The stability and electronic structure of a nanowire are studied by fi
rst-principles calculations. The wire consists of a single depassivate
d silicon dimer row on the hydrogen passivated Si(001) 2x1 surface. We
predict that sodium atoms evaporated onto this surface stick preferen
tially at the depassivated row and partially fill the empty one-dimens
ional states of this row, This leads to a thin metallic wire of atomic
size dimensions. At room temperature the sodium atoms are mobile alon
g the depassivated row; they become immobile at temperatures before si
milar to 120 K.