Atoms of the same element that have different masses are called isotopes. Different masses result when there are different numbers of neutrons. All atoms of a particular element have the same number...
Definition of absent. 1 : not present at a usual or expected place : missing was absent from class today sharing memories of absent friends She was conspicously absent [=her absence was very noticeable] at the meeting. absently. adverb He replied absently to her question. Examples of absent in a sentence.
unidentified - being or having an unknown or unnamed source; a poem by an unknown author; corporations responsible to nameless owners; an unnamed donor. nameless, unnamed, unknown. anon., anonymous - having no known name or identity or known source; anonymous authors; anonymous donors; an anonymous gift.