Chemical Bonding and Molecular Geometry

octet rule

states that atoms lose, gain, or share electrons in order to acquire the stable electron configuration of a noble gas

ionic bond

electrostatic force that holds oppositely charged particles together in an ionic compound; *TRANSFERRING ELECTRONS*

(extreme case of 2 atoms of very different electronegativities)

greater than or equal to 1.7 in difference of electronegativities

covalent bond

chemical bond that results from the sharing of valence electrons *SHARING ELECTRONS*

multiple covalent bond

bonds that involve more than one pair of electrons

coordinate covalent bond

polar covalent bond

a covalent bond with greater electron density around one of the atoms; 0.4-1.7 difference in electronegativity

non-polar covalent bond

a covalent bond in which electron density is equal around both atoms

less than 0.4 difference in electronegativity

saturated hydrocarbons

having only single bonds, ends with -ane (saturated--- lots of H)

unsaturated hydrocarbons

contain multiple bonds (enes, ynes, etc.)

difference between carbohydrate and hydrocarbon

Hydrocarbon: C*H4*

Carbohydrates: C6*H12O6*

All carbohydrates are hydrocarbons but no hydrocarbons are carbohydrates (I think)

sigma bond

form when the electron pair is shared in the area centered b/w 2 atoms (can be formed by s- and p- orbitals)

pi bond

form when parallel orbitals overlap to share electrons above and below the line connecting the two atoms (can only be formed by parallel p-orbitals)

resonance structures

one of two or more Lewis structures for a single molecule that cannot be accurately represented by only one Lewis structure

hybridization

process in which different atomic orbitals are mixed to form new/identical orbitals

sp3, sp2, and sp hybridization

isomers

molecules that have same numbers and kinds of atoms but differ in the way atoms are arranged

structural isomers

isomers that have the same formula but different structures

stereoisomers

- made up of same types and number of elements with the same sequence but different spatial arrangement

- only double bonds

- cis- and trans- isomers

optical isomers

non-superimposable mirror images (absolutely the same in chemical properties EXCEPT light thing)

dipole moment

determines shift in electron density due to electronegativity difference (how polar is the molecule)

hydrogen bond

special type of dipole-dipole interaction b/w a H atom bonded to a very electronegative atom and an unshared pair of electrons in another very electronegative atom (such as N, O, or F)

dipole-dipole attraction

attractive forces b/w polar molecules

- molecules are permanently dipole, but the *attraction* depends on proximity of polar molecules to each other

dispersion forces

- always there b/w molecules

- attractive forces that arise as a result of temporary dipoles induced in atoms or molecules

metallic bond

bond in which electrons do not belong to a particular atom but are *delocalized* over the entire metal sample

covalent network solid

- network of covalently bonded atoms that forms an extremely strong crystal

- present in carbon allotropes

allotrope

different forms of the same element

electrolyte

an ionic compound whose aqueous solution conducts electricity

non-electrolyte

an ionic compound that doesn't conduct electricity/have aqueous compound

surface tension

inward force that tends to minimize surface area of a liquid and makes a liquid act as if it had a thin "skin"

- jar trick!

- paperclip on water!

adhesion

attraction b/w unlike molecules

cohesion

intermolecular attraction of like molecules

capillary action

spontaneous rise of liquid in a thin tube

viscosity

measure of fluid's resistance to flow; liquids whose molecules are polar or can form hydrogen bonds are usually more viscous than similar nonpolar substances

What is ionic bonding? How is it formed? How does it hold atoms together? Explain and give examples.

Ionic bonding is between a metal and a nonmetal; electrons are *transferred* from one atom to another, making one a cation and another an anion. The positive charge of the cation is attracted to the negative charge of the anion, forming an ionic bond. This happens on a large scale, making a formula unit.

- Crystal structure

- Example: sodium and chlorine

What is covalent bonding? How is it formed? How does it hold atoms together? Explain and give examples.

Covalent bonding

-sharing of electrons

-attraction between an electron and a proton in nucleus (not between positive or negative)

- nuclei repels each other if they get too close

What factors determine the length and the strength of the bond?

- Nuclei repel each other if they get too close: determines length of bond (balance between attractive and repulsive forces)

- Number of bonds ( multiple or not)

- Atom size

-The distance between the two bonded nuclei at the position of maximum attraction is called bond length. It is determined by the sizes of the two bonding atoms and how many electron pairs they share.

Compare molecular and ionic compounds. Use at least 6 criteria.

*MOLECULAR*

- Covalent bonds

- Formed between two nonmetals

- Attraction between nuclei

- Low melting and boiling points/soft

- Poor conductors of electricity

- Poor conductors of heat

- At STP, found in all three states

*IONIC*

- Ionic bonds

- Formed between a metal and a nonmetal

- Attraction of opposite charges of ions (electrical attraction)

- High melting and boiling points

- Good conductor of electricity (when in aqueous state)

- Good conductor of heat (when in aqueous state)

- At STP, solids

Why do we say that there is no real boundary between polar covalent and ionic bond? Elaborate.

There is always going to be some sharing between molecules (because there is always an attraction between the nuclei). The attraction doesn't suddenly just change to ionic, it is just that the two atoms it is between have enough difference in electronegativity for the bond to be considering ionic, but there is no boundary that 1.71 difference in electronegativities would be ionic but 1.69 difference in electronegativities would not; it's more of a scale than clear boundaries.

Explain polar covalent bonds. Use electronegativity values, partial charge and dipole. Give examples.

2 elements differ in electronegativity: a covalent bond (C and O or Cl and F), the bond is polarized, meaning that the shared electrons are more likely to be in the region of the more electronegative atom. This creates a partial charge and makes it a dipole.

Using electronegativity values, determine: are the bonds ionic or covalent? Which is the most polar?

Less than/equal to 0.4= nonpolar covalent

0.4-1.7= polar covalent

Greater than or equal to 1.7= ionic

PRACTICE

Know the basics of hydrocarbon nomenclature: 10 stem words and prefixes -ane, -ene, and -yne and how they help determine bonding type. Recognize and name simple hydrocarbons.

PRACTICE

What is resonance? Give an example of a molecule with resonance structures.

Resonance is when a single molecule cannot be accurately represented by only one Lewis structure.

Example: CO3, O3

Describe two theories explaining molecular geometry: VSEPR and hybridization. What was the necessity of proposing each of them?

- Many models have been developed to visualize spatial arrangement of bonds in single molecules

VSEPR (*V*alence *S*hell *E*lectron *P*air *R*epulsion)

- Electron pairs spread out as far as possible to minimize repulsive forces. This results in certain angels at which atoms bond. (Instead of 90 degrees like on paper, think of models in class)

- Both *lone pairs* and *shared pairs* repel and account for resulting shapes

HYBDRIDIZATION

- process in which different atomic orbitals are mixed/adjusted to form new identical orbitals

- number of final orbitals= same number as orginal

What are types of hybridization we studied and how do they correspond to the bonding type?

sp3

- one s-orbital and 3 p-orbitals transform into 4 equal sp3 orbitals

- single bonds (4 sigma)

sp2

- one s-orbital and 2 p-orbitals transform into 3 equal sp2 orbitals

- typically double bonds (1pi, 3sigma)

sp

- one s orbital and one p-orbital transforms into 2 equal sp orbitals

- typically triple bonds (2 pi bonds, 2 sigma)

Not all molecules containing polar bonds are polar. How is this possible? Explain with examples. What are some common molecular shapes

-

Know basic shapes (linear, bent/angular, tetrahedral, trigonal pyramidal, trigonal planar) and an example of a real molecule having each shape.

Linear

Recognize shapes of very simple molecules given molecular formulas:

HBr

- Linear

- sp3 orbitals

- 4 sigma bonds

SO3

- trigonal planar

- sp2 orbitals

- 3 sigma bonds, one pi bond

PH3

- trigonal pyramidal

- sp3 orbitals

- 3 sigma bonds, one lone pair

H2S

- bent

- sp3 orbitals

- 2 sigma bonds

CO

- linear, triple bond

- 2 pi, one sigma

- sp hybridization (2 extra p orbitals)

CS2

- linear, double bond

- 2 pi, 2 sigma

- sp hybridization

H2O

- bent

- 2 sigma bonds

- sp3

NH3

Cl2O

CO2

- Linear (109.5 degrees)

SO2

CCl4

HCL

CH3Cl

NI3

SeF2

BF3

CH4

BeH2

CF4

What is a metallic bond? How does it hold atoms together? Explain why metals are malleable and ductile while ionic crystals are fragile. Also know other metals' properties we discussed.

A metallic bond is best described by an electron sea model:

- each atom is a cation (with a positive core) with surrounding valence electrons

- cores= pebbles in sea, cations are fixed in place

- electrons do not belong to a particular atom but are *delocalized* over the entire metal sample

- Malleable/ductile: free electrons can be deformed

- can also: conduct electricity, heat

- Ionic crystals fragile bc there is a v strong bond, electrons are not delocalized

-

Describe intermolecular attractions between molecules: dipole interactions, hydrogen bonds, dispersion (London) forces. For each, explain causes and relative strength.

WEAKEST

Dispersion forces

Dipole interactions: polar molecules

hydrogen bonds

Explain how network solids are formed. Give examples. How do diamond and graphite form and how do they differ in properties?

- network of covalently bonded atoms that form extremely strong crystals

- present in carbon allotropes

- Diamond: so hard, transparent hexagons: super hard, doesn't conduct electricity

- Graphite: flat honeycombs: soft, black, conducts electricity

Explain how the structure and polarity of water affect its properties. List properties of water that are caused by an unusually strong hydrogen bonding. Include ice, surface tension, viscosity, capillary action, high melting, boiling points and heat capacity. How is surface tension exhibited? Give several examples.

- Hydrogen bonding: 2 lone pairs

Ice: bc of H bonds, hexagons form with expanded spaces, hollow centers, decrease density so it floats

- Implications are expanding ice (water expands when freezing and can break its container)

Surface Tension

- insect

- beads of water on leaf

- inward force that tends to minimize surface are of a liquid

Snowflakes

- Honeycomb layered pattern

Capillary Action

- spontaneous rise of liquid in a thin tube

- cohesion, adhesion

Viscosity

- molasses, honey, corn syrup

High melting point and high boiling point

- rly strong H bonds

High specific heat

- water can absorb a lot of heat

- Water will *bond* to surfaces hat are polar, won't bond to surfaces that are nonpolar