Unit 3 Exam Review - Coulombic Attraction, Periodic Trends & Bonding

Exceptions to the Octet Rule

1. Types of Exceptions

   • Suboctet: Central atom has fewer than 8 valence electrons.

   • Expanded Octet: Central atom has more than 8 valence electrons.

   • Odd number of valence electrons: Central atom has an an odd number of valence electrons.

2. Valence Electron Count in Exceptions

   • Atoms demonstrating these exceptions may not follow the typical octet rule when forming bonds.

3. Molecular Examples of Exceptions

   • i. Nitrogen Dioxide (NO_2): 17 valence electrons (Odd number)

   • ii. Phosphorus Pentachloride (PCl_5): 40 valence electrons (Expanded octet)

   • iii. Boron Trifluoride (BF_3): 24 valence electrons (Suboctet)

🌐 1. Atomic Radius (size of the atom)

Trend:

• Across a period (→): decreases

• Down a group (↓): increases

Why?

• Across: more protons pull electrons closer → smaller.

• Down: more energy levels → electrons farther from nucleus → bigger.

Easy way to remember:
Atoms get bigger going down, smaller going right.

⚡ 2. Ionization Energy (energy required to remove an electron)

Trend:

• Across a period (→): increases

• Down a group (↓): decreases

Why?

• Across: atoms are smaller & hold electrons tighter → harder to remove → higher IE.

• Down: electrons are farther from nucleus → easier to remove → lower IE.

Remember:
Big atoms = electrons far away = easy to steal → low ionization energy.
Small atoms = electrons close = hard to steal → high ionization energy.

🧲 3. Electronegativity (how strongly an atom attracts electrons)

Trend:

• Across a period (→): increases

• Down a group (↓): decreases

Why?

• Across: atoms want electrons more to fill their valence shell.

• Down: nucleus is farther from bonding electrons → weaker pull.

Highest EN: Fluorine
Lowest EN: Francium (practically)

Remember:
Top–right corner (except noble gases) = most electronegative.