Lesson 6 of 12
⚛ Atoms & the Periodic Table
What You'll Learn
- Describe the structure of an atom: protons, neutrons, electrons
- Read an element's entry on the periodic table
- Understand periods, groups, and element categories
Atomic Structure
All matter is made of atoms. An atom has a central nucleus containing protons and neutrons, with electrons orbiting in shells (energy levels).
- ✙ Protons (+) — determine the element (atomic number). Carbon always has 6 protons.
- ⚪ Neutrons (neutral) — add mass; different numbers create isotopes of the same element.
- 🔹 Electrons (-) — determine chemical behavior. Atoms bond by sharing or transferring electrons.
An element's atomic number = number of protons. Atomic mass ≈ protons + neutrons.
The Periodic Table
The periodic table organizes all 118 known elements by atomic number. Periods (rows) correspond to electron shells. Groups (columns) share similar chemical properties.
- Group 1 (alkali metals) — highly reactive, 1 valence electron
- Group 17 (halogens) — very reactive nonmetals, 7 valence electrons
- Group 18 (noble gases) — very stable, full outer shells, rarely react
⚛ Virtual Lab: Atom Builder
Use the sliders to set protons and neutrons. The atom diagram and element info update instantly!
Protons (Z)
6
Neutrons (N)
6
Quick Check
What determines which element an atom is?
Two atoms of the same element with different numbers of neutrons are called: