How many neutrons does carbon-12 have? Is it stable?

Z=6 for carbon; neutrons = 12−6 = 6.

What happens when you add a neutron to make carbon-13 vs carbon-14?

Carbon-13 is stable; carbon-14 undergoes beta decay.

Why does iron-56 have the highest binding energy per nucleon?

It sits at the optimal balance of nuclear force vs. Coulomb repulsion.

Pro 🔒~20 min

Build a Nucleus

Assemble protons and neutrons to explore nuclear stability

How it works

Build a Nucleus demonstrates a key principle: Atomic nuclei are held together by the strong nuclear force, which overcomes electromagnetic repulsion between protons. Atomic nuclei are held together by the strong nuclear force, which overcomes electromagnetic repulsion between protons. Stability depends on the neutron-to-proton ratio. Too many or too few neutrons lead to radioactive decay. Binding energy per nucleon peaks near iron-56, explaining why fusion releases energy for light elements and fission releases energy for heavy elements.

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Step-by-step

Click to add protons (red) and neutrons (blue) to the nucleus.
Watch the stability indicator and binding energy chart.
Unstable nuclei show the predicted decay mode (alpha, beta+, beta−, or gamma).

Key formulas

$BE = \left[Z m_p + N m_n - M\right]c^2$ Binding energy
$BE/A$ Binding energy per nucleon
${}^A_Z X$ Nuclide notation

Frequently asked questions

How many neutrons does carbon-12 have? Is it stable?: Z=6 for carbon; neutrons = 12−6 = 6.
What happens when you add a neutron to make carbon-13 vs carbon-14?: Carbon-13 is stable; carbon-14 undergoes beta decay.
Why does iron-56 have the highest binding energy per nucleon?: It sits at the optimal balance of nuclear force vs. Coulomb repulsion.