The effects of hydrogen, oxygen, and nitrogen atomic chemisorption on capped armchair (5, 5) single-walled carbon nanotubes (SWCNT) are investigated by first-principles calculations based on the density functional theory aimed at the CNT based fuel cell applications. O or N chemisorption could break C-C bond to form doping type structure. C-C bonds are weakened from H chemisorption, favoring hydrogen storage. Both C-adatom and related C-C bond lengths fluctuate from the cap top to the tube for each type of adsorbate. There is a total amount of about 1.0 e charge transfer between N or O atom and the carbon atoms, and the catalytic activity is expected to be higher with N adsorption around the cap top. The adsorption energies and work functions also vary with the adsorption at different sites. Atomic chemisorptions are more stable on the cap than on the tube due to smaller local curvature radius. The work functions increase to above 5.0 eV with the adsorption of N and O, and drop below 4.8 eV for H adsorption, comparing with 4.89 eV for the clean tube. DOS study reveals orbital information for electrons of adatom contributed to the valence bands and the conduction bands. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.