Mass Number Examples in Chemistry
Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Mass Number.
This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.
Concept Recap
The total count of protons and neutrons (collectively called nucleons) in an atom's nucleus, always a whole number, used to identify specific isotopes of an element.
How heavy the nucleus is — each proton and neutron contributes about 1 atomic mass unit.
Read the full concept explanation →How to Use These Examples
- Read the first worked example with the solution open so the structure is clear.
- Try the practice problems before revealing each solution.
- Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea: Mass number tells you the isotope; atomic number tells you the element.
Common stuck point: Mass number is always a whole number; atomic mass (weighted average) usually isn't.
Sense of Study hint: When calculating mass number, simply add protons and neutrons. First find the atomic number Z (number of protons) from the periodic table. Then determine the number of neutrons from the problem or isotope notation. Finally, add them: A = Z + N.
Worked Examples
Example 1
easySolution
- 1 Mass number A = \text{protons} + \text{neutrons} = 17 + 18 = 35.
- 2 Atomic number Z = 17 corresponds to chlorine (Cl).
- 3 This isotope is written as {}^{35}_{17}\text{Cl}.
Answer
Example 2
mediumExample 3
mediumPractice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
easyExample 2
easyRelated Concepts
Background Knowledge
These ideas may be useful before you work through the harder examples.