Electron Configurations and Periodic Properties – Week 3 Study Notes

Electron Configurations

Introduction to Electron Configurations

Electron configurations describe the arrangement of electrons in the orbitals of an atom. The order in which sublevels are filled follows the increasing energy levels, as represented on the periodic table.

• Electron configuration is a notation that shows how electrons are distributed among the various atomic orbitals.

• Sublevels are filled in the order of increasing energy: s, p, d, f.

• The periodic table is divided into blocks (s, p, d, f) corresponding to these sublevels.

Periodic Table Blocks

The periodic table is organized into blocks based on the type of atomic orbital being filled:

• s block: Groups 1A and 2A; final electrons enter s sublevel.

• p block: Groups 3A to 8A; final electrons enter p sublevel.

• d block: Transition metals; final electrons enter d sublevel.

• f block: Inner transition metals; final electrons enter f sublevel.

Orbital Diagrams

Drawing Orbital Diagrams

Orbital diagrams visually represent how electrons occupy orbitals within an atom. Each box represents an orbital, and arrows indicate electrons with their spins.

• Draw boxes for each orbital in a sublevel (e.g., one for s, three for p).

• Fill orbitals from lowest to highest energy.

• Electrons within the same sublevel are placed one at a time in separate orbitals before pairing (Hund's rule).

Example: Carbon Orbital Diagram

Carbon has 6 electrons. Its orbital diagram is:

• 1s: two paired electrons

• 2s: two paired electrons

• 2p: two unpaired electrons (each in a separate box)

Electron Configuration Notation

Writing Electron Configurations

Electron configuration notation uses numbers and letters to indicate the energy level, type of orbital, and number of electrons.

• Format: norbitalnumber of electrons

• Example for carbon: $1s^2 2s^2 2p^2$

• Abbreviated notation uses the previous noble gas in brackets to represent core electrons.

Example Table: Period 1 Elements

Element	Atomic Number	Orbital Diagram	Electron Configuration
H	1	1s1	$1s^1$
He	2	1s2	$1s^2$

Example Table: Period 2 Elements

Element	Atomic Number	Orbital Diagram	Electron Configuration	Abbreviated Configuration
Li	3	1s22s1	$1s^2 2s^1$	[He]$2s^1$
Be	4	1s22s2	$1s^2 2s^2$	[He]$2s^2$
B	5	1s22s22p1	$1s^2 2s^2 2p^1$	[He]$2s^2 2p^1$
C	6	1s22s22p2	$1s^2 2s^2 2p^2$	[He]$2s^2 2p^2$
N	7	1s22s22p3	$1s^2 2s^2 2p^3$	[He]$2s^2 2p^3$
O	8	1s22s22p4	$1s^2 2s^2 2p^4$	[He]$2s^2 2p^4$
F	9	1s22s22p5	$1s^2 2s^2 2p^5$	[He]$2s^2 2p^5$
Ne	10	1s22s22p6	$1s^2 2s^2 2p^6$	[He]$2s^2 2p^6$

Guide to Drawing Orbital Diagrams

Steps for Drawing Orbital Diagrams

Follow these steps to construct orbital diagrams for elements:

1. Draw boxes to represent the occupied orbitals.

2. Place a pair of electrons with opposite spins in each filled orbital.

3. Place remaining electrons in the last occupied sublevel in separate orbitals (Hund's rule).

Example: Nitrogen Orbital Diagram

• Nitrogen (atomic number 7): $1s^2 2s^2 2p^3$

• 1s and 2s orbitals are filled with paired electrons.

• 2p orbitals each have one unpaired electron.

Periodic Trends and Properties

Valence Electrons and Group Numbers

Valence electrons are the electrons in the outermost energy level of an atom. The group number for representative elements (Groups 1A–8A) indicates the number of valence electrons.

• Valence electrons determine chemical properties and reactivity.

• For main group elements, valence electrons are found in the s and p sublevels of the highest energy level.

Lewis Symbols

Lewis symbols represent valence electrons as dots around the element symbol.

• Up to four valence electrons are shown as single dots.

• Five to eight valence electrons are arranged with at least one pair of electrons on each side.

Example: Lewis Symbol for Magnesium

• Mg: two valence electrons, shown as two dots.

Atomic Size

Trends in Atomic Radius

Atomic size is determined by the atomic radius, the distance from the nucleus to the outermost electrons.

• Atomic size increases down a group (more energy levels).

• Atomic size decreases across a period (increased nuclear charge pulls electrons closer).

Ionization Energy

Definition and Trends

Ionization energy is the energy required to remove one valence electron from an atom in the gas phase.

• Ionization energy decreases down a group (valence electrons farther from nucleus).

• Ionization energy increases across a period (greater nuclear charge).

• Equation: $\text{Na}(g) + \text{energy} \rightarrow \text{Na}^+(g) + e^-$

Metallic Character

Definition and Trends

Metallic character refers to the tendency of an element to lose electrons and form positive ions.

• Metallic character increases down a group.

• Metallic character decreases across a period.

• Metals are found on the left side of the periodic table; nonmetals on the right.

Summary Table: Trends in Periodic Properties

Property	Top to Bottom (Group)	Left to Right (Period)
Atomic Size	Increases	Decreases
Ionization Energy	Decreases	Increases
Metallic Character	Increases	Decreases

Additional info:

• Hund's rule states that electrons occupy orbitals singly before pairing.

• Pauli exclusion principle: no two electrons in an atom can have the same set of quantum numbers.

• Abbreviated electron configurations use noble gas symbols to simplify notation.