The periodic table, electron shells, and orbitals

Differences in chemical reactivity between elements are based on the number and spatial distribution of their electrons.

The Bohr model shows the atom as a central nucleus containing protons and neutrons, with the electrons in circular electron shells at specific distances from the nucleus, similar to planets orbiting around the sun. Each electron shell has a different energy level, with those shells closest to the nucleus being lower in energy than those farther from the nucleus.