Non-metal with high electronegativity.
These elements are most likely to bond with 1 other atom of the same element (to give a diatomic molecule) because non-metals, often only need to obtain 1 or 2 more electrons to complete their outermost energy level and therefore achieve a stable electronic configuration. As the electronegativities of both atoms in the diatomic molecules are equal (since they are atoms of the same element) they are both equally as good at attracting the electron density in the covalent bond towards themselves, so the diatomic molecules is more stable than the elements on their own.
