Oligonucleotides with a novel sugar-phosphate backbone containing at least one 2′-arabino-fluoronucleoside and an internucleoside 3′-NH—P(═O)(OR)—O-5′ linkage, where R is a positively charged counter ion or hydrogen, and methods of synthesizing and using the inventive oligonucleotides are provided. The inventive phosphoramidate 2′-arabino-fluorooligonucleotides have a high RNA binding affinity to complementary nucleic acids and are base and acid stable.