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Add a proton and you change the element. Add a neutron and you change the isotope. Add an electron and you make an ion. Everything below updates the moment you do.
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Turn it on for twelve build-this-nuclide puzzles.
What you are looking at
The proton count — the atomic number, Z — is the element. Six protons is carbon, and nothing else can be carbon. Change Z and you have transmuted one element into another, which is exactly what alchemists wanted and what nuclear reactors do.
Neutrons add mass and, crucially, dilute the electrostatic repulsion between protons. Too few or too many and the nucleus decays. Same element, different neutron count = a different isotope: carbon-12 is stable forever, carbon-14 has a 5,730-year half-life.
Chemistry is entirely a story about electrons. Remove one and you get a positive cation; add one and you get a negative anion. Atoms gain, lose and share electrons to reach a full outer shell — that single drive explains almost every reaction you will ever study.
The stability verdict
Plot every known nuclide with protons on one axis and neutrons on the other and the stable ones form a narrow curved band — the valley of stability. Light elements sit near N = Z. As Z rises, protons repel each other more strongly and the nucleus needs proportionally more neutrons to stay bound, so the band bends upward toward N ≈ 1.5 Z.
Stray above the band and the nuclide is neutron-rich: a neutron converts to a proton and it emits an electron (β⁻ decay). Stray below and it is proton-rich, so a proton converts to a neutron and it emits a positron (β⁺ decay). Past bismuth (Z = 83) the valley simply runs out — no arrangement of neutrons can hold that many protons together indefinitely.
Neutrons (vertical) against protons (horizontal) · your build is marked
Common questions
An isotope differs in neutron count — same element, different mass, same chemistry. An ion differs in electron count — same element, same mass, very different chemistry. Carbon-12 and carbon-14 are isotopes of each other; Na and Na⁺ are an atom and its ion. Change protons instead and you have changed element entirely.
Classical physics says an orbiting charge radiates energy and should collapse in about 10⁻¹¹ seconds. It does not, and that failure is what forced quantum mechanics into existence. An electron in an atom does not orbit at all: it occupies a standing wave with a fixed lowest energy. There is no lower state to fall into. The rings drawn here are a convenience — the orbital viewer on any element page shows the real probability cloud.
Shell n holds at most 2n² electrons: 2, 8, 18, 32 and so on. But shells do not fill strictly in order — 4s fills before 3d because it sits at lower energy, which is the Madelung rule this builder follows. That ordering is exactly why the transition metals form a block in the middle of the periodic table.
Iron-56 (and nickel-62) sit at the peak of the binding-energy-per-nucleon curve. Fusing anything lighter releases energy; splitting anything heavier releases energy; iron does neither. That is why fusion in a massive star stops at iron, why the core then collapses, and why every element heavier than iron in your body was made in a supernova or a neutron-star collision rather than ordinary stellar burning.
Not here, and not in a laboratory yet either. This builder stops at 118 because that is where the periodic table currently ends. Teams in Japan, Russia and the United States have been firing beams at actinide targets for over a decade trying to make 119 and 120; the nuclei fall apart faster than any detector can register them.