Chem #3 Exam
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What is the description of Ionic Bonds?
a chemical bond formed by the electrostatic attraction between positive and negative ions
Formation of Ionic Bonds
Involves the transfer of one or more electrons from the valence shell of one atom to the valence shell of another
When are Ionic Bonds most likely to occur?
Especially prevalent in compounds formed between Group 1A and 2A elements with Group 6A and 7A elements
What is Lattice Energy?
the energy required to completely separate one mole of a solid ionic compound inot gaseous ions
How can you determine Lattice Energies?
- They can be calculated from Coulomb's law or measured indirectly by the Born-Haber cycle
- **cannot be measured directly**
Allows the indirected determination of the lattice energy of an ionic solid
What is a Covalent Bond?
a chemical bond formed by the sharing of a pair of electrons between atoms
What are the multiple bonds?
- Single Bond: a single pair of electrons is shared by two atoms
- Double Bond: two pairs of electrons are shared by two atoms (often C, N, O and S)
- Triple Bond: three pairs of electrons are shared by two atoms (often C and N)
What is Bond Length?
The distance between the nuclei of two covalent atoms in a molecule
What are Intramolecular Forces?
the attractive forces that holds the atoms together within a molecule
What are Intermolecular Forces?
- the forces that operate between molecules
- *relatively weak in covalent compounds
- **very strong in ionic compounds
What is a Polar Covalent Bond?
a covalent bond in which the bonding electrons spend more time near one atom that the other
What are the Three types of Polar covalent bonds?
- Pure covalent bond: neutral atoms held together by equally shared electrons
- Polar covalent bond: partially charged atoms held toegher by unequally shared electrons
- Pure ionic bond: oppositely charged ions held together by electrostatic attraction
What is electronegativity?
a measure of the ability of an atom in a molecule to draw bonding electrons to itself
Robert S. Mulliken
- came up with the Mulliken scale which calculates average ionization potential and electron affinity.
- (no longer use the Mulliken scale)
Linus C. Pauling
- Created the Pauling Scale - derived from bond energies and is very widely used.
- X increases left to right, decreases top to bottom
What are the Five most electronegative elements?
- F : 4.0
- O : 3.5
- N~Cl : 3.0
- Br : 2.8
Electron Affinity vs. Electronegativity
- Electron Affinity: how much an isolated atom wants someone else's electrons
- Electronegativity: how much an atom wants the shared electrons in a chemical bond
Polarity of a bond depends upon the difference in electronegativity of the two bonded atoms
Bond Polarity and Bonding:
-if Delta X is < than 0.5...?
-if 0.5 is < Delta X and < 2.0...?
-if Delta X is greater than or equal to 2.0...?
- -if Delta X is < 0.5 the bond is purely covalent (nonpolar)
- -if 0.5 is < Delta X and < 2.0 the bond is polar covalent
- -if Delta X is greater than or equal to 2.0 the bond is ionic
What is a Formal Charge?
the hypothetical charge of an atom
What are Resonance Structures?
all possible Lewis Structures for a single molecule that cannot be represented accurately by only one Lewis structure
What is delocalized bonding?
a type of bonding in which a pair of bonding electrons is spread over a number of atoms rather than localized between two
What are the exceptions to the octet rule?
- -molecules having an odd number of electrons
- -molecules with an atom from Group IIA or IIIA having < 8 valence electrons around that atom
- -molecules with an atom having unfilled nd orbitals may have > 8 valence electrons around that atom
What are bond enthalpy?
the enthalpy charge Delta H required to break a particular bond in one mole of gaseous molecules
What is average bond enthalpy?
the average enthalpy change for the breaking of an A-B bond in the gas phase
What is Bond Order?
- the number of pairs of electrons in a bond
- -Bond enthalpy- increases with bond order
- -Bond length- decreases with bond order
What is Molecular Geometry?
the three-dimensional arrangements of atoms in a molecule
*Importance: a molecule's geometry affects its physical and chemical properties
Valence-Shell Electron-Pair Repulsion (VSEPR) Model
-Predicts the shape of molecules and ions
Basis: Valence-shell electron-pairs (bonding and nonbonding) are kept as far away from one another as possible
What are Electron Domains?
- a lone pair of electrons or a chemical bond
**Note: Double and Triple bonds are treated like Single bonds and count as one electron domain
VSEPR Model Definitions:
Arrangement of electron domains
- Arrangement of electron domains: determined by total number of electron domains (i.e., # of bonds AND non-bonding pairs)
- Molecular geometry: determined by number of bonds
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