Li, Na, and K Energy Levels

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This aufbau for the first three rows of the periodic table follows the Pauli exclusion principle, in that the lowest energy levels are each occupied by two electrons of opposite spin before higher lying levels fill in. The diagrams above show the allowed electron energies as horizontal lines, and represent electrons as arrows that indicate whether the electron is spin-up or spin-down. We see that the Li atom (left) has an unpaired outermost electron in the n = 2, l = 0 state; for Na (center) and K (right), this is replaced by the outer electron being in the n = 3, l = 0 and n = 4, l = 0 states, respectively. These single unpaired electrons outside an inert filled shell impart similar physical and chemical properties to these three elements, which are all in the first column of the periodic table. (Unit: 6)