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Atom Builder 2D - Online Create Elements with Protons/Electrons

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Select a mode above, then click on the atom model to add particles. Use Remove mode to delete particles.
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No Element
Awaiting particles...
● Protons (Z)
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● Neutrons (N)
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● Electrons (e⁻)
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Mass Number (A) 0
Charge 0
⚡ Quick Presets:
● Proton (positive) ● Neutron (neutral) ● Electron (negative) — — Orbit shell
Frequently Asked Questions

An atom is the smallest unit of matter that retains the properties of a chemical element. It consists of a nucleus containing protons (positively charged) and neutrons (neutral), surrounded by electrons (negatively charged) orbiting in shells. This interactive 2D builder lets you construct atoms by adding protons, neutrons, and electrons — visually demonstrating atomic structure, ion formation, and isotope creation in real time.

The number of protons in the nucleus — called the atomic number (Z) — uniquely identifies a chemical element. For example, 1 proton = Hydrogen, 6 protons = Carbon, 8 protons = Oxygen, and 26 protons = Iron. Changing the proton count transforms the element entirely. Neutrons affect the isotope but not the elemental identity.

Isotopes are variants of the same element that have the same number of protons but different numbers of neutrons. For instance, Carbon-12 has 6 protons + 6 neutrons, while Carbon-14 has 6 protons + 8 neutrons. In this builder, you can experiment with isotope creation by changing the neutron count independently. The Mass Number (A) = protons + neutrons reflects the isotope's total nucleon count.

Electrons fill shells following the 2n² rule: the K-shell (n=1) holds up to 2 electrons, the L-shell (n=2) holds up to 8, the M-shell (n=3) holds up to 18, and the N-shell (n=4) holds up to 32. Electrons fill from the innermost shell outward. The outermost electrons (valence electrons) determine chemical reactivity. In this builder, electrons automatically distribute across visible orbit rings following this pattern.

An ion is formed when an atom has an unequal number of protons and electrons, resulting in a net electrical charge. If protons > electrons, it's a cation (positive charge). If electrons > protons, it's an anion (negative charge). Use this builder to create ions by adjusting the electron count independently of protons — the charge badge will update automatically (e.g., Na⁺, Cl⁻, O²⁻).

Nuclear stability depends on the neutron-to-proton (N/Z) ratio. For light elements (Z ≤ 20), a ratio near 1:1 is ideal. For heavier elements, the ratio increases to about 1.5:1 to offset proton repulsion. When the ratio deviates significantly, the nucleus becomes radioactive and may undergo decay (alpha, beta, or gamma emission). This builder displays a real-time stability indicator to help you explore these nuclear physics concepts.
Key Atomic Concepts to Explore:
  • Atomic Number (Z): The proton count — defines the element's identity on the periodic table.
  • Mass Number (A): Total nucleons (protons + neutrons) — used to identify isotopes like ¹²C or ¹⁴C.
  • Valence Electrons: Electrons in the outermost shell — determine bonding behavior and chemical properties.
  • Octet Rule: Atoms tend to gain, lose, or share electrons to achieve 8 valence electrons (a full outer shell).
  • Noble Gases: Elements like He, Ne, and Ar have full outer shells, making them chemically inert.
  • Nuclear Force: The strong force binds protons and neutrons together, overcoming proton-proton electrostatic repulsion.
  • Beta Decay: An unstable neutron can transform into a proton, emitting an electron and an antineutrino.
  • Alpha Decay: Heavy unstable nuclei may emit a helium-4 nucleus (2 protons + 2 neutrons).
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