What the Bohr model shows
In 1913 Niels Bohr pictured the atom as a tiny, dense nucleus surrounded by electrons travelling in fixed circular shells (energy levels). Each shell sits at a set distance and holds only so many electrons before the next one begins to fill. The builder above draws this for any element: choose an atomic number (Z, the number of protons) with the slider or the +/− buttons, and the nucleus label and concentric rings of electron dots update instantly. The default is carbon (Z = 6), with shells holding 2, 4 electrons.
Bohr’s picture is a simplification — modern chemistry replaces fixed orbits with fuzzy orbitals (see electron configuration). But the shell count and the outer-shell electron number it gives are reliable for the main-group (s- and p-block) elements students meet first, which is why the Bohr model remains the first diagram every student draws. (Transition metals are subtler — their inner d-electrons also take part in bonding.)
Reading the diagram
- The center is the nucleus. It carries the element’s Z protons (plus neutrons, which set the atomic mass).
- Each ring is one shell, filled from the inside out. The dot counts come straight from verified data, so they follow the real filling order rather than a naive 2-8-18 guess — copper and chromium, for instance, are genuinely irregular.
- The outer ring is highlighted. For main-group elements these outer electrons are the valence electrons that decide how an atom bonds and reacts (transition metals are more involved — their d-electrons join in).
Patterns to look for
Step through a whole period (a row of the periodic table) and watch one shell fill one electron at a time. Step down a group (a column) and watch a new shell appear while the outer-electron count stays the same — that shared outer count is why a group behaves alike. Jump to the noble gases like neon or argon to see a full, stable outer shell, which drives the broader periodic trends in reactivity and size.
Compare a few atoms directly: hydrogen with its single electron, oxygen with 6 outer electrons hungry to bond, sodium with a lone outer electron it readily gives away, or iron. Every element links to its own page from the full elements list.
Using this with a class
Project the builder and have students predict the shell pattern before you change Z, then check it together — a fast formative-assessment loop. Or ask them to find every element with exactly one outer-shell electron. The widget is free to embed on your own site or LMS with the snippet below.