Someone refers to an unspecified member of a group of people. Some one refers to an unspecified member of a group of items or people that is being selected for individual attention.
Someone arrived at the meeting late.
We know that some one of the new robots has
a defect, but we're not sure which.
In the latter usage, some can often be omitted without loss of clarity.
We know that one of the robots has a defect, but we're not
sure which.See also anyone/any one and everyone/every one.
