An electron shell is a region around an atom's nucleus where electrons are likely to be found. These shells are organized in layers, with each layer corresponding to a specific energy level. The first shell can hold up to 2 electrons, the second can hold up to 8, and so on. The arrangement of electrons in these shells determines the chemical properties of the element. Electrons in the outermost shell, known as the valence shell, play a crucial role in chemical bonding and reactions. Elements in the same group of the periodic table, such as sodium and potassium, have similar valence shell configurations, which leads to similar chemical behaviors.