Chemistry
Confusing Atomic Number and Mass Number
Using the mass number to count protons or to identify the element.
Quick check
Why students make it
Both are numbers attached to an element symbol, so it is easy to mix up their roles.
The rule done right
Atomic number is the proton count and defines the element. Mass number is protons plus neutrons for a specific atom. Neutrons equal mass number minus atomic number.
Related resources
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Atomic Structure Infographic -
Chemistry: Isotopes and Average Atomic Mass Infographic -
Isotopes & Ions Infographic -
Periodic Table Basics Infographic - Atomic Structure & Periodic Trends Cheat Sheet
- Periodic Table Trends Visual Master Cheat Sheet
- Atomic Orbital Filling Diagram Reference Cheat Sheet
- Science: Atoms Elements and the Periodic Table Worksheet
- Chemistry: Introduction to the Periodic Table Worksheet
- Science: Atomic Structure and Electron Configuration Worksheet
- Science: Periodic Table Trends Worksheet
- Chemistry: Reading the Periodic Table: Groups and Periods Worksheet
- Atoms, Molecules & Ions Builder Tool
- Periodic Table Explorer Tool
- Electron Configuration Generator Tool
- Periodic Table Families Explorer Tool