Chem Exam: Electron Config, Lewis Structures, VSEPR

Aufbau principle

Electrons fill orbitals from lowest energy to highest energy

Pauli exclusion principle

No two electrons in an atom can have the same four quantum numbers (max 2 e⁻ per orbital, opposite spins)

Hund's rule

Electrons fill degenerate orbitals singly with parallel spins before pairing up

Heisenberg uncertainty principle

You can't know both the exact position and momentum of an electron at the same time

Orbital filling order

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p

Principal quantum number (n)

Shell / energy level: 1, 2, 3, …

Angular momentum quantum number (l)

Subshell shape: 0 to n−1 (s = 0, p = 1, d = 2, f = 3)

Magnetic quantum number (mₗ)

Orbital orientation: whole numbers from −l to +l

Spin quantum number (mₛ)

+½ or −½

Forming a negative ion (anion)

Add electrons to the next open orbital (O²⁻ = 1s²2s²2p⁶)

Forming a positive ion (cation)

Remove electrons from the highest n first (Sr²⁺ = [Kr])

Transition metal cations: which electrons leave first?

The ns (e.g., 4s) electrons leave before the (n−1)d electrons

Isoelectronic

Species with the same number of electrons (e.g., N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ all have 10)

Valence electrons: alkali metals (Group 1)

1

Valence electrons: alkaline earth metals (Group 2)

2

Valence electrons: halogens (Group 17)

7

Valence electrons: noble gases (Group 18)

8 (except He, which has 2)

Ionization energy (definition + equation)

Energy needed to remove an electron from a gaseous atom: X(g) → X⁺(g) + e⁻

Electron affinity (definition + equation)

Energy change when a gaseous atom gains an electron: X(g) + e⁻ → X⁻(g)

Ionization energy trend

Increases going UP and to the RIGHT

Electron affinity trend

Gets larger (more negative) going UP and to the RIGHT (generally)

Atomic radius trend

Increases going DOWN and to the LEFT

Metallic character trend

Increases going DOWN and to the LEFT

Penetration

An orbital that penetrates into the core is LESS shielded, feels more nuclear charge, and is LOWER in energy (s < p < d in the same shell)

Formal charge formula

FC = valence e⁻ − nonbonding (lone pair) e⁻ − number of bonds

Best Lewis structure

Formal charges closest to zero; any negative formal charge on the most electronegative atom

Which atom goes in the center of a Lewis structure?

The least electronegative atom (never H)

Which central atoms can have an expanded octet?

Atoms in period 3 or below (e.g., P, S, Cl, Xe)

How do double/triple bonds count in VSEPR?

As ONE electron group

2 electron groups

sp hybridization · linear · 180°

3 electron groups

sp² hybridization · trigonal planar · 120°

4 electron groups

sp³ hybridization · tetrahedral · 109.5°

5 electron groups

sp³d hybridization · trigonal bipyramidal · 90° and 120°

6 electron groups

sp³d² hybridization · octahedral · 90°

Bent (3 electron groups)

2 bonding, 1 lone pair · sp² · <120°

Trigonal pyramidal

3 bonding, 1 lone pair · sp³ · ~107°

Bent (4 electron groups)

2 bonding, 2 lone pairs · sp³ · ~104.5°

Seesaw

4 bonding, 1 lone pair · sp³d

T-shaped

3 bonding, 2 lone pairs · sp³d

Linear (5 electron groups)

2 bonding, 3 lone pairs · sp³d

Square pyramidal

5 bonding, 1 lone pair · sp³d²

Square planar

4 bonding, 2 lone pairs · sp³d²

Where do lone pairs go in a trigonal bipyramid?

Equatorial positions

Carbons in benzene (C₆H₆)

sp² · 120°