kòng is the everyday word for a free slot — in time (有空, 没空, 抽空) or in space (空白, 空格, 空缺, 空隙). Northern speech adds 儿 to get 空儿 — same meaning, more colloquial.
mistakes
Watch the tone-by-meaning split. 空想 (daydream — abstract 'empty thought') is 1st tone, but 空缺 (a vacancy you could fill) is 4th tone. 落空 'to fall through' takes kōng (becoming nothing), while 填空 'fill in the blank' takes kòng (filling a slot).
Cave radical on top — pictures a dwelling-pit roofed by 宀 with two leg-strokes for the opening. It marks 空 as a member of the cave/hollow family alongside 穿 (pierce through), 窗 (window) and 究 (probe deeply). The hollow inside the cave gave 空 its 'empty' meaning.
工 sits under the cave to give the sound, which then nasalised into kōng. The same 工-phonetic threads through 红, 江, 项 and 攻 — a useful little family. Here it is purely sonic; the meaning of 'empty' all comes from the 穴 above.
