The structure of an atom
An atom is a tiny, dense nucleus of positively charged protons and neutral neutrons, surrounded by negatively charged electrons. The number of protons — the atomic number Z — defines the element and, in a neutral atom, equals the number of electrons. Almost all the mass sits in the nucleus, while the electrons occupy the vastly larger surrounding volume in energy levels or shells.
From the Bohr model to quantum mechanics
Niels Bohr's 1913 model pictured electrons in fixed circular orbits with quantized energies, and it successfully explained the spectral lines of hydrogen. It is the clear, teachable picture this simulator uses. The modern quantum-mechanical model refines it: electrons are not on sharp orbits but in orbitals — three-dimensional probability clouds where an electron is most likely to be found. Each shell n holds up to 2n² electrons.
How to use the simulator
Set the element or the proton count. The tool assembles the nucleus and fills the shells outward (2, 8, 18, …), showing the electron configuration. Watching the shells fill makes the structure of the periodic table — periods and groups — visible.
Note: the Bohr model is a simplified, semi-classical picture. Real atoms obey quantum mechanics, where electrons occupy orbitals (probability distributions), not sharp circular orbits; the Bohr shells remain an excellent first model of chemical periodicity.
Frequently asked questions
What is the structure of an atom?
An atom has a dense central nucleus of protons and neutrons, surrounded by electrons. The number of protons (the atomic number) defines the element, and in a neutral atom it equals the number of electrons. Nearly all the mass is in the nucleus.
What is the Bohr model?
The Bohr model, proposed by Niels Bohr in 1913, pictures electrons orbiting the nucleus in fixed circular shells with quantized energies. It explained the hydrogen spectrum and is still a useful teaching model, though quantum mechanics gives the fuller picture.
What is an electron shell?
An electron shell is an energy level around the nucleus where electrons reside. Shells are labelled by the principal quantum number n (n = 1, 2, 3, ...), with higher shells farther from the nucleus and at higher energy.
How many electrons fit in each shell?
A shell with principal quantum number n holds up to 2n^2 electrons: 2 in the first shell, 8 in the second, 18 in the third, and so on. This pattern underlies the rows of the periodic table.
References
- N. Bohr (1913), "On the Constitution of Atoms and Molecules", Philosophical Magazine 26(151):1–25.
- D. J. Griffiths, Introduction to Quantum Mechanics (Cambridge University Press) — the quantum model of the atom.
- IUPAC, Periodic Table of the Elements and electron-configuration conventions.