Chemical Bonding and Molecular Geometry

Question 1

What are ionic bonds?

Answer 1

Ionic bonds are the electrostatic forces of attraction between cations and anions.

Question 2

What are some properties of ionic compounds?

Answer 2

• They have a crystalline structure.
• They tend to be rigid and brittle.
• They have high melting and boiling points (because the bonds are very strong).
• Ionic solids are poor conductors of electricity (because the strength of the bonds prevent the free movement of ions).
• Most dissolve readily in water.
• Once dissolved, ionic compounds are excellent conductors of heat and electricity (because the ions can move about more freely).

Question 3

What are binary ionic compounds?

Answer 3

Binary ionic compounds are ionic compounds composed of just two elements (a metal and a nonmetal).

Question 4

What explains the formation of ionic compounds?

Answer 4

• Many metallic atoms have relatively low ionization potentials and lose electrons easily.
• Nonmetals have relatively high electron affinities, and thus more readily gain electrons lost by metal atoms, thereby filling their valence shells.

Question 5

Why is it incorrect to refer to sodium chloride as a molecule?

Answer 5

• There is no single ionic bond, per se, between any specific pair of sodium and chlorine ions.
• The attractive forces between ions are isotropic (meaning they are the same in all directions). Thus, any particular ion is equally attracted to all of the nearby ions of opposite charge.
• This results in the arrangement of tightly-bound, three-dimensional lattice structures of ions.

Question 6

What is the inert pair effect?

Answer 6

The inert pair effect reflects the relatively low energy of valence s-electron pairs for atoms of heavy elements of groups 13, 14, and 15, which results in valence s-electrons not being lost during ionization.

Question 7

What are covalent bonds?

Answer 7

Covalent bonds are the mutual attraction of atoms for a “shared” pair of electrons.

Question 8

How do covalent bonds form?

Answer 8

Covalent bonds form when two atoms have similar tendencies to attract electrons to themselves (i.e., when both atoms have identical or fairly similar ionization energies and electron affinities).

Question 9

What are some physical properties of covalent compounds?

Answer 9

• Covalent compounds are electrically neutral, so the attraction between molecules is weaker. Thus, they have lower melting and boiling points than ionic compounds.
• Many are liquids or gases at room temperature, and are softer in their solid states.
• Most are insoluble in water.
• Since they are electrically neutral, they are poor conductors of electricity in any state.

Question 10

Describe how a covalent bond is formed between two hydrogen atoms.

Answer 10

• As the atoms approach each other, their valence orbitals (1s) begin to overlap.
• The single electrons on each hydrogen atom then interact with both atomic nuclei, occupying the space around both atoms.
• The strong attraction of each shared electron to both nuclei stabilizes the system, and the potential energy decreases as the bond distance decreases.
• If the atoms continue to approach each other, the nuclei will begin to repel each other, and the potential energy increases.

Question 11

What is bond length?

Answer 11

Bond length is the internuclear distance at which the lowest potential energy is achieved.

Question 12

What is a pure covalent bond?

Answer 12

A pure covalent bond is a covalent bond in which atoms are identical and electrons in the bond are shared equally. Electrons that are shared have equal probability of being near the nucleus. (Examples include H₂ and Cl₂.)

Question 13

What is a polar covalent bond?

Answer 13

A polar covalent bond is an unequal distribution of electrons between the bonded atoms. The bonded electrons are more attracted to one atom than the other, creating a partial positive charge on one atom and a partial negative charge on the other. (δ+ and δ- designate partial positive and partial negative charges.)

Question 14

What is electronegativity?

Answer 14

Electronegativity is the measure of the tendency of an atom to attract electrons (or electron density) towards itself. It determines how the shared electrons are distributed between the two atoms in a bond.

Question 15

What are the general trends in electronegativity in the periodic table?

Answer 15

In general, electronegativity increases from left to right across a period, and decreases down a group.

Question 16

Which element is the most electronegative?

Answer 16

Fluorine is the most electronegative element.

Question 17

What is the difference between electronegativity and electron affinity?

Answer 17

• Electron affinity of an element is a measurable physical quantity of the energy released or absorbed when an isolated gas-phase atom acquires an electron (measured in kJ/mol).
• Electronegativity describes how tightly an atom attracts electrons in a bond. It is a dimensionless quantity that is calculated, not measured (on a scale of 0 to 4).

Question 18

How do differences in electronegativity of bonded atoms determine the type of bond?

Answer 18

When the absolute value of the difference in electronegativity of two bonded atoms is very small or zero, the bond is nonpolar covalent. When it is large, the bond is polar covalent or ionic.

Question 19

What are the general values of electronegativity for each type of bond?

Answer 19

• Pure covalent bonds have an electronegativity of less than 0.4.
• Polar covalent bonds have an electronegativity of between 0.4 and 1.8.
• Ionic bonds have an electronegativity of greater than 1.8.
• ***There are exceptions to these guidelines***

Question 20

What is a lone pair?

Answer 20

A lone pair is a pair of electrons that are not used in bonding.

Question 21

What is a single bond?

Answer 21

A single bond is a single shared pair of electrons.

Question 22

What is a double bond?

Answer 22

A double bond consists of two pairs of shared electrons.

Question 23

What is a triple bond?

Answer 23

A triple bond consists of three pairs of shared electrons.

Question 24

What is the octet rule?

Answer 24

The octet rule is the tendency of main group atoms to form enough bonds to obtain eight valence electrons.

Question 25

What are the categories of covalent molecules that have central atoms without eight electrons in their Lewis structures?

Answer 25

• odd-electron molecules
• electron deficient molecules
• hypervalent molecules

Question 26

What are odd-electron molecules?

Answer 26

Odd-electron molecules are molecules that have an odd number of valence electrons, and therefore have an unpaired electron.

Question 27

What are electron deficient molecules?

Answer 27

Electron deficient molecules are molecules with a central atom that has fewer electrons than needed for a noble gas configuration. Generally, the central atom is from group 2 or group 13.

Question 28

What are hypervalent molecules?

Answer 28

Hypervalent molecules are molecules with a central atom that has more electrons than needed for a noble gas configuration. This arises because elements in periods 3 and higher have more than four valence orbitals, and they can share more than four pairs of electrons with other atoms because they have empty d orbitals in the same shell.

Question 29

What are free radicals?

Answer 29

Free radicals are molecules with an odd number of electrons.

Question 30

Why is an atom like boron in BF₃ very reactive?

Answer 30

An atom like boron in BF₃ is very reactive because boron does not have eight valence electrons, so it readily combines with molecules containing atoms with lone pairs of electrons. For example, NH₃ reacts with BF₃ because the lone pair on nitrogen can be shared with the boron atom.

Question 31

What is formal charge?

Answer 31

The formal charge of an atom in a molecule is the hypothetical charge the atom would have if we could redistribute the electrons in the bonds evenly between atoms.

Question 32

What is the formula for calculating formal charge?

Answer 32

formal charge = the number of valence electrons the atom would normally have — (the number of bonds attached to the atom + the number of unpaired electrons)

Question 33

What are some guidelines involving formal charge that can help decide which of the possible molecular structures is most likely for a particular molecule or ion?

Answer 33

• A molecular structure in which all formal charges are zero is preferable to one in which some formal charges are not zero.
• If the Lewis structure must nonzero formal charges, the arrangement with the smallest nonzero formal charges is preferable.
• Lewis structures are preferable when adjacent formal charges are zero or of the opposite sign.
• When we must choose among several Lewis structures with similar distributions of formal charges, the structure with the negative formal charges on the more electronegative atoms is preferable.

Question 34

What is resonance?

Answer 34

Resonance is the situation in which one Lewis structure is insufficient to describe the bonding in a molecule and the average of multiple structures is observed.

Question 35

What are resonance forms?

Answer 35

Resonance forms are two or more Lewis structures that have the same arrangement of atoms but different arrangements of electrons.

Question 36

What is resonance hybrid?

Answer 36

Resonance hybrid is the actual electronic structure of a molecule (the average of the resonance forms). It is indicated by a double-headed arrow between Lewis structures. All bonds are actually exactly the same, even if the resonance forms show multiple types of bonds. The resonance hybrid never possesses the electronic structure described by any of the resonance forms.

Question 37

What is bond energy, D?

Answer 37

Bond energy, D, is the energy required to break a specific covalent bond in one mole of gaseous molecules.

Question 38

What is the formula for the enthalpy change, ΔH, for a chemical reaction?

Answer 38

ΔH = Σ D_{bonds broken} – Σ D_{bonds formed}

Question 39

When there are multiples of the same bond, why are their bond energies not equal when they are being broken?

Answer 39

Once the first bond is broken, the remaining bonds are easier to break.

Question 40

What is lattice energy (ΔH_lattice)?

Answer 40

Lattice energy (ΔH_lattice) of an ionic compound is the energy required to separate one mole if the solid into its component gaseous ions.

Question 41

What is the formula to calculate lattice energy?

Answer 41

ΔH_lattice = C(Z⁺)(Z^-)/R_o, where C is a constant that depends on the type of crystal structure, Z⁺ and Z^- are the charges of the ions, and R_o is the interionic distance (the sum of the radii of the cation and the ion).

Question 42

What is the Born-Haber cycle?

Answer 42

The Born-Haber cycle is an application of Hess’s law that breaks down the formation of an ionic solid into a series of individual steps:

• ΔH_f^º, the standard enthalpy of formation of the compound =
• IE, the ionization energy of the metal +
• EA, the electron affinity of the nonmetal +
• ΔH_s^º, the enthalpy of sublimation of the metal +
• D, the bond dissociation energy of the nonmetal +
• ΔH_lattice, the lattice energy of the compound

Question 43

What is bond angle?

Answer 43

Bond angle is the angle between any two bonds that include a common atom.

Question 44

What is bond distance (bond length)?

Answer 44

Bond distance (bond length) is the distance between nuclei of two bonded atoms along the straight line joining the nuclei.

Question 45

What is VSEPR (valence shell electron-pair repulsion) theory?

Answer 45

VSEPR (valence shell electron-pair repulsion) theory is a theory used to predict the bond angles in a molecule based on positoning regions of high electron density as far apart as possible to minimize electrostatic repulsions.

Question 46

What geometry do two regions of electron density around a central atom in a molecule form, and what are the bond angles?

Answer 46

linear (angle of 180º)

Question 47

What geometry do three regions of electron density around a central atom in a molecule form, and what are the bond angles?

Answer 47

trigonal planar (all angles are 120º)

Question 48

What geometry do four regions of electron density around a central atom in a molecule form, and what are the bond angles?

Answer 48

tetrahedral (all angles are 109.5º)

Question 49

What geometry do five regions of electron density around a central atom in a molecule form, and what are the bond angles?

Answer 49

trigonal bipyramidal (90º and 120º angles)

Question 50

What geometry do six regions of electron density around a central atom in a molecule form, and what are the bond angles?

Answer 50

octahedral (90º and 180º angles)

Question 51

What is the order of electron-pair repulsions from greatest to least repulsion?

Answer 51

• lone pair-lone pair repulsions
• lone pair-bonding pair repulsions
• bonding pair-bonding pair repulsions

Question 52

What is the order of the sizes of space occupied by regions of electron density (from largest to smallest)?

Answer 52

• lone pair
• triple bond
• double bond
• single bond

Question 53

In trigonal bipyramidal molecules, which location do lone pairs occupy and why?

Answer 53

In trigonal bipyramidal molecules, lone pairs occupy the equatorial positions because they are more spacious than axial positions, so they can more easily accomodate the larger lone pairs.

Question 54

What is a bond dipole moment?

Answer 54

A bond dipole moment is the separation of charge in a bond that depends on the difference in electronegativity and the bond distance represented by partial charges or a vector.

Question 55

What is the formula for bond dipole moment?

Answer 55

μ = Qr, where μ is the bond dipole moment, Q is the magnitude of the partial charges (determined by the electronegativity difference), and r is the distance between the charges.

Question 56

What is a polar molecule?

Answer 56

A polar molecule is a molecule with a dipole moment.

Question 57

What is a dipole moment?

Answer 57

A dipole moment is a property of a molecule that describes the separation of charge determined by the sum of the individual bond moments based on the molecular structure.