The charge on an atom at a metallic surface in an electric field is defined as the field-derivative of the force on the atom, and this is consistent with definitions of effective charge and screening charge. This charge can be found from the shift in the potential outside the surface when the atoms are moved. This is used to study forces and screening on surface atoms of Ag(001) c$(2\times 2)$ -- Xe as a function of external field. It is found that at low positive (outward) fields, the Xe with a negative effective charge of -0.093 $|{e}|$ is pushed into the surface. At a field of 2.3 V Å$^{-1}$ the charge changes sign, and for fields greater than 4.1 V Å$^{-1}$ the Xe experiences an outward force. Field desorption and the Eigler switch are discussed in terms of these results.

4 pages, 1 figure, RevTex (accepted by PRL)