Short answers, pulled from the story.
Swedish chemist Carl Gustaf Mosander discovered lanthanum in 1839 while examining cerium nitrate. Another discovery occurred that same year when Axel Erdmann found the element in a mineral from Låven island in Norway.
Scientists finally isolated pure lanthanum metal in 1923 after decades of complex separation processes. Pure metal remained elusive for many years following its initial discovery in 1839.
The stable isotope lanthanum-139 makes up 99.911 percent of all natural lanthanum found in the Earth's crust. Scientists produce this isotope through the s-process inside low to medium mass stars and during the r process within core collapse supernovae.
Reaction with water produces lanthanum hydroxide accompanied by heat evolution and a hissing sound. The metal corrodes completely within a year as its oxide flakes off like iron rust when exposed to air.
Mining operations target minerals such as monazite and bastnäsite where lanthanum composes about a quarter of the lanthanide content. Bastnäsite usually lacks thorium making purification of light lanthanides less involved compared to other sources.