Which element has the atomic number 11?

The element with the atomic number 11 is Sodium (Na). Atomic numbers are assigned based on the number of protons in an atom's nucleus, which defines the identity of the element. Sodium is located in group 1 of the periodic table, which is known as the alkali metals. This group is characterized by elements that have a single electron in their outermost shell, leading to high reactivity.

The atomic number is crucial for determining the electron configuration of an element. For Sodium, with an atomic number of 11, the electron configuration is 1s² 2s² 2p⁶ 3s¹. This indicates that Sodium has 11 electrons distributed among the energy levels, with the outermost shell containing one electron, making it prone to losing that electron and forming positive ions.

Understanding that atomic numbers correlate directly to the number of protons and electrons helps in identifying elements based on their position in the periodic table. Meanwhile, the other options present different elements with lower atomic numbers: Potassium has an atomic number of 19, Lithium has an atomic number of 3, and Magnesium has an atomic number of 12.