4
What rules must be applied for the concept of formal charge?
1. The sum of all formal charges in a neutral molecule must be zero 2. The sum of all formal charges in an ion must equal the charge of the ion 3. Small (or zero) formal charges on individual atoms are better than large ones 4. When formal charge cannot be avoided, negative formal charge should reside on the most electronegative atom
What is the first step in writing Lewis structures for molecular compounds?
1. Write the correct skeletal structure for the molecule The atoms must be in the correct position.
Write the Lewis structure for CO?
211
Write the Lewis structure for NH3?
211
Write the Lewis structure for the hypochlorite ion ClO-
212
Write the Lewis Structure for the NO2- ion. Include the resonance structures.
214
Write the Lewis structure for the NO3- ion. Include resonance structures.
214
Assign formal charges to each atom in the resonance forms of the cyanate ion (OCN-).
216
What is the number of moles of Cl in 38.5 mol of CCl2F2?
38.5 mol CCl2F2 x 2 mol Cl/ 1 mol CCl2F2= 77.0 mol Cl
Write the Lewis Structure for NH4+
5+ 4(1) -1 =8 212
What is true regarding ionic compound formulas? 3
-Ionic compounds always contain positive and negative ions -In a chemical formula, the sum of the charges of the positive ions (cations) must equal the sum of the negations ions (anions) -The formula of an ionic compound reflects the smallest whole-number ratio of ions
How would we find the number of CO2 molecules in a sample of dry ice (solid CO2) with a mass of 10.8g?
-the molar mass of CO2 is a conversion factor between mass (in grams) and amount (in moles) 10.08 g CO2 x 1 mol CO2/44.01 g CO2 x 6.022x10^23/1 mol CO2= 1.48 x 10^23
What mass (in grams) of iron (III) oxide contains 58.7g of iron? Iron (III) oxide is 69.94% iron by mass.
...
What are the formal charges of hydrogen and fluorine in HF?
...The calculated charges ignoring their differences in electronegativity are both zero
What is delocalization?
In the Lewis model, electrons are localized either on one atom (lone pair) or between atoms (bonding pair). Yet, in nature, electrons in molecules are often delocalized over several atoms or bonds. The delocalization of electrons lowers their energy; it stabilizes them.
A chemical formula, in combination with the molar masses of its constituent elements?
Indicates the relative quantities of each element in a compound, which is useful info
Why is formal charge useful?
It can help to distinguish between competing skeletal structures or competing resonance structures
Why is He unique?
It contains two dots, a duet, represents a stable electron configuration because n=1 quantum level fills with only two electrons
What is true regarding the chemical formula?
It gives us a relationship between the amounts (in moles) of substances, not between the masses (in grams) of them
What is important to know regarding hydrogen atoms when writing Lewis structures for molecular compounds?
It is important to remember that hydrogen atoms are always TERMINAL. Central atoms must form at least two bonds, hydrogen has only 1 valence electron to share and requires only a duet EX, for water HOH
Why most the ionic compound composed of K and F must be KF?
K always forms 1+ cations and F always forms 1- anions. In order for the compound to be charge neutral, it must contain one K+ cation to every on F- anion.
What happens the atoms K and Cl bond?
K gives transfers its valence electron to Chlorine. The transfer of electron gives chlorine an octet and leaves K without any valence electrons but with an octet in the previous principal energy level K- cation Cl-anion the positive and negative charges attract each other
What is the mass in grams of Cl contained in 25g CCl2F2?
The relationship inherent in the chemical formula (2 mol Cl : 1 mol CCl2F2) 25g CCl2F2 x 1 mol CCl2F2/120.91 g x 2 mol Cl/1 mol 1 mol CCl2F2 x 35.45 g Cl/ 1 mol Cl = 14.7 g 184
What is resonance stabilization?
The resulting stabilization of the electrons--that is lowering of their potential energy due to delocalization. This makes an important contribution to the stability of many molecules
What is the general form for solving problems where we are asked to find the mass of an element present in a given mass of a compound?
mass compound -> moles compound -> moles element -> mass element
What is the formula we use to calculate the mass percent of element X?
mass of element X in 1 mol of compound/mass of 1 mol of the compound x 100%
What is resonance structure?
one of two or more Lewis structures shown with double-headed arrows between them to indicate that the actual structure of the molecule is intermediate between them. One or two or more Lewis structures that have the same skeletal formula (the atoms are in the same locations) but different electron arrangements
If someone consumed 22 g of sodium chloride per day, what mass (in grams) of sodium does that person consume? Sodium chloride is 39% sodium by mass?
pg 184
What is molar mass?
the mass in grams of one mole of it atoms, numerically equivalent to its atomic mass
What is a formula unit?
the smallest, electrically neutral collection of ions in an ionic compound the basic unit of an ionic compound part of a larger lattice NaCl- formula unit is composed of NA+ and Cl- ions 166
What is the octet rule?
the tendency for most bonded atoms to possess or share eight valence in their outer shell in order to obtain stable electron configurations and lower their potential energy
How would we write the Lewis Structure for CO2.
1. carbon is the less electronegative atom, so it gets the central position O C O 2. Take the sum of the valence electrons of each atom in the molecule 4+2(6)= 16 3. Distribute the electrons among the atoms, giving octets (duets for H). Start with the bonding electrons, then proceed to lone pairs on terminal atoms and then to lone pairs on the central atom 4. If any atom lacks an octet, form double or triple bonds 5. Check
Calculate the mass of Cl in 1.00 kg CCl2F2?
1.00kg CCl2F2 x 1000g/1 kg x 58.64 g Cl/ 100 g CCl2 = 5.86 x 10^2
The U.S. Food and Drug Administration (FDA) recommends that a person consume less than 2.4 g of sodium per day. What mass of sodium chloride (in grams) can you consume and still be within the FDA guidelines? Sodium chloride is 39% sodium by mass.
100 g NaCl/39 g Na 2.4g Na x 100g NaCl/39 g Na= 6.2g NaCl
What is the formula mass of carbon dioxide, CO2?
12.01 amu + 2(16.00 amu)= 44.01 amu
What is the mass of a sample of water containing 3.55 x 10^22 H20 molecules?
181
Hydrogen may replace gasoline as fuel in the future. Most major automobile companies are developing vehicles that run on hydrogen. These cars are environmentally friendly because their only emission is water vapor. One way to obtain hydrogen for fuel is to use an emission-free energy source such as wind power to form elemental hydrogen from water. What mass id hydrogen (in grams) is contained in 1.00 gallon of water? (the density of water is 1.00 g/mL)
185
Butane (C4H10) is the liquid fuel in lighters. How many grams if carbon are present within a lighter containing 7.25 mL butane? (The density of liquid butane is 0.601 g/mL)
186
Determine the mass of oxygen in a 7.2 g sample of Al2 (SO4)2
186
What is the mass percent composition of Cl in chlorofluorocarbon CCl2F2?
2 x molar mass Cl/ molar mass CCl2F2 x 100% -We multiple the molar mass by two because the chemical formula has a subscript of 2 for Cl, indicating that 1 mol of CCl2F2 contains 2 mol Cl atoms -Molar mass of CCl2F2: 12 g/mol + 2(35 g/mol) + 2 (19.00 g/mol)= 120.91 g/mol 2 x 35.45 g.mol/120.91 g/mol x 100% =58.64%
What is a triple bond?
A bond that forms when three electron pairs are shared between two atoms. Triple bonds are even shorter and stronger than double bonds. Ex. Nitrogen, N2 relatively unreactive molecule, difficult to break
What is formal charge?
A fictitious charge assigned to each atom in a Lewis structure that helps to distinguish among competing Lewis structures. the charge an atom in a Lewis Structure would have if all the bonding electrons were shared equally between the bonded atoms* Formal charge is the calculated charge for an atom in a molecule if we ignored electronegativity
What is an octet?
A lewis symbol with 8 dots, signifying a filled outer electron shell for s and p block elements. The atoms with 8 valence electrons are particularly stable because they have a full outer principal level
What is a structural formula?
A molecular formula that shows how the atoms in a molecule are connected or bonded to each other uses lines to represent covalent bonds and can also indicate a sense of the molecules geometry
What is a space-filling molecular model?
A representation of a molecule that shows how the atoms fill the space between them is scaled to visible size
What is the Lewis model?
A simple model of chemical bonding that uses diagrams to represent bonds between atoms as lines or pairs of dots. In this model, atoms bond together to obtain stable octets (eight valence electrons) Valence electrons are represented as dots (Lewis Structures)
What is a chemical formula?
A symbolic representation of a compound that indicates the elements present in the compound and the relative number of atoms of each.
What is the mass percent composition (mass percent)?
An element's percentage of the total mass of a compound containing the element A way to express how much of an element is in a given compound
Why does the Lewis model focus on valence electrons?
Because chemical bonding involves the transfer or sharing of valence electrons between two or more atoms
Chemical formulas contain within them inherent relationships such as?
Between atoms (or moles of atoms) and molecules (or moles of molecules). Ex. CCl2F2 tells us that I mol of CCl2F2 contains 2 mol of Cl atoms
In the Lewis model, two atoms may share more than one electron to get octets. How is this achieved?
By a double bond or triple bond Each oxygen atom now has an octet because the additional bonding pair counts toward the octet of both atoms
An aspirin tablet contains 325 mg of acetylsalicylic acid (C9H8O4). How many acetylsalicylic acid molecules does it contain?
C9H8O4 9(12)+ 8(1)+ 4(16)= 180.15 g/mol 325 mg C9H8O4 x 10^-3/1 mg x 1 mol C9H8O4 / 180.15 g x 6.022x10^23 C9H8O4 molecules/ 1 mol C9H8O4 =1.09 x 10^21 molecules
What is the second step in writing Lewis structures for molecular compounds?
Calculate the total number of electrons for the Lewis structure for the Lewis structure by summing the valence electrons of each atom in the molecule
What are ionic compounds compounds composed of?
Cations (usually a metal) and anions (usually on or more nonmetals)
What is the fifth step in writing Lewis structures for molecular compounds?
Check. Count the number of electrons in the Lewis structure and make sure it matches the number from step 2
What is the third step in writing Lewis structures for molecular compounds?
Distribute the electrons among the atoms, giving octets (or duets in the case of hydrogen) to as many atoms as possible. Start by placing two electrons between every two atoms, this represents the minimum number of bonding electrons. The remaining electrons should then be distributed as lone pairs, first to terminal atoms and then to the central atom
Calculate the formal charge for Hydrogen in HF, then for fluorine
H: 1-[0 + 1/2(2)]= 0 F: 7- [6+ 1/2(2)]= 0
What is meant by term hybrid?
Hybrid comes from breeding and means the offspring of two animals or plants of different varieties or breeds. Similarly, the actual structure of a resonance hybrid is intermediate between the two resonance structures. The only structure that actually exists is the hybrid structure. The individual resonance structures do not exists and are merely a convenient way to describe the actual structure.
What is the fourth step in writing Lewis structures for molecular compounds?
If any atoms lack an octet, form double or triple bonds as necessary to give them octets. This can be done by moving lone electron pairs from the terminal atoms into the bonding region with the central atom.
What is a covalent bond?
If electrons are shared as occurs between two nonmetals
What is an ionic bond?
If electrons are transferred as occurs between a metal and a nonmetal
What happens when we can write two or more valid Lewis structures for the same molecule?
In nature, the molecule exists as an average of the two Lewis structures
Why most the ionic compound composed of Mg and F must be MgF2?
Mg always forms 2+ cations and F always forms 1- anions. In order for this compund to be charge neutral it must contain one Mg2+ cation for every two F- anions.
In the previous calculation, it was found that the mass percent composition of Cl in CCl2F2 is 58.64% what does this mean?
Percent means per hundred, there are 58.64 grams Cl per hundred grams CCl2F2 58.64 g Cl/100 g CCl2F2
Refer to the table on page 216, which structure is the better Lewis structure?
Structure A The sum of the formal charges for each structure is zero The best skeletal structure usually has the least electronegative atom in the central position While both HCN and HNC exist, by assigning formal charges, HCN is more stable
Why is water H2O and and not H3O?
The Lewis model also accounts for why particular combinations of atoms form molecules and others do not. H2O should be stable, while H3O is not (unless we remove an electron)
How does the Lewis model account for why covalent bonds are highly directional?
The attraction between two covalently bonded atoms is due to the sharing of one or more electron pairs in the space between them. Each bond links just one specific pair of atoms---in contrasts to ionic bonds, which are nondirectional and hold together an entire array of ions
What is formula mass?
The average mass of a molecule of a compound in amu = (number of atoms of 1st element in chemical formula x atomic mass of 1st element) + (number of atoms of 2nd element in chemical formula x atomic mass of 2nd element) (aka molecular mass or molecular weight)
What is a double bond?
The bond that forms when two electron pairs are shared between two atoms In general double bonds are shorter and stronger than single bonds Ex. oxygen
What happens when our idealized ionic solid dissolves in water?
The cations and anions dissociate forming free ions in solution. These ions can move in response to electrical forces creating an electrical current. Ionic compounds conduct electricity
While writing Lewis structure for polyatomic ions, it is important to pay close attention to?
The charge of the ion when calculating the the number of electrons for the Lewis structure. Add one electron for each negative charge and substract one electron for each positive charge. For polyatomic ion, write the lewis structure with brackets, with the charge of the ion in the upper right-hand corner, outside the bracket.
What is important to remember if you are writing a Lewis structure for a polyatomic ion?
The charge of the polyatomic ion should be considered when calculating the total number of electrons. Add one electron for each negative charge and subtract one electron for each positive charge Which atom the electron came from should not be of concern, only the total number is important
What accounts for the stability of ionic substances?
The formation of a crystalline lattice as a result of the attractions between the cations and anions
Where can the number of valence electrons be found for main group elements?
The group number on the periodic table, is equal to the number of valence electrons
In covalently bonded molecular compounds, what is weaker, the interactions between molecules (intermolecular forces) or the bonding interactions within a molecule (intramolecular forces)?
The interactions between molecules (intermolecular forces) are generally much weaker than the bonding interactions within a molecule (intramolecular forces)
What is the molar mass of a compound?
The mass in grams of 1 mol of its molecules or formula units, numerically equivalent to its formula mass Ex. the formula mass of CO2 is 44.01 amu
What happens when a molecular compound melts or boils?
The molecules remain intact. Only the relatively weak interaction between molecules must be overcome ...as a result molecular compounds tend to have lower melting and boiling points than ionic compounds
Where should the more electronegative elements be, less electronegative?
The more electronegative elements should be in terminal positions and the less electronegative elements (Other than hydrogen) in the central position
In the Lewis model, a chemical bond is what?
The sharing or transfer of electrons to attain stable electron configurations for the bonding atoms.
In a Lewis symbol what is represented?
The valence electrons of the main-group elements are represented as dots surrounding the abbreviation for the element Ex. the electron configuration for O 1s2 2s2 2p4 --there are 6 valence electrons
What is true of a double bond compared to a single bond?
They are generally stronger and shorter than a single bond. Also, a double bond corresponds to two shared electron pairs, a single bond corresponds to one shared electron pair.
Why are bonding theories important in chemistry?
They explain how atoms bond together to form molecules. They explain why some combinations of atoms are stable and others are not. They also predict the shape of molecules.
What is true regarding nonequivalent resonance structures?
They may be weighted differently in their contributions to the true overall structure of a molecule
Why do we use the molar mass in combination with Avogadro's number?
To determine the number of atoms in a given mass of the element. The same concept applied to compounds.
How can the Lewis model be used?
To predict whether a particular set of atoms will form a stable molecule and what that molecule might look like
What does resonance depicts?
Two or more structures with electrons in different places in an attempt to more accurately reflect the delocalization of electrons. In the real hybrid structure-an average between the resonance structures- the electrons are more spread out (or delocalized) than in any of the the resonance structures
Where are the valence electrons?
Valence electrons are in the element's outermost principal energy level.
What is with the Lewis Structure for O3?
While both the Lewis structure for O3 predict that O3 contains two different bonds (one double bond and one single bond), when O3 structure is experimentally examined the bonds in the O3 molecule are equivalent and that each bond is intermediate in length and strength between a double bond and a single bond
In the Lewis model, how can we represent covalent bonding?
With a Lewis structure, which depicts neighboring atoms as sharing some (or all) of their valence electrons in order to attain octets (or duets for hydrogen)
What is a molecular formula?
a chemical formula that shows the actual number of atoms of each element in a molecule of a compound
What is lone pair (nonbonding electrons)?
a pair of electrons associated with only one atoms
What is a bonding pair?
a pair of electrons shared between two atoms a bonding pair of electrons is often represented by a dash to emphasize that the pair constitutes a chemical bond
What is a ball-and-stick molecular model?
a representation of the arrangement of atoms in a molecule that shows how the atoms are bonded to each other and the overall shape of the molecule a more accurate way to specify the compound represents atoms as ball and chemical bonds as sticks
How do we calculate formal charge?
formal charge= number of valence electrons (number of nonbonding electrons + 1/2 number of bonding electrons )
What are the diatomic elements?
hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine The Lewis model helps us explain what we see in nature
What is the fundamental unit of covalently bonded compounds?
individual molecules
What is a resonance hybrid?
the actual structure of a molecule that is intermediate between two or more resonance structures
What is atomic mass?
the average mass of an atom of an element
