Mendeleev corrected uranium’s valence to 6 in his periodic system.
Before Mendeleev’s revision, uranium was commonly assigned a valence of 3 and an atomic weight near 120. Those values did not fit the periodic relationships he was developing. Mendeleev therefore doubled the estimated atomic weight to about 240 and changed the valence to 6.
The correction was significant because valence helped determine where an element belonged among chemically related groups. Uranium’s revised values placed it more consistently within Mendeleev’s arrangement and showed how the periodic system could expose errors in accepted measurements and formulas.
Mendeleev’s revised atomic weight was close to the modern value: uranium’s relative atomic mass is approximately 238. The historical valence should not be confused with every oxidation state uranium can display today. Modern chemistry commonly discusses uranium in oxidation states such as +4 and +6, but the specific correction in Mendeleev’s early periodic work was from 3 to 6.