I'm talking about the chemical shift. In XPS test, oxidation leads to the increase of binding energy. This is determined by: 1. attraction between nuclear and the electron 2. shielding effect from outer-shell electrons My question is : What does shielding effect mean? Shield what? The positive charge in nuclear attract 1s electron. Also, 1s electron will be repelled by outer shell electrons. It seems both of these two forces have the same direction. I'm a bit confused despite having understood this .... can anyone help?