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MCAT Chem > Chem #3 > Flashcards

Chem #3 Flashcards

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1
Q

what electrons interact to form bonds?

A

valence electrons

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2
Q

bond length ______ with larger differences between the elements involved

A

decreases

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3
Q

the octet rule

A

an atom tends to bond with other atoms so that it has eight electrons in its outermost shell, thereby forming a stable configuration similar to that of the noble gases.
o There are more elements that are exceptions to the octet rule than follow it. Exceptions:
 Hydrogen
 Lithium and beryllium bond (2 and 4 valence e-, respectively)
 Boron (6 valence e-)
 All elements in period 3 or greater: can expand the shell to include more than eight electrons by incorporating d orbitals.
o Easy method of exception memorizing:
 Incomplete octet: these elements are stable with fewer than 8 electrons in their valence shell and include hydrogen (2), helium (2), lithium (2), beryllium (4) and boron (6)
 Expanded octet: any element in period 3 and greater can hold more than 8 electrons, including phosphorus (10), sulfur (12), and chlorine (14) and more.
 Odd number of electrons: any molecule with an odd number of valence electrons cannot distribute those electrons to give eight to each atom.
• Ex: NO: has 11 valence e-.

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4
Q

which elements can form an incomplete octet?

A

these elements are stable with fewer than 8 electrons in their valence shell and include hydrogen (2), helium (2), lithium (2), beryllium (4) and boron (6)

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5
Q

which elements can form an expanded octet?

A

any element in period 3 and greater can hold more than 8 electrons, including phosphorus (10), sulfur (12), and chlorine (14) and more.

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6
Q

which elements can have an odd number of electrons?

A

any molecule with an odd number of valence electrons cannot distribute those electrons to give eight to each atom.
• Ex: NO: has 11 valence e-.

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7
Q

ionic bonding

A

one or more electrons from an atom with a low ionization energy, typically a metal, are transferred to an atom with a high electron affinity, typically a nonmetal.
 Ex: NaCl: NA has a low IE, easily releasing an electron; Cl has a high electron affinity, easily absorbing that electron into its valence shell, both atoms achieve the octet.
• Creates crystal lattice
 Resulting electrostatic charges between the two atoms holds them together.

• Bonds
o Form between atoms that have significantly different EN (greater than 1.7).
o Cation: atom that loses the e-
o Anion: atom that gains the e-
o Strength of electrostatic charges gives them very high melting and boiling points.
o Dissolve readily in water and other polar solvents and are good conductors of electricity.
o Form crystalline lattice in solid state: consists of repeating positive and negative ions.
 Attractive and repulsive forces in this are optimized.

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8
Q

covalent bonding

A

: an electron pair is shared between two atoms, typically nonmetals, that have relatively similar values of electronegativity.
 Degree of polarity of bond depends determines how equally the electrons are shared.
 Coordinate covalent: both of the shared electrons are contributed by one of the two atoms.
 Covalent compounds consist of individually bonded molecules.

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9
Q

coordinate covalent bond

A

both of the shared electrons in a covalent bond are contributed by one of the two atoms.

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10
Q

compare ionic or covalent compounds

A

ionic: creates crystal lattice
covalent: compounds consist of individually bonded molecules.

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11
Q

what is the EN boundary for determining ionic bonds?

A

difference of 1.7

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12
Q

are covalent or ionic bonds stronger?

A

ionic

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13
Q

in covalent bonds, atoms ____ electrons

A

share

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14
Q

are ionic or covalent bonds better conductors of electricity?

A

ionic

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15
Q

what makes up nonpolar covalent bond?

A

when atoms have nearly identical EN (<0.5 difference in EN)

• Diatomics: H, O, F, Br, I, N, Cl

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16
Q

what makes up polar covalent bond?

A

: atoms that differ moderately in EN, share electrons unevenly (0.5-1.7).
• Slight separation of charge across the bond.
• Partial negative charge: to more EN atom
• Partial positive charge: to less EN atom

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17
Q

coordinate covalent bond

A

 Both of the shared electrons originated on the same atom.
 Lone pair on one atom attacked another atom with an unhybridized p-orbital
 Typically found in Lewis acid-base reactions
• Lewis base: donate lone pair

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18
Q

bonding vs. nonbonding electrons

A

 Bonding electrons: the electrons involved in a covalent bond are in the valence shell
 Nonbonding electrons: electrons in the valence shell not involved in covalent bonds.
• Unshared electron pairs are lone pairs.

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19
Q

formal charge

A

difference between atom’s actual valence electrons and the electrons attributed to it in the Lewis structure.
o Best structure minimizes this.
o Formal charge = (valence e-) - (nonbonding e- + ½ bonding e-)
o The charge of an ion or compound is the sum of formal charges of the atoms in the compound.

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20
Q

how to draw out lewis dot structure

A

o The least EN atom is the central atom.
o Hydrogen and halogens occupy a terminal position
o Count the # valence e-
o Draw single bonds
o Octet rule
o Left over e-, place on the central atom either as lone pairs or try to double or triple bond it to give it 8.

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21
Q

resonance structures

A

: when it is possible to draw 2 or more Lewis structures that demonstrate the same arrangement of atoms but that differ in the specific placement of electrons.
• Actual distribution is a hybrid, called resonance hybrid
o More stable the structure, the more it contributes to the resonance hybrid
• A Lewis structure with less separation between opposite charges is preferred over a Lewis structure with a larger separation of charges.
• More stable if negative formal charges are placed on more EN atoms.

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22
Q

which resonance structures are most stable?

A

fewest formal charges, formal charges are closer to each other, formal charges go on the more EN atoms.

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23
Q

Valence shell electron repulsion

A

 Valence shell electron repulsion (VSEPR): reflects the geometric arrangement of atoms in a compound. Uses Lewis dot structures to predict the molecular geometry of covalently bonded molecules.
• Based on number of bonding and nonbonding electron pairs in the valence shell of the central atom.
• Position to reduce electron repulsion
• Electronic geometry: describes the spatial arrangement of all pairs of electrons around the central atom, including both the bonding and the line pairs
o Ideal bond angle: -2.5 degree to the bond angle for every lone pair (away from the ideal bond angle which minimizes repulsion for a structure of only bonding electrons)
• Molecular geometry: describes the spatial arrangement of only the bonding pairs
• Coordination number: the number of atoms that surround and are bonded to a central atom.

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24
Q

compare bonding electrons and nonbonding electrons and repulsion in VSEPR

A

Nonbonding electrons exert more repulsion than bonding electrons becasue they reside closer to the nucleus

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25
Q

what happens to the bond angle in VSEPR for every lone pair?

A

-2.5 degree to the bond angle for every lone pair (away from the ideal bond angle which minimizes repulsion for a structure of only bonding electrons)

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26
Q

coordination number

A

the number of atoms that surround and are bonded to a central atom (not including lone pairs)

27
Q

shape of s and p orbitals

A
  • s orbital: sphere around x-y-z axis

* p orbital: barbell along z, y, and z axis (3 of them)

28
Q

molecular orbital

A

when two atoms bond to form a compound, the atomic orbitals interact

29
Q

bonding orbital

A

signs of the two atomic orbitals are the same

30
Q

antibonding orbital

A

the signs of the two atomic orbitals are different

31
Q

sigma bond

A

: when orbitals overlap head to head
 allow for free rotation about their axes because the electron density of the bonding orbital is a single linear accumulation between the atomic nuclei

32
Q

pi bond

A

the orbitals overlap such that there are two parallel electron cloud densities.
 Do not allow for free rotation

33
Q

intermolecular forces

A

: attractive forces between molecules (strongest, H-bond, has 1/10 strength of covalent bond)

34
Q

consider covalent bonds and the different types of ionic bonds/ionic interactions

A

ionic bonds can be between two atoms (intramolecular) or there can also be ionic interactions which are intermolecular (between molecules). One or the other is stronger and weaker than covalent bonds.

35
Q

London dispersion forces

A

the attractive or repulsive interactions of the short-lived and rapidly shifting dipoles. Can induce dipoles in nearby molecules and can be induced by other charge molecules/magnet
 Weakest
 Only significant when molecules are in close proximity.
 Larger molecules have greater LD forces.
 Transient molecular dipole

36
Q

dipole-dipole interactions

A

the positive region of one molecule is close to the negative region of another molecule.
 Electrostatic force between two molecules.
 Only present in the solid and liquid phase (gases are too far apart)
 Permanent molecular dipole.

37
Q

hydrogen bonds

A

: specific form of dipole-dipole interaction that is very strong and can be intramolecular or intermolecular.
 H is bonded to N, O, or F (which all have high EN)
 Acts as naked proton and interacts with partial negative charge on other molecules N, O, or F

38
Q

what are the intermolecular forces?

A

london dispersion
dipole dipole
hydogen bond

39
Q

is hydrogen partial positive or partial negative in H bond?

A

partial positive, as N, O, or F pulls away the electron density.

40
Q

which elements always abide by the octet rule?

A

carbon, nitrogen, oxygen, fluorine, sodium, and magnesium

41
Q

in the sodium chloride bond, the components are held together by ____

A

electrostatic charges after the sodium donates an electron (making it a cation) and the chloride accepts the electron (making it an anion)

42
Q

ionic bonding molecules form _____ unlike covalent compounds which consist of individually bonded molecules.

A

crystal lattices

43
Q

ionic bonds have very high ___ and _____

A

melting points and boiling point

44
Q

a _____ is formed when ionic bonds are in their solid state which minimizes repulsive forces.

A

crystalline lattice

45
Q

____ are created before ionic bonds form

A

ions

46
Q

what is the bond order of a triple bond

A

3

47
Q

bond energy

A

the energy required to break a bond by separating its components into their isolated, gaseous atomic states.

48
Q

list the diatomics

A

H, O, F, Br, I, N, Cl

49
Q

compare lewis acid and lewis base

A

lewis base will donate a pair of electrons while a lewis acid will accept a pair of electrons.

50
Q

resonance forms of a compound

A

same connectivity but different arrangement of electrons,

If there is different connectivity then it is a different molecule altogether.

51
Q

formal charge vs. oxidation number

A

formal charge underestimates the effect of electronegativity differences
oxidation numbers overestimate the effects of electronegativity differences

52
Q

when calculating formal charges, you assume that ______

A

the electrons in the bond are shared equally.

53
Q

the actual electronic distribution in a compound is a ______ of all of the possible resonance structures

A

hybrid/average

54
Q

Do Lewis dot structures reflect the actual geometric arrangement of atoms in a compound?

A

No, that is what VSEPR is for.

55
Q

go over VSEPR chart

A

https://www.templateroller.com/template/86292/vsepr-theory-molecular-shapes-chart.html

56
Q

Subtract ___ degrees for the bond angle from the ideal bond angle for every lone pair

A

2.5 degrees

57
Q

a molecule with only nonpolar bonds must be ____ while a molecule with polar bonds is ________

A

nonpolar or polar

58
Q

for london dispersion forces, _____ molecules are more easily polarizable and thus have stronger forces

A

larger

59
Q

for london dispersion and dipole dipole interactions, need the molecules to be _____

A

close in proximity

60
Q

Is a hydrogen bond or a covalent bond stronger?

A

A hydrogen bond is not an actual bond and even though it is the strongest of the intermolecular interactions, it is nowhere close to as strong as a covalent bond.

61
Q

molecular geometry refers to the position of only the _____ of electrons in a molecule.

A

bonding pairs

62
Q

ionic bonds are ____ than covalent bonds

A

stronger

63
Q

when considering which compound has a larger dipole. look at the direction of the ______

A

vectors

64
Q

more electronegative elements have ____ atomic radii so they have ___ bonds

A

shorter

MCAT Chem (12 decks)

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