Chapter 4 Flashcards
Ionic Compounds
- Consist of positive ions
(cations) and negative ions (anions) - In the chemical formula, the sum of the
positive charges must equal the sum of the
negative charges - The formula of ionic compounds reflects
the smallest whole-number ratio of ions - In the formula, the cations are written first,
the anions are written second
+ Hydrate
+ hydrate is H20 ex. tetrahydrate is 4H2O
Covalent Bond
- Full name: [cation name][anion name]
✓ NaSCN, sodium thiocyanate
✓ Cu2O, copper(I) oxide
✓ FeCl3, iron(III) chloride
✓ Al2(SO4)3, aluminum sulfate - Some ionic compounds have waters of hydration in their 3D structures
✓ These are called hydrates - Name: [cation name][anion name] [prefix]hydrate
✓ BaCl2·6H2O, barium chloride hexahydrate
Pure Covalent bonds
Covalent bonding between atoms with same
electronegativities results in equal sharing of
the electrons: pure or nonpolar covalent
bond
✓ Same electron density on both atoms
Polar Covalent bonds
Covalent bonding between atoms with
different electronegativities results in
unequal sharing of the electrons: polar
covalent bond
✓ Higher electron density on more electroneg. atom
✓ Lower electron density on less electroneg. atom
✓ Ex: HF
✓ The larger the electroneg. difference, the more
polar the covalent bond
Octet rule
- Octet rule: ach atom wants to complete its
octet by sharing pairs of electrons w/ its
neighbors
Octet rule exceptions
- Odd-electron species
✓have 1 unpaired electron
✓called radicals
✓very reactive
✓Ex: NO - Incomplete octets
✓most common for Be & Group 13 elements (B, Al, Ga…)
✓Ex: BeH2, BF3
✓react with molecules with lone pairs (ex, NH3)
Hypervalent Molecules
- Elements in 2nd row can only have 4 atoms around them
- Heavier (3rd row & below) main-group elements can have 5-6 atoms around them
- When the central atom is connected to 5 or 6 terminal atoms, the most accepted LDS requires expanding beyond the octet: expanded valence shell or hypervalent molecule
✓ terminal atoms always of high EN (F, Cl, O)
✓ an alternative with just an octet is possible, but looks ugly, rarely used
✓ Ex: PCl5, SF6, IF5
Formal Charge
formal charge = (# valence e–) – (# e– it “owns”)
formal charge =
(# valence e–) – (# nonbonding e– + ½ # bonding e–)
Always explicitly show formal charges on atoms!
Always explicitly show formal charges on atoms!!!!
Resonance Structure
Some molecules are not adequately described by a single Lewis dot structure
* Resonance structures or resonance forms differ only in the position of the electrons
✓same connectivity, same number of electrons
* The actual electron distribution is an average of the resonance forms: resonance hybrid
Equivalent Resonance Structure
Equivalent resonance structures: an equal mixture
of all resonance structures
✓ are equally good as judged by all criteria:
➢ satisfy the octet rule
➢ same number & magnitude of formal charges, on same element types
✓ the bond order is averaged over all resonance structures
* Ozone (O3) has 2 equivalent resonance structures
✓ O-O bond order is 1.5
* Benzene (C6H6) has 2 equivalent resonance structures
✓ C-C bond order is 1.5
Nonequivalent Resonance Structures
- Nonequivalent resonance structures: ✓ an unequal mixture (weighted average) of different resonance structures
✓ resonance structures have different number and/or magnitude of formal charges, or placed on atoms of different elements
✓ one resonance structure contributes more to the resonance hybrid
Electron vs molecular geometry
Molecular geometry: The ideal geometry without the electron lone pairs
Electron geometry: the real geometry
Dipole moment
Dipole moment is directly prop. to the magnitude of charges (Q) & distance between them (r):
m= Qr
* Dipole moments are vectorquantities
✓have a magnitude & direction
✓represented by an arrow
✓arrowhead has the d– charge, other end has d+ charge
