I was reading about atomic orbitals. From Bohr's model, electron orbitals are considered circular. But as I saw the sub-orbitals like 2px$2p_x$, 3d(x^2-y^2)$3d_{x^2-y^2}$ I noticed something strange. These sub-orbitals aren't circular. How can that be possible if 2p$2p$ and 3d$3d$ are circular? Also why is 3d$3d$ defined like this (x^2-y^2)$(x^2-y^2)$? What does this actually mean?
I don't know how to write mathjax. Also I couldn't decide where to post it.