An element’s electrons are located in shells around the nucleus. The number of electron shells an element can have is equal to its atomic number. The periodic table arranges elements by atomic number, therefore an element’s position on the periodic table indicates how many electron shells it has. The number of electrons in each shell is determined by the shell’s energy level. The first shell can hold up to two electrons, the second shell can hold up to eight, and so on.
Knowing how many electrons are in an element is important because it helps to determine the element’s chemical properties. For example, the number of electrons in an element’s outermost shell determines its valence, which is its ability to combine with other elements. Elements with a full outermost shell are unreactive, while elements with an incomplete outermost shell are more reactive.
