Electron Configurations and Atomic Structure: Study Notes

Electron Configurations

Introduction to Electron Configuration

The arrangement of electrons in an atom is described by its electron configuration. The most stable configuration is the one with the lowest possible energy, known as the ground state. Electron configuration notation consists of:

• Energy level (number)

• Type of orbital (letter: s, p, d, f)

• Number of electrons in the orbital (superscript)

Example: $4p^5$

Orbital Box Diagrams

Orbital box diagrams visually represent electron configurations:

• Each box = one orbital

• Half-arrows = electrons

• Arrow direction = electron spin

For example, lithium (Li) has one electron in the 1s orbital, shown as a single arrow in a box labeled "1s".

Hund's Rule

Filling Degenerate Orbitals

Hund's Rule states that when filling orbitals of equal energy (degenerate orbitals), the lowest energy configuration is achieved when the maximum number of unpaired electrons with parallel spins is present. This means:

• Each orbital in a sublevel gets one electron before any pairing occurs

• Unpaired electrons have the same spin as much as possible

Example: For the p orbitals, fill each box with one electron before pairing.

Table: Electron Configurations of Several Lighter Elements

Element	Electron Configuration
H (Z=1)	$1s^1$
He (Z=2)	$1s^2$
Li (Z=3)	$1s^2 2s^1$
Be (Z=4)	$1s^2 2s^2$
B (Z=5)	$1s^2 2s^2 2p^1$
C (Z=6)	$1s^2 2s^2 2p^2$
N (Z=7)	$1s^2 2s^2 2p^3$
O (Z=8)	$1s^2 2s^2 2p^4$
F (Z=9)	$1s^2 2s^2 2p^5$
Ne (Z=10)	$1s^2 2s^2 2p^6$

Condensed Electron Configuration

Core and Valence Electrons

Elements in the same group of the periodic table have the same number of electrons in their outermost shell, called valence electrons. The filled inner shell electrons are core electrons, which may include completely filled d or f sublevels.

• Condensed electron configuration uses brackets around a noble gas symbol to represent core electrons, listing only valence electrons.

Example: Sodium (Na, Z=11): Full: $1s^2 2s^2 2p^6 3s^1$ Condensed: [Ne] $3s^1$

Types of Electrons

Classification of Electrons

• Core electrons: Those the atom has in common with the previous noble gas

• Outer electrons: Those in the highest energy level

• Valence electrons: Those involved in forming bonds - Main group: valence = outer - Transition metals: valence = outer + number of d electrons

Periodic Table and Electron Configuration

Periodic Table Blocks

The periodic table is divided into blocks corresponding to orbital types:

• s-block: Groups 1 and 2

• p-block: Groups 13-18

• d-block: Transition metals (Groups 3-12)

• f-block: Lanthanides and actinides

Electron configurations follow the order of filling from lowest to highest energy orbitals, which can be determined using the periodic table.

Examples of Electron Configurations

Neon (Ne, Z=10)

Electron configuration: $1s^2 2s^2 2p^6$ Orbital box diagram:

• 1s: two paired electrons

• 2s: two paired electrons

• 2p: six electrons, paired in three boxes

Sodium (Na, Z=11)

Electron configuration: $1s^2 2s^2 2p^6 3s^1$ Condensed: [Ne] $3s^1$

Key Rules for Electron Configuration

Aufbau Principle

Electrons fill orbitals starting from the lowest energy level and move to higher levels as lower ones are filled.

Pauli Exclusion Principle

No two electrons in an atom can have the same set of four quantum numbers; each orbital can hold a maximum of two electrons with opposite spins.

Hund's Rule

Maximize the number of unpaired electrons in degenerate orbitals by filling each orbital singly before pairing.

Summary Table: Electron Configuration Notation

Notation	Meaning
1s2	2 electrons in the 1s orbital
2p6	6 electrons in the 2p orbital
[Ne]	Core electrons represented by neon's configuration

Additional info:

• These notes are based on lecture slides and textbook sections relevant to General Chemistry, specifically atomic structure and electron configuration.

• For transition metals, remember that the d sublevel is filled after the s sublevel of the next higher principal quantum number.