Final answer:
The element with the electron configuration [Ne]3s²3pµ is chlorine (Cl), which is in the third period of the periodic table.
Step-by-step explanation:
An element that has the electron configuration [Ne]3s23p5 is in the third period of the periodic table. This is because the electron configuration notation starts with [Ne], which is the noble gas configuration for Neon, indicating that the first 10 electrons (which fill the 1s, 2s, and 2p orbitals) are identical to those in Neon.
Following this, the electrons start to fill the 3s and 3p orbitals (3 indicating the third energy level), which corresponds to the third period on the periodic table. The electron configuration [Ne]3s23p5 pertains to the element chlorine (Cl), which has 17 electrons, and is in the third period and group 17 (or VIIA, the halogens)
Mendeleev predicted that the gaps in his periodic table represented elements that were yet to be discovered. The discovery of gallium matched his prediction for eka-aluminum, providing support for his periodic table.