Build an Atom
Drop protons, neutrons and electrons into the nucleus and shells. The element, atomic number, mass number, charge, ion name and isotope all update as you build.
An atom is built from protons and neutrons in the nucleus with electrons in shells around it. The number of protons is the atomic number Z and decides which element it is; protons plus neutrons give the mass number A. Sodium has Z = 11 and A = 23, so it has 23 − 11 = 12 neutrons and, when neutral, 11 electrons.
Particles
Use + / −, or drag a bead from the tray onto the atom. Tap any particle in the picture to take it away.
Drag trayTry an example
Ready-made atoms and ions from the Class 9–11 syllabus.
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Charge & ion
Charge = protons − electrons.
Isotope
Same element, different number of neutrons.
Electron shells, configuration & valency
Shells fill in the order K (2), L (8), M (18), N (32) — the Bohr–Bury scheme.
How chemists write it
The mass number goes on top left, the atomic number below it, and the charge on the top right.
The three subatomic particles
Learn this table — it is asked almost every year in Class 9.
| Particle | Symbol | Charge | Mass | Found | Discovered by |
|---|
Protons decide the element, electrons decide the charge
Three counters, one atom. Everything a Class 9 or 10 chapter on the structure of the atom asks for is derived live from the particles you add.
Add protons
The proton count is the atomic number Z — it tells you which element you have made.
Add neutrons
Protons + neutrons = mass number A. Change only the neutrons and you get an isotope.
Add electrons
They fill shells K, L, M, N. Equal to the protons means a neutral atom; unequal means an ion.
Read and check
Switch to Challenge mode and build what the question asks — the tool marks it and explains.
Atomic number and mass number
Z = number of protons · A = protons + neutrons
Number of neutrons = A − Z. For sodium, A = 23 and Z = 11, so it has 23 − 11 = 12 neutrons.
In a neutral atom the number of electrons is also equal to Z.
Bohr–Bury shell rules
- Shell n holds at most 2n² electrons → K = 2, L = 8, M = 18, N = 32.
- The outermost shell can never hold more than 8 electrons.
- A new shell starts only after the one before it has 8 (or 18) electrons.
Potassium (Z = 19) is therefore 2, 8, 8, 1 — not 2, 8, 9.
Ions: cation vs anion
Lose electrons → fewer negatives → positive ion (cation), e.g. Na → Na⁺ + e⁻.
Gain electrons → negative ion (anion), e.g. Cl + e⁻ → Cl⁻.
Anions of single elements are named with an -ide ending: oxide (O²⁻), chloride (Cl⁻), nitride (N³⁻).
Isotopes and isobars
Isotopes — same Z, different A (same element). Hydrogen has three: protium ¹H, deuterium ²H and tritium ³H.
Isobars — same A, different Z (different elements), e.g. ⁴⁰Ar and ⁴⁰Ca.
Because chlorine is a mix of 75.76% ³⁵Cl and 24.24% ³⁷Cl, its average atomic mass works out to 35.45 u.
Valency from the outer shell
An atom gains, loses or shares electrons to reach 8 in its outermost shell (an octet) — or 2 if it has only the K shell (a duplet).
Magnesium (2, 8, 2) loses 2 → valency 2. Oxygen (2, 6) gains 2 → valency 2. Neon (2, 8) needs nothing → valency 0.
Who found what
1897 J. J. Thomson discovers the electron (plum-pudding model).
1911 Rutherford's alpha-particle scattering shows a tiny, dense, positive nucleus.
1913 Bohr puts the electrons in fixed shells of definite energy.
1932 James Chadwick discovers the neutron, explaining the missing mass.
Build an Atom — your questions
How do you find the number of neutrons in an atom?
Subtract the atomic number from the mass number: neutrons = A − Z. Chlorine-35 has A = 35 and Z = 17, so it has 18 neutrons. If a question gives you the mass number and the number of neutrons instead, it is really giving you Z, because protons = A − neutrons.
What is the Bohr-Bury rule for filling electron shells?
Shell number n holds at most 2n² electrons, so K holds 2, L holds 8, M holds 18 and N holds 32. The outermost shell can never hold more than 8 electrons, and a new shell starts only after the one before it has 8 or 18. Potassium (Z = 19) is therefore 2, 8, 8, 1 and not 2, 8, 9.
What is the difference between a cation and an anion?
An atom that loses electrons has more protons than electrons, so it becomes a positive ion called a cation: Na → Na⁺ + e⁻. An atom that gains electrons becomes a negative ion called an anion: Cl + e⁻ → Cl⁻. Charge equals protons minus electrons, and single-element anions take an -ide ending, such as oxide and chloride.
What are isotopes and isobars?
Isotopes are atoms of the same element with the same atomic number but different mass numbers, such as protium, deuterium and tritium. Isobars have the same mass number but different atomic numbers, so they are different elements — argon-40 and calcium-40. Natural chlorine is 75.76% chlorine-35 and 24.24% chlorine-37, which averages to 35.45 u.
How do you find valency from the electronic configuration?
An atom gains, loses or shares electrons to reach 8 in its outermost shell (an octet), or 2 if it only has the K shell (a duplet). Magnesium is 2, 8, 2 so it loses 2 and has valency 2. Oxygen is 2, 6 so it gains 2, valency 2. Neon is 2, 8 already, so its valency is 0.
Who discovered the electron, the nucleus and the neutron?
J. J. Thomson discovered the electron in 1897 and proposed the plum-pudding model. Rutherford's alpha-particle scattering experiment in 1911 showed that an atom has a tiny, dense, positively charged nucleus. Niels Bohr placed electrons in fixed shells of definite energy in 1913, and James Chadwick discovered the neutron in 1932.
Why is nearly all the mass of an atom in the nucleus?
A proton is about 1836 times heavier than an electron, and a neutron is heavier still, so the electrons contribute almost nothing to the mass even though they fill most of the volume. One unified atomic mass unit (1 u) equals 1.66 × 10⁻²⁷ kg, roughly the mass of a single proton or neutron.