Final answer:
The electronegativity difference between two atoms with values of 2.6 and 4.0 is 1.4, which typically signifies a polar covalent bond, where the electrons are unequally shared between the atoms.
Step-by-step explanation:
The electronegativity difference between two atoms is a measure of how strongly they attract electrons in a bond. In the case where one atom has an electronegativity of 2.6 and the other 4.0, the difference is 4.0 - 2.6 = 1.4. This difference falls into the range typically associated with polar covalent bonds, as bonds with electronegativity differences greater than approximately 1.7 are generally considered to be ionic. The bond between two atoms with an electronegativity difference of 1.4, such as O and H (oxygen and hydrogen), would therefore be expected to be very polar, but still covalent.