Groups (Columns)

Elements in the same group share similar chemical properties and have the same number of valence electrons. Groups are numbered 1-18.

• Group 1: Alkali Metals (highly reactive)
• Group 2: Alkaline Earth Metals
• Groups 3-12: Transition Metals
• Group 17: Halogens (very reactive nonmetals)
• Group 18: Noble Gases (inert)

Periods (Rows)

Elements in the same period have the same number of electron shells. Period number = number of electron shells.

Blocks (s, p, d, f)

Blocks indicate which orbital is being filled with electrons:

• s-block: Groups 1-2 (valence electrons in s orbital)
• p-block: Groups 13-18 (filling p orbitals)
• d-block: Groups 3-12 (transition metals, filling d orbitals)
• f-block: Lanthanides & Actinides (filling f orbitals)

Electron Configuration

Shows how electrons are distributed in atomic orbitals. Format: [Noble Gas] orbital^electrons

• Example: Carbon = [He] 2s² 2p² (2 electrons in 2s, 2 in 2p)
• Aufbau Principle: Electrons fill lowest energy orbitals first
• Order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
• Orbital Capacity: s=2, p=6, d=10, f=14 electrons
• Valence Electrons: Outermost shell electrons determine chemical properties

Oxidation States

The charge an atom would have if all bonds were ionic. Indicates electron loss/gain in compounds.

• Positive (+): Metals lose electrons (e.g., Na⁺, Ca²⁺, Fe³⁺)
• Negative (−): Nonmetals gain electrons (e.g., Cl⁻, O²⁻, N³⁻)
• Multiple States: Transition metals can have several (e.g., Fe: +2, +3)
• Group Patterns: Group 1=+1, Group 2=+2, Group 17=−1, Group 18=0
• Variable States: Allow formation of different compounds (FeCl₂ vs FeCl₃)

Electronegativity

Ability of an atom to attract electrons in a chemical bond (Pauling scale: 0.7-4.0).

• Increases →: Left to right across periods (more protons attract electrons)
• Decreases ↓: Top to bottom down groups (electrons farther from nucleus)
• Highest: Fluorine (3.98) - most electronegative element
• Lowest: Francium (0.7) - least electronegative
• Predicts Bonding: Large difference = ionic, small = covalent

Ionization Energy

Energy required to remove an electron from a gaseous atom (measured in kJ/mol).

• Increases →: Left to right across periods (stronger nuclear attraction)
• Decreases ↓: Top to bottom down groups (electrons farther, easier to remove)
• Highest: Helium (2372 kJ/mol) - hardest to ionize
• Lowest: Francium (380 kJ/mol) - easiest to ionize
• Noble Gases: Highest in each period (stable electron configuration)

Atomic Radius

Distance from nucleus to outermost electron shell (measured in picometers, pm).

• Decreases →: Left to right across periods (more protons pull electrons closer)
• Increases ↓: Top to bottom down groups (more electron shells added)
• Largest: Francium/Cesium (~298 pm) - most electron shells
• Smallest: Helium (31 pm) - only one electron shell
• Cations: Smaller than parent atom (lost electrons)
• Anions: Larger than parent atom (gained electrons)