It can be one word or two words. Both ways are right. Cannot and can not. Or can't for short.
While both forms are acceptable, some dictionaries recognize only the combined form, except when the "not" is not part of another construction (e.g. not only, not do something). In short, cannot cannot be considered wrong, while can not can.
It is one word.
Its one word
One, it's a compound word.
It is one word.
one word
Schoolchildren, one word.
It is one word
One word. 'Sandstorm.'
It is one word.
one word
It is one word.
It is one word.