Mass number (A) = protons + neutrons. Always a whole number. Electrons don’t count.
3 steps, 1 worked example, 4 practice problems. About 10 minutes. Free.
The route · 3 steps
Protons come from here. Tap any element.
Tap an element to see its protons.
Swipe sideways for the full table.
Asked for "mass number" or "nucleons," with the element (or protons) and neutrons given.
Clue words marked:
An atom of sodium has 12 neutrons. What is its mass number?
Not this type?
The idea
| What you see | Example | Mass number |
|---|---|---|
| Isotope name | carbon-14 | after the dash: 14 |
| Isotope symbol | 146C | top number: 14 |
| Particle counts | 6 protons, 8 neutrons | 6 + 8 = 14 |
| Atom | Protons (Z) | Neutrons | Mass number |
|---|---|---|---|
| lithium | 3 | 4 | 7 |
| carbon | 6 | 6 | 12 |
| carbon | 6 | 8 | 14 |
| oxygen | 8 | 10 | 18 |
| chlorine | 17 | 20 | 37 |
Mass number (A)
One isotope. Whole number.
Chlorine-37: A = 37
Average atomic mass on the table
Average of isotopes. Decimal.
Chlorine: 35.45
On the periodic table: Li 3, C 6, O 8, Na 11, Cl 17 protons. Neutrons aren’t on the table.
Worked example · 3 steps
An atom of sodium has 12 neutrons. What is its mass number?
Given.
| Protons | + Neutrons | = Mass number |
|---|---|---|
| 11 | 12 | 23 |
23: sodium-23, 2311Na.
Try first, then check. Hints if you need them.
Problem 1 of 4 · warm-up
An atom has 8 protons and 9 neutrons. What is its mass number?
Both nucleus counts are given.
Add the protons and the neutrons.
Your path:
Answer: 17
What you know: 8 protons, 9 neutrons. What you want: the mass number.
8O16.00On the periodic table: oxygen (O) = 8 protons. Oxygen-17.
Answer: 17
Problem 2 of 4
An atom of chlorine has 20 neutrons. What is its mass number?
Protons aren’t given. The periodic table has them.
Chlorine’s atomic number from the periodic table, plus 20.
Your path:
Answer: 37
What you know: chlorine, 20 neutrons. What you want: the mass number.
Watch out: 35.45 is an average, not this atom’s mass number.
17Cl35.45On the periodic table: chlorine (Cl) = 17 protons.
Answer: 37
Problem 3 of 4 · exam-style wording
A neutral atom contains 26 electrons and 30 neutrons. How many nucleons does it have (what is its mass number)?
Nucleons = protons + neutrons. Neutral: electrons = protons.
26 protons + 30 neutrons.
Your path:
Answer: 56
What you know: neutral atom, 26 electrons, 30 neutrons. What you want: the mass number.
Watch out: don’t add the electrons too (82).
26Fe55.85On the periodic table: 26 is iron (Fe). Iron-56.
Answer: 56
Problem 4 of 4 · watch for the trap
A magnesium ion, Mg2+, has 10 electrons and 12 neutrons. What is its mass number?
It’s an ion: electrons ≠ protons. Use the element.
Magnesium’s atomic number from the periodic table, plus 12. Ignore the electrons.
Your path:
Answer: 24
What you know: Mg2+, 10 electrons, 12 neutrons. What you want: the mass number.
Watch out: electrons aren’t protons here (22 is wrong).
12Mg24.31On the periodic table: magnesium (Mg) = 12 protons.
Answer: 24
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Next up: Run it backward: how to find the number of neutrons from the mass number.
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