Lesson 4: Atomic Structure & Electron Configuration
Atoms are the fundamental building blocks of matter. The modern atomic model describes a dense nucleus of protons and neutrons surrounded by electrons organized into energy levels (shells) and subshells.
Key Concepts
Subatomic Particles
Protons (+1 charge, in nucleus) determine the element. Neutrons (0 charge, in nucleus) determine the isotope. Electrons (-1 charge, in shells) determine chemical behavior.
Electron Shells
Electrons occupy energy levels. Shell 1 holds 2 electrons. Shell 2 holds 8. Shell 3 holds 8 (for the first 18 elements). Electrons fill from the lowest energy level first.
Electron Configuration Notation
Written as 1s² 2s² 2p⁶ 3s²... Each letter (s,p,d,f) is a subshell type. The superscript is the electron count in that subshell.
Valence Electrons
The outermost shell electrons are valence electrons. They determine bonding behavior. Elements with 8 valence electrons (full outer shell) are noble gases — very stable and unreactive.
Periodic Trends
Elements in the same column (group) of the periodic table have the same number of valence electrons and similar chemical properties. That's why the table has its structure.
🔬 Virtual Lab: Electron Configuration Builder
Use the slider to set the atomic number (1–20). The Bohr model diagram draws the correct electron shells automatically.
✅ Check Your Understanding
1. How many electrons can the second electron shell hold?
2. Valence electrons are located in the:
3. Elements in the same group (column) of the periodic table have: