The interaction which occurs when a hydrogen atom, covalently bonded to an electronegative atom (as in A–H), interacts with another atom to form the aggregate A–H…Y. The shortest and strongest bond is indicated as A–H, while the secondary and weaker interaction is written as H…Y. Thus A–H is a proton donor, while (Y) is a proton acceptor which often contains lone pair electrons and can act as a base. The strongest hydrogen bonds are formed between the most electronegative (A) atoms such as fluorine, nitrogen, and oxygen which interact with (Y) atoms having electronegativity greater than that of hydrogen (C, N, O, S, Se, F, Cl, Br, I). The weakest of hydrogen bonds are formed by acidic protons of C–H groups, as in chloroform and acetylene, and by olefinic and aromatic π-electrons acting as (Y).