Group-V elemental monolayers including phosphorene are emerging as promising 2D materials with semiconducting electronic properties. Here, we present the results of first principles calculations on stability, mechanical and electronic properties of 2D antimony, antimonene. Our calculations show that {\alpha} and \b{eta} allotropes of antimonene are stable and semiconducting. The {\alpha}-Sb has a distorted atomic structure with two atomic sub-layers and \b{eta}-Sb has a buckled hexagonal lattice. The calculated Raman spectra and STM images have distinct features identifying in-plane and out-of-plane vibrating modes in both allotropes. The \b{eta}-Sb has nearly isotropic mechanical properties while {\alpha}-Sb shows strongly anisotropic characteristics. An indirect-direct band gap transition is expected with moderate tensile strains applied to antimonene monolayers. Since the mechanical exfoliation (scotch tape) approach will be difficult to fabricate antimonene due to large binding energy of bilayers, the standard chemical techniques will play a prominent role in synthesis of antimonene.