Thoughtlessness is the noun — the quality itself, or a single act of it. 'His thoughtlessness hurt her.' 'It was an act of pure thoughtlessness.' Making it a noun changes the temperature of a complaint. 'You were thoughtless' points a finger at a person; 'the thoughtlessness of it' puts the fault on the table where everyone can look at it. So this word turns up a lot in writing that wants to be serious but not personal — complaint letters, newspaper columns, school reports: 'the sheer thoughtlessness of leaving litter on the beach'. Usually it is uncountable ('such thoughtlessness'), but you can say 'a thoughtlessness' or 'small thoughtlessnesses' when you mean particular acts. That plural is rare and a little literary; if unsure, stay with the uncountable form. Common partners: sheer thoughtlessness, pure thoughtlessness, out of thoughtlessness ('He did it out of thoughtlessness, not malice' — a sentence that shows the whole point of the word, since it is defined most clearly by what it is NOT). It is a long, formal-sounding word. In speech people usually escape it: 'It was so thoughtless of him' rather than 'His thoughtlessness was striking'. Teach students to recognise it in reading first, and to use the adjective when they talk.