Mythical woman after whom element 41 is named / MON 2-22-21 / Key for Debussy's fille aux cheveux de lin

Constructor: Barbara Lin

Relative difficulty: Normal Monday except for KYIV, what the hell was that!?

THEME: POOL (64A: Game suggested by the ends of 20-, 38- and 55-Across and 11- and 34-Down) — themers end in words related to billiards:

Theme answers:

• GIMME A BREAK (20A: "Jeez, lighten up, will ya!")
• ON CUE (38A: As expected)
• OUT OF POCKET (55A: Like medical expenses you pay for yourself)
• TOWEL RACK (11D: Bar from the bathroom?)
• ON THE BALL (34D: Alert)

Word of the Day: Niobium (6D: Mythical woman after whom element 41 is named) —

Niobium, also known as columbium, is a chemical element with the symbol Nb (formerly Cb) and atomic number 41. Niobium is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to that of pure titanium, and it has similar ductility to iron. Niobium oxidizes in the earth's atmosphere very slowly, hence its application in jewelry as a hypoallergenic alternative to nickel. Niobium is often found in the minerals pyrochlore and columbite, hence the former name "columbium". Its name comes from Greek mythology, specifically Niobe, who was the daughter of Tantalus, the namesake of tantalum. The name reflects the great similarity between the two elements in their physical and chemical properties, making them difficult to distinguish. (wikipedia)

Disappointing on all levels, this one. The theme is a remedial type that feels like it was probably done a hundred times in 90s and 00s puzzles. Last words are all related to [X] ... ok, well, the answers you use should be sizzling and the revealer should probably be colorful and the grid should definitely be whistle-clean ... and none of these things are true. BREAK isn't even like the others, in that it's an act, not a piece of equipment. ON CUE adds nothing and should probably have been ditched. All it does is make the fill in the center of the grid really awful. No one's gonna be begging for a fifth themer that's five letters long. There is no such desire among solvers. Make your themers hot and your grid immaculate and no one's gonna notice if you have four themers or three themers or five themers. ON CUE adds nothing, takes away a ton. That ON CUE section (i.e. the middle) is the worst part of the grid, fill-wise (except perhaps the SW, about which, more below). No good way to clue POCUS, so you get a laughably easy fill-in-the-blank. EAUS!??!!? Wow, really Really thought the plural of EAU was EAUX ('cause it is). EAUS is garbage, though only slightly more garbage-y than OHIOU, which looks ridiculous. It's a Monday puzzle with a simple theme, why is this grid not totally smooth? No excuse. Theme is boring, executed poorly, and the fill is subpar all over.