Ch 2: Chemical Bonding and Lewis Structures Flashcards
What are compounds?
- chemicals that are combinations of two or more different elements in defined proportions
- no overall charge, “neutral”
What are ions?
species possessing either an overall positive or negative charge
What are cations?
ions carrying an overall positive charge
What are anions?
ions carrying an overall negative charge
What are simple ions?
ions that contain a single atom
What are complex/polyatomic/molecular ions?
ions that contain multiple atoms
Why are cations/anions isolable?
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
What holds atoms in compounds together?
chemical bonds
What do chemical bonds consist of?
electrons that simultaneously feel the electrostatic forces of attraction for both of the positively charged nuclei of adjacent atoms
What do elements achieve by forming bonds?
elements achieve a lower overall energy than they would in their free elemental form
What is an ionic bond?
oppositely charged ions held together by electrostatic forces
What is a covalent bond?
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
What is a metallic bond?
electrons are shared between many atoms simultaneously and are free to “flow” between the atoms
Where is the most important type of bond, the metallic bond?
in elemental metals or metallic alloys
What are metallic alloys?
mixtures of two or more elemental metals
What determines whether a bond is covalent or ionic?
the electronegativity difference of the bonding atoms
What is electronegativity?
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
How is electronegativity measured?
it cannot be directly measures, it must be calculated from other molecular properties
What may electronegativity values be used for?
to predict the nature of a bond formed between any two elements
What are homonuclear diatomic species?
examples of molecules containing nonpolar covalent bonds, because the atoms in the bond have the exact same electronegativity
What is the polarity of heteronuclear covalent bonds?
polar to varying degrees
What is a polar covalent bond?
electrons are polarized towards the atom with a greater electronegativity, so the electrons are shared unequally
What is bond polarity used for?
to understand molecular properties and chemical reactivity
What bonds may be ionic?
bonds between atoms with very different electronegativity values
ie. NaCl (ΔX = 2.2)
Describe the periodic trend of ionization energy, electron affinity, and electronegativity.
increases across a period and decreases down a group
Describe the periodic trend of atomic radius.
decreases across a period and increases down a group
What happens when an ionic compound is melted? Covalent compound?
ionic: cations and ions move freely past one another, and the resulting liquid conducts electronegativity
covalent: resulting liquid does not conduct electricity
What are ions able to do?
whether as liquids or dissolved in solution, they’re able to carry charge from one place to another
What does the motion of charged ions result in?
an electric current
What do scientists use electrical conductivity for?
as evidence that a structure contains ionic bonds
What are salts?
ionic compounds are commonly referred to as salts
What is a redox reaction?
‘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
What is a bond pair?
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
What are lone pairs?
valence electrons that do not participate in covalent bonds
What is a Lewis structure?
type of chemical representation that uses lines as bond pairs and pairs of dots for lone pairs
Why will elements form covalent bonds?
in general, to achieve a closed shell
What is a closed shell?
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
What is a valence shell?
the outermost shell
What is a closed shell for p-block elements?
eight electrons, an OCTET
What is a closed shell for H2?
two electrons, a DUET
What is bond order?
number of bond pairs between two atoms
What is bond order useful for?
indicating how closely and tightly two atoms are held together
What is bond length?
distance between the two nuclei participating in the bond
What is bond dissociation energy?
determines the strength of a bond, it is an experimental value describing how much energy needs to be added to break a particular bond
What is the difference between homonuclear and heteronuclear diatomic molecules
homonuclear: electrons are shared equally between the atoms
heteronuclear: electrons are not shared equally
What gives rise to a bond dipole?
in polar covalent bonds, electrons are polarized towards the atom with a greater electronegativity
Writing partial positive and negative charges in Lewis structures.
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)
What is percent ionic character?
ratio of actual bond dipole (measured) to the bond dipole if the bond pair was purely ionic
How do you calculate percent ionic character?
measured dipole moment of X-Y
percent ionic character = —————————————- x100
calculated dipole moment of X+Y-
Why is the specific arrangement of atoms and bonds in a molecule important?
it allows us to predict the physical properties and chemical reactivity of different chemical compounds
What does ‘electron deficient’ mean?
compounds in which an atom has an incomplete octet is referred to as ‘electron deficient’
What is an expanded octet?
species containing elements of the third and later periods of the p-block sometimes have more than an octet of electrons
What is hypervalent?
central atoms in species with an expanded octet
What are ‘free radicals’?
species that contain unpaired electrons
What are radical species important for?
- roles in many chemical reactions
- messenger molecules in living systems
- reactive species in atmospheric chemistry
What can nitric oxide do?
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
How is formal charged determined?
by imagining bonding electrons are shared exactly equally between atoms
When will a bonded atom have a negative formal charge? Positive formal charge?
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
What difference is formal charge used to keep track of?
number of electrons surrounding an atom in a particular Lewis structure may differ from the number of valence electrons in an unbonded atom
How do you calculate and indicate the formal charge for an atom in a Lewis structure?
- determine number of valence electrons in neutral unbonded atom
- count outer shell electrons surrounding the bonded atom in the compound or ion
- formal charge = #1 - #2
- 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-
Why are formal charges always circled?
helps to distinguish formal charges from overall charges of ions and from oxidation states
What is a ‘valid’ resonance structure?
any structure where the octet rule is satisfied on all atoms (other than known exceptions)
What is the ‘best’ resonance structure?
structure that has minimal formal charges and has negative formal charges on more electronegative elements, and positive formal charges on less electronegative atoms
What is a ‘chemically reasonable’ structure?
any valid resonance structure where the magnitudes of the formal charges are small (but not necessarily minimized)
What do chemically reasonable structures represent?
bonding that is typical of various elements
ie. fluorine never participates in double bonds
What is a diatomic species?
molecules or molecular ions that consist of two atoms
What is a polyatomic species?
chemical species that consists of more than two atoms
How do you draw Lewis structures for simple species?
- arrange atom symbols
- add valence electrons
- draw single bonds
- adjust for cations and anions
- form additional bonds from unpaired electrons
- rearrange electrons for octets
- redraw
- label formal charges
- check electron count
- decide on the best structure
What is a central atom?
bonded to two or more atoms, generally the atom with the lowest electronegativity
How do you adjust for cations and anions in a Lewis structure?
- 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
What is a resonance structure?
each individual structure when two or more valid Lewis structures can represent the arrangement of electrons in the same molecule or molecular ion
What is identical in resonance structures?
the positions and connectivity of the atomic nuclei
What is different in resonance structures?
the arrangement of electrons
Although resonance structures are drawn as separate structures, what is the real structure?
a weighted average of the set of resonance structures
What is the difference between an isomer and a resonance structure?
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
What is a hypervalent species?
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
How are hypervalent species best represented?
by a set of resonance structures instead of a single Lewis diagram
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
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
What is the chemical rationale?
Characteristic:
all electrons are assigned, either as bond pairs or lone pairs on each atom in a species
this is the consequence of quantum mechanics (ch 4-5)
What is the chemical rationale?
Characteristic:
in a Lewis structure, atoms typically are surrounded by an octet of electrons
an octet of electrons results in the same number of valence electrons as the noble gas in the same row as the periodic table
What is the chemical rationale?
Characteristic:
when two or more Lewis structures are possible, the best one will have the minimum formal charge
the creation of formal charge requires a redistribution of electrons from their atomic states
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
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
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
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
What is oxidation state?
a measure of the degree of oxidation (electron loss) for a chemical species
What is oxidation state for ionic compounds?
the same as the charge on the ionic form
What is oxidation state for covalent compounds?
the charge an atom would have if both electrons in a bond were “given” to the more electronegative element
What is the sum of oxidation states of the atoms in a neutral molecule?
zero
What is the sum of oxidation states of the constituent atoms of an atom?
must be equal to the charge on the ion
What are the 3 rules for determining oxidation state?
- oxidation state of a free element (uncombined element) is zero
- for a simple (monatomic) ion, the oxidation state is equal to the net charge on the ion
- 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)
What is similar between formal charge and oxidation state?
- they are both forms of “electron bookkeeping”
- neither adequately describes the charge distribution in most covalently bonded molecules
Describe formal charge compared to oxidation state?
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
What are partial charges?
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
What is the Bronsted-Lowry acid-base theory?
- acid is a proton (H+) donor
- base is a proton acceptor
