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+ CHEM 121 > Ch 2: Chemical Bonding and Lewis Structures > Flashcards

Ch 2: Chemical Bonding and Lewis Structures Flashcards

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1
Q

What are compounds?

A
  • chemicals that are combinations of two or more different elements in defined proportions
  • no overall charge, “neutral”
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2
Q

What are ions?

A

species possessing either an overall positive or negative charge

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3
Q

What are cations?

A

ions carrying an overall positive charge

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4
Q

What are anions?

A

ions carrying an overall negative charge

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5
Q

What are simple ions?

A

ions that contain a single atom

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6
Q

What are complex/polyatomic/molecular ions?

A

ions that contain multiple atoms

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7
Q

Why are cations/anions isolable?

A

cations cannot be isolated in the condensed phases (liquid/solid) without anions present to balance the charge to form a net-neutral compound

and vice versa

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8
Q

What holds atoms in compounds together?

A

chemical bonds

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9
Q

What do chemical bonds consist of?

A

electrons that simultaneously feel the electrostatic forces of attraction for both of the positively charged nuclei of adjacent atoms

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10
Q

What do elements achieve by forming bonds?

A

elements achieve a lower overall energy than they would in their free elemental form

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11
Q

What is an ionic bond?

A

oppositely charged ions held together by electrostatic forces

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12
Q

What is a covalent bond?

A

atoms are held together by mutual attraction of a pair (or pairs) of electrons to the nuclei of adjacent atoms

electrons are “shared” between the atoms

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13
Q

What is a metallic bond?

A

electrons are shared between many atoms simultaneously and are free to “flow” between the atoms

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14
Q

Where is the most important type of bond, the metallic bond?

A

in elemental metals or metallic alloys

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15
Q

What are metallic alloys?

A

mixtures of two or more elemental metals

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16
Q

What determines whether a bond is covalent or ionic?

A

the electronegativity difference of the bonding atoms

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17
Q

What is electronegativity?

A

the tendency of an atom in a molecule to attract electrons to itself

cannot discuss electronegativity of an atom unless it is part of a molecule or molecular ion

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18
Q

How is electronegativity measured?

A

it cannot be directly measures, it must be calculated from other molecular properties

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19
Q

What may electronegativity values be used for?

A

to predict the nature of a bond formed between any two elements

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20
Q

What are homonuclear diatomic species?

A

examples of molecules containing nonpolar covalent bonds, because the atoms in the bond have the exact same electronegativity

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21
Q

What is the polarity of heteronuclear covalent bonds?

A

polar to varying degrees

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22
Q

What is a polar covalent bond?

A

electrons are polarized towards the atom with a greater electronegativity, so the electrons are shared unequally

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23
Q

What is bond polarity used for?

A

to understand molecular properties and chemical reactivity

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24
Q

What bonds may be ionic?

A

bonds between atoms with very different electronegativity values

ie. NaCl (ΔX = 2.2)

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25
Q

Describe the periodic trend of ionization energy, electron affinity, and electronegativity.

A

increases across a period and decreases down a group

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26
Q

Describe the periodic trend of atomic radius.

A

decreases across a period and increases down a group

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27
Q

What happens when an ionic compound is melted? Covalent compound?

A

ionic: cations and ions move freely past one another, and the resulting liquid conducts electronegativity
covalent: resulting liquid does not conduct electricity

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28
Q

What are ions able to do?

A

whether as liquids or dissolved in solution, they’re able to carry charge from one place to another

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29
Q

What does the motion of charged ions result in?

A

an electric current

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30
Q

What do scientists use electrical conductivity for?

A

as evidence that a structure contains ionic bonds

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31
Q

What are salts?

A

ionic compounds are commonly referred to as salts

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32
Q

What is a redox reaction?

A

‘reduction-oxidation’ reaction

the total number of electrons lost must equal to the total number of electrons gained

ie. one electron is transferred from the alkali metal atom to the halogen atom, thus the alkali metal is oxidized and the halogen is reduced

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33
Q

What is a bond pair?

A

pair of electrons in which one electron comes from each atom of a covalent bond

represented by a solid line between the symbols of the two atoms involved in the bond

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34
Q

What are lone pairs?

A

valence electrons that do not participate in covalent bonds

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35
Q

What is a Lewis structure?

A

type of chemical representation that uses lines as bond pairs and pairs of dots for lone pairs

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36
Q

Why will elements form covalent bonds?

A

in general, to achieve a closed shell

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37
Q

What is a closed shell?

A

when an element achieves the same number of electrons in its valence shell as the noble gas within the same row on the periodic table

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38
Q

What is a valence shell?

A

the outermost shell

39
Q

What is a closed shell for p-block elements?

A

eight electrons, an OCTET

40
Q

What is a closed shell for H2?

A

two electrons, a DUET

41
Q

What is bond order?

A

number of bond pairs between two atoms

42
Q

What is bond order useful for?

A

indicating how closely and tightly two atoms are held together

43
Q

What is bond length?

A

distance between the two nuclei participating in the bond

44
Q

What is bond dissociation energy?

A

determines the strength of a bond, it is an experimental value describing how much energy needs to be added to break a particular bond

45
Q

What is the difference between homonuclear and heteronuclear diatomic molecules

A

homonuclear: electrons are shared equally between the atoms
heteronuclear: electrons are not shared equally

46
Q

What gives rise to a bond dipole?

A

in polar covalent bonds, electrons are polarized towards the atom with a greater electronegativity

47
Q

Writing partial positive and negative charges in Lewis structures.

A

S+ (delta plus)
S- (delta minus)

S+ S-
H — Cl
–|——-> (arrow with crossed tail is sometimes included to indicate the direction of the bond dipole, and the arrow points towards excess negative charge)

48
Q

What is percent ionic character?

A

ratio of actual bond dipole (measured) to the bond dipole if the bond pair was purely ionic

49
Q

How do you calculate percent ionic character?

A

measured dipole moment of X-Y
percent ionic character = —————————————- x100
calculated dipole moment of X+Y-

50
Q

Why is the specific arrangement of atoms and bonds in a molecule important?

A

it allows us to predict the physical properties and chemical reactivity of different chemical compounds

51
Q

What does ‘electron deficient’ mean?

A

compounds in which an atom has an incomplete octet is referred to as ‘electron deficient’

52
Q

What is an expanded octet?

A

species containing elements of the third and later periods of the p-block sometimes have more than an octet of electrons

53
Q

What is hypervalent?

A

central atoms in species with an expanded octet

54
Q

What are ‘free radicals’?

A

species that contain unpaired electrons

55
Q

What are radical species important for?

A
  • roles in many chemical reactions
  • messenger molecules in living systems
  • reactive species in atmospheric chemistry
56
Q

What can nitric oxide do?

A

due to its small size, it can easily penetrate cell walls and triggers relaxation of vascular smooth muscle tissue, this causes blood vessel dilation and thus increases blood flow

57
Q

How is formal charged determined?

A

by imagining bonding electrons are shared exactly equally between atoms

58
Q

When will a bonded atom have a negative formal charge? Positive formal charge?

A

negative: bonded atom has a greater number of outer shell electrons than its free form
positive: bonded atom has fewer electrons than its free form

59
Q

What difference is formal charge used to keep track of?

A

number of electrons surrounding an atom in a particular Lewis structure may differ from the number of valence electrons in an unbonded atom

60
Q

How do you calculate and indicate the formal charge for an atom in a Lewis structure?

A
  1. determine number of valence electrons in neutral unbonded atom
  2. count outer shell electrons surrounding the bonded atom in the compound or ion
  3. formal charge = #1 - #2
  4. label the formal charges on the Lewis structure by:
    - show only non-zero formal charges
    - non-zero formal charges must be indicated by placing the formal charge within a CIRCLE next to the appropriate term
    - indicate a formal charge of +1 as +, -1 as -
    - indicate a formal charge of +2 as 2+, -2 as 2-
61
Q

Why are formal charges always circled?

A

helps to distinguish formal charges from overall charges of ions and from oxidation states

62
Q

What is a ‘valid’ resonance structure?

A

any structure where the octet rule is satisfied on all atoms (other than known exceptions)

63
Q

What is the ‘best’ resonance structure?

A

structure that has minimal formal charges and has negative formal charges on more electronegative elements, and positive formal charges on less electronegative atoms

64
Q

What is a ‘chemically reasonable’ structure?

A

any valid resonance structure where the magnitudes of the formal charges are small (but not necessarily minimized)

65
Q

What do chemically reasonable structures represent?

A

bonding that is typical of various elements

ie. fluorine never participates in double bonds

66
Q

What is a diatomic species?

A

molecules or molecular ions that consist of two atoms

67
Q

What is a polyatomic species?

A

chemical species that consists of more than two atoms

68
Q

How do you draw Lewis structures for simple species?

A
  1. arrange atom symbols
  2. add valence electrons
  3. draw single bonds
  4. adjust for cations and anions
  5. form additional bonds from unpaired electrons
  6. rearrange electrons for octets
  7. redraw
  8. label formal charges
  9. check electron count
  10. decide on the best structure
69
Q

What is a central atom?

A

bonded to two or more atoms, generally the atom with the lowest electronegativity

70
Q

How do you adjust for cations and anions in a Lewis structure?

A
  • cations: remove one or more electrons from the central atom
  • anions: add one or more electrons to the most electronegative atoms that have incomplete octets (if you can’t add the extra electrons to the terminal atoms, add it to the central atom
71
Q

What is a resonance structure?

A

each individual structure when two or more valid Lewis structures can represent the arrangement of electrons in the same molecule or molecular ion

72
Q

What is identical in resonance structures?

A

the positions and connectivity of the atomic nuclei

73
Q

What is different in resonance structures?

A

the arrangement of electrons

74
Q

Although resonance structures are drawn as separate structures, what is the real structure?

A

a weighted average of the set of resonance structures

75
Q

What is the difference between an isomer and a resonance structure?

A

isomer: chemical species that have the same molecular formula and a different arrangement of atoms
resonance structures: differ in the placement of electrons, and not in the arrangement of atoms

76
Q

What is a hypervalent species?

A

has a central atom with more electrons in its valence shell than predicted by the octet rule

they have an “expanded octet”

TERMINAL ATOMS CANNOT HAVE EXPANDED OCTETS

77
Q

How are hypervalent species best represented?

A

by a set of resonance structures instead of a single Lewis diagram

78
Q

What is the chemical rationale?

Characteristic:
the number of valence electrons for all the atoms in the species, taking into account overall charge, must be shown as lines or dots in the Lewis structure

A

when atoms bond to form molecules or ions, the total number of electrons is conserved

valence electrons are involved in bonding and in chemical reactions, therefore they are the only ones shown in the Lewis structure

79
Q

What is the chemical rationale?

Characteristic:
all electrons are assigned, either as bond pairs or lone pairs on each atom in a species

A

this is the consequence of quantum mechanics (ch 4-5)

80
Q

What is the chemical rationale?

Characteristic:
in a Lewis structure, atoms typically are surrounded by an octet of electrons

A

an octet of electrons results in the same number of valence electrons as the noble gas in the same row as the periodic table

81
Q

What is the chemical rationale?

Characteristic:
when two or more Lewis structures are possible, the best one will have the minimum formal charge

A

the creation of formal charge requires a redistribution of electrons from their atomic states

82
Q

What is the chemical rationale?

Characteristic:
when two or more lewis structures are possible, and non-zero formal charges are necessary, the best structure will have the negative formal charge on the most electronegative atom and the positive formal charge on the least electronegative atom

A

since more electronegative elements have a greater ability to attract electrons within a covalent bond, they also have a greater ability to attract additional electrons and take on a negative formal charge

the opposite is true for less electronegative elements

83
Q

What is the chemical rationale?

Characteristic:
when two or more resonance forms are possible, the “actual” molecular structure is considered to be a hybrid of these

A

lewis structures are limited in a way that we may not be able to draw a single structure that closely represents the “actual” valence electron distribution within the species

84
Q

What is oxidation state?

A

a measure of the degree of oxidation (electron loss) for a chemical species

85
Q

What is oxidation state for ionic compounds?

A

the same as the charge on the ionic form

86
Q

What is oxidation state for covalent compounds?

A

the charge an atom would have if both electrons in a bond were “given” to the more electronegative element

87
Q

What is the sum of oxidation states of the atoms in a neutral molecule?

A

zero

88
Q

What is the sum of oxidation states of the constituent atoms of an atom?

A

must be equal to the charge on the ion

89
Q

What are the 3 rules for determining oxidation state?

A
  1. oxidation state of a free element (uncombined element) is zero
  2. for a simple (monatomic) ion, the oxidation state is equal to the net charge on the ion
  3. H has an oxidation state of +1 and O has an oxidation state of -2 in most compounds
    - exceptions: hydrogen has an oxidation state of -1 in hydrides of active metals (ie. LiH) and oxygen has an oxidation state of -1 in peroxides (ie. H2O2)
90
Q

What is similar between formal charge and oxidation state?

A
  • they are both forms of “electron bookkeeping”

- neither adequately describes the charge distribution in most covalently bonded molecules

91
Q

Describe formal charge compared to oxidation state?

A

formal charge is the charge an atom would have if the electrons in a covalent bond are considered to be shared equally

in covalently bonded species, oxidation state can be considered the charge an atom would have if both electrons in a bond were “given” to the more electronegative atom

92
Q

What are partial charges?

A

the most representative of the actual charge distribution in a bond

in reality, atoms in polar covalent bonds do not share the bonding electrons equally due to their different electronegativity values

93
Q

What is the Bronsted-Lowry acid-base theory?

A
  • acid is a proton (H+) donor

- base is a proton acceptor

+ CHEM 121 (5 decks)

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