Mass number — Science definition
The total number of protons plus neutrons in an atom's nucleus.
Where it is taught
- Atomic Structure — Properties of Matter, Physical Science: Matter, Forces & Energy, Grade 8
In use
The mass number counts the protons and neutrons together, since those two supply essentially all of an atom's mass.
Atomic Structure
Questions using mass number
Problem 1
A sodium (Na) atom has atomic number 11 and mass number 23. How many neutrons does it have?
Answer and explanation
12
Neutrons = mass number − atomic number = 23 − 11 = 12.
Problem 2
A magnesium atom has 12 protons and 12 neutrons. What is its mass number?
Answer and explanation
24
Mass number = protons + neutrons = 12 + 12 = 24.
Problem 3
An atom of chlorine has 17 protons and a mass number of 37. How many neutrons does it have?
Answer and explanation
20
Neutrons = mass number − atomic number = 37 − 17 = 20.
Problem 4
An atom has 8 protons, 9 neutrons and 10 electrons. Identify the element, mass number and charge. Explain the reasoning in your own words.
Answer and explanation
An oxygen-17 ion with charge −2.
Atomic number 8 identifies oxygen.
Mass number is 8+9=17; 8 positive and 10 negative charges give a net charge of −2.
Response criteria
- Use the stated conditions.
- Show the evidence or calculation.
- Explain the conclusion and any stated limitation.