Lesson 7: Atoms & the Periodic Table

Atoms & Periodic Table Reference

Atomic Structure

• Protons: positive charge, in nucleus, defines the element (atomic number Z).
• Neutrons: no charge, in nucleus. Mass number A = protons + neutrons.
• Electrons: negative charge, orbit nucleus in shells/orbitals.

Electron Configuration

Electrons fill shells in order: 1s → 2s → 2p → 3s → 3p. Valence electrons (outermost shell) determine chemical behavior.

• Shell 1: max 2 electrons
• Shell 2: max 8 electrons
• Shell 3: max 18 electrons (first 8 for elements 11-18)

The Periodic Table

• Period: horizontal row — same number of electron shells.
• Group: vertical column — same number of valence electrons, similar properties.
• Metals: left and center; conduct electricity, malleable.
• Nonmetals: right side; poor conductors, form anions.
• Noble gases: Group 18; full valence shells, very unreactive.

Periodic Trends

• Atomic radius: increases down a group, decreases left to right across a period.
• Electronegativity: increases right and up (F is highest).
• Ionization energy: increases right and up.