CH301 Unit 2 Exam Review

Duet Rule

Hydrogen achieves the greatest stability when it’s valence shell contains 2 electrons. Different from the octet rule where other elements become stable upon achieving 8 valence electrons.

Bond Polarity

Is proportional to the length of the dipole between 2 atoms. Greater ΔEN, the greater polarity of the bond.

Bond Polarity v. Molecular Polarity

Molecules can have polar bonds and be nonpolar molecules. Molecular polarity is the overall distribution of electron density in the molecule, while bond polarity is how uneven the sharing of electrons are between 2 atoms in the molecule.

Coulomb’s Law

Attraction gets stronger when charges are larger and ionics can compact closer together, which is possible when ionic radius is smaller.

Lattice Energy

Energy in the bonds of a crystalline solid salt. Compare charge first, THEN compare ionic radius ONLY IF charges are the same. Higher charges and smaller ions means stronger attraction and more LE.

Drawing 2D Lewis Structures

1. Add up the valence electrons in a molecule

2. Spread out perimeter atoms around the central atom

3. Put either 2 (for H) or 8 (for all others) electrons around perimeter atoms

4. Dump the rest of the unused valence electrons in the middle (central atom)

5. If the central atom does not have 8 or more electrons (Octet rule), share e- from perimeter atoms to make multiple bonds (double or triple)

DO NOT DO S = N -- A. It does not work all the time.

Octet Rule

Atoms prefer to obtain noble gas configuration (filled valence shells).

Electronegativity Trend

Increases and highest at the UPPER RIGHT of the table. Shielding increases down a group, meaning electrons aren’t pulled as closely to the nucleus, which directly affects EN.

In what cases is ΔEN = 0?

Br2, I2, N2, Cl2, H2, O2, F2 = All exist as diatomics.

Ionic Radius Trend

Decreases from left to right, and increases down a group. Largest in the LOWER LEFT corner of the table.

Metallic Character Trend

Most metallic in the LOWER LEFT corner of the table. Cs is the most metallic and F is the least metallic (excluding noble gases).

ENC Trend

Increases left to right, and remains constant down a group. ENC equation: # of protons - # of shielding electrons.

Ionization Energy Trend

Increases left to right, and decreases down a group. Largest in the UPPER RIGHT of the periodic table.

Naming Ionic Compounds

Leave cation as is + polyatomic ion + anion base name ending in -ide.

Transition Metal charges to know.

Ag —> 1+

Zn —> 2+

Al —> 3+

Hypovalency

Electron deficiency around the central atom. Will commonly be Be and B. Boron forms up to 3 bonds (3ve-), and Beryllium forms up to 2 bonds (2ve-).

Radicals

Odd number of valence electrons. Add up the number of valence electrons to determine if the molecule is radical, They are VERY reactive, and therefore unstable, so it’s correct for the central atom to not achieve the octet rule. Note that the radical electron is on the LESS EN atom.

Hypervalency

Some elements can have expanded octet rules. Elements with an n value of 3 or higher have access to the d-orbitals. Think 3d^10 for n=3 shells.

Regarding charges when drawing Lewis Structures

With anions, add the charge to the total e- count, and subtract the charge from the total e- count in the case of cations.

Deciding Central Atom of Lewis Structures

- Typically first element in the formula.

- The element that is the LEAST electronegative.

- Halogens are usually perimeter atoms

- H always outer/perimeter atom.

Formal Charges

# of valence electrons - # of dots - # lines = FC

- FC of 0 is ideal.

- FC of +2/-2 is not ideal.

- FC of -1 i preferable on the relatively more electronegative atom in the molecule if unavoidable.