He or I, but not she

ELEMENT

Think of a basic part of chemistry that starts with the same letter as "he."

This clue plays on the chemical symbols for elements. "He" is the symbol for helium (He), "I" is iodine (I), and "she" isn't a valid element symbol. The answer "ELEMENT" ties it together by referring to the periodic table entries these symbols represent. The wordplay hinges on recognizing these abbreviations as part of chemistry.