Inside the atom
Atoms contain protons, neutrons and electrons. Almost all their mass is concentrated in a tiny nucleus.
Three particles, different roles
Protons and neutrons form the nucleus. Electrons occupy the space around it. A neutral atom has equal numbers of protons and electrons.
2 protons + 2 neutrons → mass number 4. Both isotopes are helium because they have two protons.
What defines an element?
Atomic number Z is the number of protons and defines the element. Mass number A counts protons plus neutrons. Isotopes have the same Z but different neutron counts. Ion formation changes electron count rather than the nucleus.
Relative atomic mass is a weighted mean of isotope masses, so it is not generally an integer. A mass spectrum separates ions by mass-to-charge ratio. For singly charged atomic ions, the peaks can reveal isotopic masses and relative abundances.
A weighted atomic mass
Explore the relationship
A two-isotope example: the lighter isotope has mass 20 u. Abundances sum to 100%.
Tap or focus a plotted point to read its values.
View plotted data as a table
| Series | Heavier-isotope abundance / % | Mean atomic mass / u |
|---|---|---|
| Mean atomic mass / u | 0 | 20 |
| Mean atomic mass / u | 1.66667 | 20.0333 |
| Mean atomic mass / u | 3.33333 | 20.0667 |
| Mean atomic mass / u | 5 | 20.1 |
| Mean atomic mass / u | 6.66667 | 20.1333 |
| Mean atomic mass / u | 8.33333 | 20.1667 |
| Mean atomic mass / u | 10 | 20.2 |
| Mean atomic mass / u | 11.6667 | 20.2333 |
| Mean atomic mass / u | 13.3333 | 20.2667 |
| Mean atomic mass / u | 15 | 20.3 |
| Mean atomic mass / u | 16.6667 | 20.3333 |
| Mean atomic mass / u | 18.3333 | 20.3667 |
| Mean atomic mass / u | 20 | 20.4 |
| Mean atomic mass / u | 21.6667 | 20.4333 |
| Mean atomic mass / u | 23.3333 | 20.4667 |
| Mean atomic mass / u | 25 | 20.5 |
| Mean atomic mass / u | 26.6667 | 20.5333 |
| Mean atomic mass / u | 28.3333 | 20.5667 |
| Mean atomic mass / u | 30 | 20.6 |
| Mean atomic mass / u | 31.6667 | 20.6333 |
| Mean atomic mass / u | 33.3333 | 20.6667 |
| Mean atomic mass / u | 35 | 20.7 |
| Mean atomic mass / u | 36.6667 | 20.7333 |
| Mean atomic mass / u | 38.3333 | 20.7667 |
| Mean atomic mass / u | 40 | 20.8 |
| Mean atomic mass / u | 41.6667 | 20.8333 |
| Mean atomic mass / u | 43.3333 | 20.8667 |
| Mean atomic mass / u | 45 | 20.9 |
| Mean atomic mass / u | 46.6667 | 20.9333 |
| Mean atomic mass / u | 48.3333 | 20.9667 |
| Mean atomic mass / u | 50 | 21 |
| Mean atomic mass / u | 51.6667 | 21.0333 |
| Mean atomic mass / u | 53.3333 | 21.0667 |
| Mean atomic mass / u | 55 | 21.1 |
| Mean atomic mass / u | 56.6667 | 21.1333 |
| Mean atomic mass / u | 58.3333 | 21.1667 |
| Mean atomic mass / u | 60 | 21.2 |
| Mean atomic mass / u | 61.6667 | 21.2333 |
| Mean atomic mass / u | 63.3333 | 21.2667 |
| Mean atomic mass / u | 65 | 21.3 |
| Mean atomic mass / u | 66.6667 | 21.3333 |
| Mean atomic mass / u | 68.3333 | 21.3667 |
| Mean atomic mass / u | 70 | 21.4 |
| Mean atomic mass / u | 71.6667 | 21.4333 |
| Mean atomic mass / u | 73.3333 | 21.4667 |
| Mean atomic mass / u | 75 | 21.5 |
| Mean atomic mass / u | 76.6667 | 21.5333 |
| Mean atomic mass / u | 78.3333 | 21.5667 |
| Mean atomic mass / u | 80 | 21.6 |
| Mean atomic mass / u | 81.6667 | 21.6333 |
| Mean atomic mass / u | 83.3333 | 21.6667 |
| Mean atomic mass / u | 85 | 21.7 |
| Mean atomic mass / u | 86.6667 | 21.7333 |
| Mean atomic mass / u | 88.3333 | 21.7667 |
| Mean atomic mass / u | 90 | 21.8 |
| Mean atomic mass / u | 91.6667 | 21.8333 |
| Mean atomic mass / u | 93.3333 | 21.8667 |
| Mean atomic mass / u | 95 | 21.9 |
| Mean atomic mass / u | 96.6667 | 21.9333 |
| Mean atomic mass / u | 98.3333 | 21.9667 |
| Mean atomic mass / u | 100 | 22 |
Explore: Change isotope abundance. Why does the mean move closer to the more abundant isotope?
A ²⁴Mg²⁺ ion has atomic number 12.
- It has 12 protons and 24 − 12 = 12 neutrons.
- It has lost two electrons, leaving 10.
Assumed knowledge
Atomic charge and percentages.
Learning checkpoints & source
YOUR LEARNING CHECKPOINT- Count protons, neutrons and electrons in atoms and ions.
- Calculate relative atomic mass from isotope abundances.