  According to topological quantum chemistry, a class of electronic materials
have been called obstructed atomic insulators (OAIs), in which a portion of
valence electrons necessarily have their centers located on some empty
$\textit{Wyckoff}$ positions without atoms occupation in the lattice. The
obstruction of centering these electrons coinciding with their host atoms is
nontrivial and results in metallic boundary states when the boundary is
properly cut. Here, on basis of first-principles calculations in combination
with topological quantum chemistry analysis, we propose two dimensional
MA$_2$Z$_4$ (M = Cr, Mo and W; A = Si and Ge, Z = N, P and As) monolayer family
are all OAIs. A typical case is the recently synthesized MoSi$_2$N$_4$.
Although it is a topological trivial insulator with the occupied electronic
states being integer combination of elementary band representations, it has
valence electrons centering empty $\textit{Wyckoff}$ positions. It exhibits
unique OAI-induced metallic edge states along the (1$\bar{1}$0) edge of
MoSi$_2$N$_4$ monolayer and the in-gap corner states at three vertices of
certain hexagonal nanodisk samples respecting C$_3$ rotation symmetry. The
readily synthesized MoSi$_2$N$_4$ is quite stable and has a large bulk band gap
of 1.94 eV, which makes the identification of these edge and corner states most
possible for experimental clarification.
