Lesson 3.2: Lewis Structures
*the steps in drawing a Lewis structure:* if the general formula for the compound is "AXn," the central atom, _____, will be the one with the lower _____ (and usually the lower _____)
"A", group number, electronegativity
*how many total valence electrons are present in the following molecules?* NF₃
26
*how many total valence electrons are present in the following molecules?* SF₆
48
*how many total valence electrons are present in the following molecules?* H₂Se
8
give the formal charge of each atom in an HF molecule
H = 1 - (0 + 1) = 0 F = 7 - (6 + 1) = 0
dihydrogen monosulfide gas smells like rotten eggs with the formula, H₂S. explain why the Lewis structure skeleton cannot be S-H-H
H has only one valence electron that it can share in a single covalent bond do it can only form one bond
N₂O has three resonance forms whose structures are given below. use formal charges to identify which is the dominant form, A, B, or C: N//N//O N///N/O N/N///O
N///N/O
carbon dioxide has two possible structures that obey the octet rule. draw the Lewis structures and determine which is the most likely by calculating formal charges.
O//C//O
the greater _____ between the atoms in a multiple bond also brings the atoms closer together, so double and triple bonds are _____ than single bonds
attraction, shorter
double and triple bonds involve more _____ the single bonds as so are _____
attractive forces, stronger
for nonmetals, Lewis structures representing the anions include _____ pairs of dots surrounding the element's symbol enclosed in _____ with the negative charge written as a superscript to the right and outside the _____
four, brackets, brackets
molecules that have Lewis structures with unpaired electrons
free radicals
unpaired electrons, VERY reactive
free radicals
representations of atoms of elements based on their number of valence electrons
electron dot diagram/Lewis structure
_____ get brackets and charge
polyatomic ions
*electron dot diagram* NCl₃
-the molecule NCl₃ has three Cl atoms with 7 valence electrons each -the N atom has 5 electrons for a total of 26 valence electrons
*electrons remaining that need to be arranged* NCl₃
in NCl₃ 26 - 6 = 20 valence electrons remaining that need to be arranged in the structure
*the central atom* NCl₃
in NCl₃, N is in Group 15 while Cl is in Group 17, so N is the central atom
what makes free radical reactive?
its single electron
small or zero formal charges on individual atoms are _____ than larger ones
better
the amount of energy required to break a mole of these bonds is called _____ and is measured in _____
bond energy, kJ/mol
sometimes fewer electros are available than needed to complete the octets of the atoms in a molecule. the most important of these is _____ which forms compounds with only _____ around B, rather than _____
boron, 6, 8 ex: BF₃ lacks an octe for B
an expanded octet is only possible for an atom when _____ electrons are available, so they are only possible for elements in the _____ period or _____ periods
d orbital, third, higher
expanded octet only for elements with _____
d sublevels
electrons that are not associated with any one pair of bonded atoms, but are spread out equally between the three pairs of atoms
delocalized
_____ are depicted by showing the number of possible structures and connecting each structure with a _____. the diagrams are called ____.
delocalized electrons, double-headed arrow; resonance structures
two free radicals that combine
dimers
free radicals may form _____
dimers (N₂O₄)
when a molecule or ion contains _____ next to _____, there are often _____
double bonds, single bonds, several possible structures that we can draw that are all correct
two or three pairs of electrons may also be shared between certain atoms to form _____
double or triple bonds
for nonmetals, atoms need to gain enough electrons to have _____ (_____) when forming an anion OR the number of electrons the atoms needs to _____
eight dots (stable octet), share when forming covalent bonds
*the steps in drawing a Lewis structure:* count the total number of _____ in the bonds and the non-bonding electrons
electrons
*the steps in drawing a Lewis structure:* make sure that the number of electrons used in the structure matches the number of _____ counted in step one. if the numbers don't match the structure probably as _____
electrons; double or triple bonds,
*the steps in drawing a Lewis structure:* although there are a few exceptions, most molecules and polyatomic ions have an _____ number of valence electrons
even
there are situations where more electrons are available than needed. this often results in _____ associated with the _____ in the Lewis structure
expanded octets, central atom
a ficticios charge that ignores electronegativity
formal charge
the charge than an atom would have if all bonding electrons are shared equally between the bonded atoms
formal charge
lower _____/lower _____ is the central atom
group number, electronegativity
dimers are _____ reactive than free radicals
less
if you get the same numbers for formal charge in both structures, the more negative charge is assigned to the _____
more electronegative element
with carbon, there is sometimes _____ central atom(s)
more than one
when formal charge cannot be avoided, _____ should reside on the most electronegative atom
negative formal charge
for metals in Groups 1, 2, and 3, the Lewis structure for the stable octet has _____ dots
no
many polyatomic ions contain _____ atoms contently bonded to each other
nonmetal
what does the element's symbol in a Lewis structure represent?
nucleus and core electrons
the element's symbol represents the _____ and _____ and dots arranged around the symbol represent the _____
nucleus, core electrons, valence electrons
1/2 number of bonding electrons =
number of bonds
formal charge =
number of valence electrons - (number of nonbonding electrons + 1/2 number of bonding electrons)
some compounds have formulas in which the total number of valence electrons is an odd number. in such cases, it is impossible to construct a Lewis structure with an _____ around each atom
octet ex: NO₂
*the steps in drawing a Lewis structure:* assume that all of the atoms in the molecule obey the _____ (except _____)
octet rule, hydrogen
halogen can only bond in _____ direction (no _____ bonds)
one, double or triple
*the steps in drawing a Lewis structure:* F atoms always form _____ bond(s), as do other _____ most of the time. exceptions occur when these _____ are central atoms bonded to other smaller _____ or to _____
one, halogens, halogens, halogen, oxygen
*the steps in drawing a Lewis structure:* all bonding electrons are _____
paired
an unpaired electron makes the molecules very _____
reactive
several possible correct structures
resonance structures
*the steps in drawing a Lewis structure:* avoid creating _____ or _____ when drawing Lewis structures
rings, cyclic structures
*Lewis structure* XeF₄
see 3.2 study guide
*the structure* NCl₃
see 3.2 study guide
*Lewis structure* SO₄²⁻
see 3.2 study guide, "Lewis Structures for Polyatomic Ions"
*Lewis structure* CO₃²⁻
see 3.2 study guide, "Resonance"
*draw the Lewis structure for the following polyatomic ions:* CrO₄²⁻
see 3.2 study guide, #11
*draw the Lewis structure for the following polyatomic ions:* C₂H₃O₂¹⁻
see 3.2 study guide, #11
*draw the Lewis structure for the following polyatomic ions:* NH₄¹⁺
see 3.2 study guide, #11
*draw the Lewis structure for the following polyatomic ions:* SO₄²⁻
see 3.2 study guide, #11
*draw Lewis structures for the following molecules or ions* ICl₄¹⁻
see 3.2 study guide, #12
*draw Lewis structures for the following molecules or ions* SeBr₄
see 3.2 study guide, #12
*draw Lewis structures for the following* CS₂
see 3.2 study guide, #13
*draw Lewis structures for the following* HCN
see 3.2 study guide, #13
*draw Lewis structures for the following* O₃
see 3.2 study guide, #13
write the Lewis structures for the main group elements in period 4
see 3.2 study guide, #4
*write the Lewis structure for the following atom and its ion* Al
see 3.2 study guide, #5
*write the Lewis structure for the following atom and its ion* Ba
see 3.2 study guide, #5
*write the Lewis structure for the following atom and its ion* Bi
see 3.2 study guide, #5
*write the Lewis structure for the following atom and its ion* I
see 3.2 study guide, #5
*draw the Lewis structure for each of the following molecule:* CCl₄
see 3.2 study guide, #8
*draw the Lewis structure for each of the following molecule:* H₂O
see 3.2 study guide, #8
*draw the Lewis structure for each of the following molecule:* NH₃
see 3.2 study guide, #8
*draw the Lewis structure for each of the following molecule:* OF₂
see 3.2 study guide, #8
*draw the Lewis structure of a molecule for:* bromine trifluoride, BrF₃
see 3.2 study guide, #9
*draw the Lewis structure of a molecule for:* carbon monoxide, CO
see 3.2 study guide, #9
*draw the Lewis structure of a molecule for:* formaldehyde, CH₂O
see 3.2 study guide, #9
*draw the Lewis structure of a molecule for:* hydrogen cyanide, HCN
see 3.2 study guide, #9
one pair of valence electrons shared between two atoms is a _____
single covaent bond
*the steps in drawing a Lewis structure:* construct the skeleton of te molecule using lines to indicate _____ between atoms using the guidelines which apply to most molecules and polyatomic ions
single covalent bonds
formal charge is used to distinguish between competing _____ or competing _____
skeletal structures, resonance structures
the elements symbol is placed in _____ with the _____ written to the right as a superscript outside the _____
square brackets, cation's charge, brackets
some of all formal charges in a polyatomic ion = _____
the charge of the ion
the magnitude of the positive charge is equal to _____
the number of dots (electrons) that were lost by the neutral atom
the magnitude of the negative charge is equal to _____
the number of electrons gained by the neutral atom in order to have four pairs
for metals, the number of dots represents _____
the number of electrons lost when forming a cation
what do the number of dots in main group metals tell us about the charges of the ions formed by these metals?
the number of electrons that the metal loses when it form an ion, so the magnitude of the positive charge
each element in a group has _____ number of _____ so the Lewis structure for each element in the same group has ____ number of _____
the same, valence electrons, the same, dots
the single line between the atoms represents _____ in a _____ bond. the other pairs of electrons are _____
the shared pair of electrons, covalent, non-bonding or lone pairs
*the steps in drawing a Lewis structure:* nitrogen atoms form _____ bonds and carbon atoms form _____ bonds
three, four
*the steps in drawing a Lewis structure:* oxygen atoms normally form _____ bonds and don't often bond to _____ in compounds with other elements (except _____--_____)
two, each other, hydrogen peroxide, H₂O₂
*the steps in drawing a Lewis structure:* a Lewis structure must account for all of the _____ in the atoms that make up the molecule
valence electrons
*the steps in drawing a Lewis structure:* determine the total number of _____ in the molecule
valence electrons
what do the dots in a Lewis structure represent?
valence electrons
*the steps in drawing a Lewis structure:* arrange the remaining _____ to satisfy the _____ for all atoms
valence electrons, octet rule
*the steps in drawing a Lewis structure:* subtract the number of _____ used for bonding in the skeleton. each lines represents _____
valence electrons, two electrons
*how many total valence electrons are present in the following molecules?* CCl₄
32
for any main group (representative) element we can write Lewis structures using the following steps:
-write the element symbol -determine the number of valence electrons -imagine a square around the element symbol. place one dot for each valence electron on each side of the square, starting with the side of the square to the right of the element's symbol. proceed clockwise around the square. -if more than four valence electrons are present, begin pairing the dots only after all four sides of the square have one dot (remember Hund's rule: electrons get their own orbital and only pair up when each orbital in a sublevel has one electron).
some of all formal charges = _____ in a neutral molecule
0
*how many electrons would be needed by the following nonmetals elements to acquire a stable octet?* F
1
*how many electrons would be needed by the following nonmetals elements to acquire a stable octet?* O
2
*how many electrons would be needed by the following nonmetals elements to acquire a stable octet?* N
3
which molecule would you expect to be a free radical? CO CO₂ N₂O NO
NO
*three possible Lewis structures are shown for the thiocyanate ion below.* *a. determine the formal charges in each structure* *b. based on the formal charges, which Lewis structure is the dominant one?* [N/C///S]¹⁻ [N//C//S]¹⁻ [N///C/S]¹⁻
[N//C//S]¹⁻
the sulfate ion (SO₄²⁻) has two resonance structures shown below. based on the formal charges, which one is the most dominant? a. four single bonds b. two single bonds, two double bonds
b
