The Bohr model, proposed by Niels Bohr in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits (shells) at fixed energy levels. Each shell can hold a specific maximum number of electrons, determined by the formula 2n², where n is the shell number. Although later replaced by quantum mechanical models, the Bohr model remains a powerful visual tool for understanding atomic structure and electron configuration.

According to the 2n² rule: K shell (n=1) holds up to 2 electrons, L shell (n=2) holds up to 8, M shell (n=3) holds up to 18, and N shell (n=4) holds up to 32 electrons. However, for the first 20 elements shown in this viewer, the outermost shell never exceeds 8 electrons due to the octet rule governing chemical stability.

Electrons occupy discrete energy levels (shells) because they can only exist at certain quantized energy states. Lower shells (closer to the nucleus) have lower energy and fill first. This is known as the Aufbau principle. Electrons in higher shells possess more energy and are less tightly bound to the nucleus, making them more available for chemical bonding.

Valence electrons are the electrons in the outermost shell of an atom. They determine an element's chemical properties, reactivity, and bonding behavior. Elements with the same number of valence electrons (same group in the periodic table) exhibit similar chemical characteristics. For example, all alkali metals (Li, Na, K) have exactly 1 valence electron, making them highly reactive.

While brilliant for its time, the Bohr model has several limitations: it cannot accurately describe multi-electron atoms beyond hydrogen; it fails to explain the Zeeman effect (splitting of spectral lines in magnetic fields); it does not account for electron wave-particle duality; and it incorrectly assumes electrons orbit in fixed circular paths rather than existing in probability clouds (orbitals) as described by modern quantum mechanics.

Electron configuration notation lists the distribution of electrons across shells. For example, Sodium (Na, atomic number 11) has the configuration 2, 8, 1 — meaning 2 electrons in the K shell, 8 in the L shell, and 1 in the M shell. This viewer displays configurations in this simple comma-separated format, which is ideal for understanding the Bohr model and predicting an element's chemical behavior.