  Unpaired electronic spins at diamond surfaces are ubiquitous and can lead to
excess magnetic noise. They have been observed in several studies to date, but
their exact chemical nature is still unknown. We propose a simple model to
explain the existence and chemical stability of surface spins associated with
the $sp^3$ dangling bond on the (100) diamond surface using density functional
theory. We find that the (111) facet, which is naturally generated at a step
edge of (100) crystalline diamond surface, can sterically protect a spinful
defect. Our study reveals a mechanism for annihilation of these surface spins
upon annealing, consistent with recent experimental results. We also
demonstrate that the Fermi-contact term in the hyperfine coupling is not
negligible between the surface spins and the surrounding nuclear spins, and
thus \textit{ab initio} simulation can be used to devise a sensing protocol
where the surface spins act as reporter spins to sense nuclear spins on the
surface.
