1: bonding, shared, or bonded Blank 2: nonbonding, non-bonding, lone pair, unshared, lone, unpaired, unbonded, nonbonded, or unbonding

To classify molecular shapes, a structure is assigned a specific AXmEn designation, where A is the _____atom, X is a(n) ________ atom, and E represents a(n) _______ valence electron group that is usually a lone _____.

Blank 1: central, center, or middle Blank 2: surrounding, bonded, peripheral, terminal, substituent , or outer Blank 3: nonbonding, non-bonding, unbonded, or unshared Blank 4: pair or electron pair

In a single, double, or triple bond, the shared electron pairs are localized between the bonded atoms. In a resonance hybrid, electrons are_________________ or spread out over two or more adjacent atoms. Instead of a double or triple bond between two such atoms, the bond is considered a(n) _________________double or triple bond with a fractional bond order.

Blank 1: delocalized or shared Blank 2: partial

Some elements do not obey the octet rule because they have more than eight valence electrons, i.e., they have a(n) _____ valence shell. This is only possible for elements that have available ______ orbitals, i.e., elements from period ______ of the periodic table onward.

Blank 1: expanded or hypervalent Blank 2: d or empty Blank 3: 3 or three

A molecule that contains covalent bonds and has a(n) ________ arrangement of electron groups will have an overall ______ polarity, which is measured as a dipole ______. Unsymmetrical, Molecular, Moment.

Blank 1: unsymmetrical, non-symmetrical, unbalanced, uneven, asymmetric, unequal, asymmetrical, nonlinear, or imbalanced Blank 2: molecular Blank 3: moment

The formal charge assigned to an atom is calculated by subtracting the number of electrons the atom "owns" from the total number of ______ electrons. The number of electrons "owned" by a specific atom is defined as being the sum of the ________ valence electrons and half the ______ valence electrons.

Blank 1: valence or outer Blank 2: unshared, nonbonding, unbonded, nonbonded, lone, lone pair, non-bonded, or non-bonding Blank 3: shared, bonding, bond pair, or bonded

According to VSEPR theory, each group of _____ electrons around a central atom will be located as far away from the others as possible, in order to minimize electron-electron ________.

Blank 1: valence, outer, or valence-shell Blank 2: repulsions or repulsion

Which of the following statements correctly describe the "normal" bonding pattern for a neutral atom of each element, assuming that the octet rule is obeyed?

C usually forms four covalent bonds. F always forms a single covalent bond. H forms one covalent bond and is generally not found as a central atom in covalent species.

When drawing Lewis structures it is useful to recall the "normal" bonding patterns of common elements. Match each element with the number of covalent bonds the neutral atom usually has if the octet rule is obeyed.

CNOF 4,3,2,1

Which of the following elements may have an expanded valence shell?

Cl, S, Xe

Arrange the steps involved in drawing a Lewis structure in the correct order, starting with the first step on top.

Count the valence electrons fro all atoms, and add or subtract electrons according to the charge. Place the atoms relative to each other. Place a bond between every pair of atoms, using two valence electrons for each bond. Distribute the remaining electrons in pairs to give each atoms an octet Form multiple bonds if all electrons have been used and any atom des not have an octet

Which statement correctly describes the basic principle of VSEPR theory?

Each group of valence electrons around a central atom is located as far from the others as possible.

Because lone pairs exert larger repulsions than those from bonding pairs, lone pairs will occupy________________ (equatorial/axial) positions in a trigonal bipyramidal arrangement to minimize repulsions between electron groups.

Equatorial

Which of the lettered options correspond to equatorial positions in the image shown?

Equatorial groups are approximately horizontal, but actually somewhat distorted from that (slightly up or slightly down)

True or false: Any molecule in which all of the bonds are identical will be nonpolar.

F For a molecule to be nonpolar, all bond dipoles must cancel and the electron distribution must be symmetrical throughout. A molecule with identical bonds will still be polar if the individual bond dipoles do not cancel. This may occur if there are one or more lone pairs on the central atom. As an example, water contains only O-H bonds but has an overall dipole moment because of its two lone pairs on the central oxygen.

True or false: Resonance structures are different possible structures for the same species that interconvert rapidly.

F Resonance structures ARE different Lewis structures of a species, but no individual resonance structure actually exists. There is no interconversion between forms. Rather the true structure is a single structure that is an average of all possible resonance forms.

Which of the following statements correctly describe formal charge?

Formal charges can be used to select the most important resonance structures for a species. The formal charge of an atom is the charge it would have if all bonding electrons were shared equally. *The formal charges in a species must add up to the actual charge on the species, which is not always zero.

Select the correct Lewis structure for HCN. (Lone pairs of electrons are omitted.)

H-C≡N

Which of the following statements correctly describe how to determine whether a given molecule is polar or nonpolar?

If a molecule contains no dipole moments across any bonds, it will be nonpolar. For a molecule that has polar bonds, the geometry must be known in order to predict the overall polarity. If the individual bond dipoles do not cancel, the molecule is polar. *A molecule may have a symmetrical geometry but nonidentical bonds. If any of these bonds are polar, the molecule will be polar since individual bond dipoles will not cancel.

Match each description of molecular shape to the correct implication for polarity.

Individual bond dipoles will cancelfor a species that has identical bonds and a symmetrical geometry.Individual bond dipoles will not cancelfor a species with an unsymmetrical geometry.

Which of the following statements correctly describe KrBr2?

Kr is surrounded by 5 electron pairs. Kr is surrounded by 3 lone pairs. KrBr2 has linear geometry with bond angles of 180°. The 3 lone pairs occupy the equatorial positions, and the 2 bonding pairs occupy the axial positions.

Which of the following options correctly defines resonance structures?

Lewis structures for the same species that differ in the placement of electrons

A covalent species such as the molecule H3NO has more than one central atom. Which of the following options correctly describe how to draw a Lewis structure for this species?

N and O are central atoms because both elements form more than one covalent bond. There will be three bonds to the N atom. The O atom will be bonded to both H and N.

True or False: Two molecules with the same electron-group geometry can have different molecular shapes.

T

True or false: When using VSEPR theory to determine molecular shape, a triple bond counts as a single electron domain even though it consists of three shared electron pairs.

T

The Lewis structure for the cyanide ion is shown.

The formal charge on carbon is 4 - (2 + 3) = -1. The formal charge on the nitrogen atom is 5 - (2 + 3) = 0.

Which option correctly describes a covalent species that has four electron groups around the central atom?

The ideal bond angle for a four-electron system is 109.5 If all four electron pairs are bonding pairs, the shape of the system is tetrahedral. A bent shape is observed if there are two bonding pairs and two lone pairs.

A given species has a bond order of 1 1/2. What does this fractional bond order indicate? Select all that apply.

The species is a resonance hybrid. The species contains delocalized electrons. -Fractional bond orders are not an indication of instability in a compound. Fractional bond orders only indicate that partial bonding, as in resonance hybrids, is present in the compound.

The nitrite ion, NO2-, is a resonance hybrid and has two resonance forms, as shown. Select all the statements that correctly describe the bonding in this species.

Two electron pairs are delocalized over the entire species. The bond order for this species is 1.5. Each N-O bond is a partial double bond. *Both N-O bonds in this ion are in fact partial double bonds and are the same length. *Neither of these structures actually exists. There is only ONE structure for the ion, which is an average of these two resonance forms.

Calculate the formal charges on each of the nitrogen atoms in the N3- ion shown. The overall charge of the ion has been omitted in the structure.

a = -1; b = +1; c = -1 *Remember that formal charge is given by valence e- - [all unshared e- + 1/2 shared e-].

Resonance structures are Lewis structures that have the same relative placement of _____ but a different arrangement of _____.

atoms; electrons

The angle formed by the nuclei of two surrounding atoms with the nucleus of the central atom in a structure is called a(n) _____ angle. The value predicted for such an angle using the VSEPR theory would be 180 degrees, based upon geometry alone. This is referred to as the _______ bond angle. In practice, this value often deviates from the predicted value for various reasons.

bond; ideal

Lone pairs and double bonds are more spread out in space than a bonding pair. The result is that they exert greater repulsion on bonding pairs that leads to a(n) ______.

decrease in the angle between bonding pairs

The polarity of a molecule can be expressed in terms of its _______ moment (symbol μ), which is the product of the partial _____ in the molecule and the ______ between their centers.

dipole; charges; distance

The formal charge of an atom in a Lewis structure is the charge the atom would have if all bonding electrons were shared ____ between the atoms. The formal charge does not necessarily reflect an actual charge on the atom.

equally

Partial bonding, for example, as part of a resonance hybrid, often results in structures with _____.

fractional bond orders

Formal charge is calculated by assigning each atom _____ the bonding electrons that it shares. Oxidation number is determined by assigning all the shared electrons of a particular bond to the atom with the _____ electronegativity.

half; higher

A resonance structure with more charges will be _____ stable than a resonance structure with fewer charges. For two resonance forms with the same formal charges, the resonance structure that has the negative formal charge on the more _________ atom will be more stable.

less; electronegative

A structure containing a central atom with two electron groups, designated AX2, has a _____ shape with a bond angle of _____ degrees.

linear; 180

Molecular shape is determined by the number of electron groups around a central atom, where a "group" consists of any number of electrons that occupy a _____ region around an atom. A double bond contains _____ electron pair(s) but is considered _____ electron group because these electrons remain near each other.

localized; two; one

In general, a lone pair repels bonding electron pairs _____ than bonding pairs repel each other. A lone pair will therefore _____ the bond angle between bonding pairs. A double bond has a similar effect because a double bond has a _____ electron density than a single bond.

more; decrease; greater

Consider the molecule CCl4. each C-Cl bond in this molecule is _____ because the electronegativity difference between C and Cl is _________ than 0.4. Since CCL4 is tetrahedral in shape and symmetrical, the individual bond dipoles ______ and the molecule is _____ overall.

polar; greater; cancel; non polar

In general, the smaller the bond angle, the _____ the electron repulsions for the groups concerned. In a five-electron group system, lone pairs prefer to occupy _____ positions because equatorial-equatorial repulsions are _____ than axial-equatorial repulsions.

stronger; equatorial; weaker