Lewis Structures

Question 1

What are types of chemical bonds? Explain each briefly.

Answer 1

ionic: electron transferred from metal to non-metal

covalent: electron shared between non-metals

metallic: valence electrons delocalized over many metal nuclei

Question 2

What are the steps to drawing a lewis structure?

Answer 2

1. calculate total number of valence electrons
2. identify central atom
3. draw a signle bond between each atom
4. calculate remaining electrons
5. distribute remaining electrons in pairs around the terminal atoms, then central atom
6. if any atoms don’t have octets then make double or triple bonds

Question 3

How can you identify the central atom?

Answer 3

atom with lowest group number

if in same group then atom with higher period is central

Question 4

How many valence electrons are in each group?

Answer 4

group 1: 1 ve
group 2: 2 ve
group 13: 3 ve
group 14: 4 ve
group 15: 5 ve
group 16: 6 ve
group 17: 7 ve

Question 5

When drawing lewis structures of polyatomic ions what is different?

Answer 5

add or subtract electrons to total ve count

draw square bracket to indicate charge

Question 6

What is the born-haber cycle used for?

Answer 6

estimate lattice energy

Question 7

What are the five reactions used in born haber cycle?

Answer 7

1.sublimation reaction of metal (solid to gas)
2. bond energy (breaking bond between diatomic molecule)
3. ionization of gaseous metal (removing electron)
4. electron affinity (adding electron to nonmetal)
5. formation of ionic compound from gaseous ions

Question 8

How do you solve a born haber cycle?

Answer 8

find enthalpy of formation for the compound from solid metal and gas nonmetal

rearrange equation to find lattice energy

Question 9

What is equation used to solve born haber cycle?

Answer 9

enthalpy of formation=(enthalpy of sublimation)+(enthalpy of IE)+(enthalpy of BE)+(enthalpy of EA)+(enthalpy of LE)

rearrange to find LE

Question 10

What is trend for lattice energy?

Answer 10

magnitude decreases going down a column (less exothermic)
as magnitude of charges increases so does lattice energy

Question 11

What is bond energy?

Answer 11

amount of energy required to homolytically break 1 mol of bonds between two atoms in the gas phase

homolytically means electrons in covalent bond are divided equally

Question 12

Is bond energy positive or negative? Why?

Answer 12

positive

takes energy to break a bond

Question 13

How can you use bond energy to caluclate enthalpy of a reaction?

Answer 13

break all bonds in the reactants and find all the values and add them together

break all bonds in products and find values and add them together and then make it negative

add these two values together

Question 14

What happens when a bond get longer?

Answer 14

generally it gets weaker

Question 15

What causes a polar covalent bond?

Answer 15

unequal sharing of electrons (one atom is more electronegative)

Question 16

What is electronegativity?

Answer 16

measure of an atom’s ability to attract
electron towards itself in a chemical bond

Question 17

What is the trend for electronegativity?

Answer 17

is decreases down a column and increases from left to right

Question 18

What are the values for a difference in electronegativity for each bond?

Answer 18

0-0.4 is covalent
0.4-2.0 is polar covalent
above 2 is ionic

Question 19

What are resonance structures? What are resonance hybrids? How are they drawn?

Answer 19

resonance structures are the different possibilities for the compound structure (drawn with arrow pointing between)

resonance hybrid have characteristics of all resonance structures (drawn with dashes to represent the possible bonds)

Question 20

What is electron delocalization?

Answer 20

electrons go around the entire molecule

Question 21

How can you calculate formal charge?

Answer 21

of ve-# of bonding electrons-1/2 bonded electrons

(take number of ve and count backwards for each electron/bond)

Question 22

What can formal charges be used for?

Answer 22

comparing resonance structures to figure out the most dominant one

Question 23

What is the optimal formal charge rules?

Answer 23

as many atoms as possible have full octets

magnitudes of formal charges as small as possible

negative formal chrages on most electronegative atom

Question 24

What is a free radical? What does this usually mean?

Answer 24

an electron that is unpaired

more reactive

Question 25

Why do some compounds have incomplete octets when resonance structure do not?

Answer 25

incomplete octet is more favourable (formal charge)

Question 26

What are hypervalent compounds? What can do this?

Answer 26

exceed octet rule

third row or lower

usually large central atom with more electronegative terminal atoms

Question 27

What are ionic resonance strucutes?

Answer 27

can be drawn without expanded octet but introduce charges