In the molecule HCCH, also known as acetylene, the central atom is carbon. In this particular structure, each carbon atom is bonded to one hydrogen atom and involved in a triple bond with the other carbon atom.
The hybridization of the carbon atoms in HCCH is sp. This is because each carbon is forming two σ bonds (one with hydrogen and one with carbon) and has two unhybridized p orbitals that are involved in forming two π bonds in the triple bond.
In terms of molecular geometry, the geometry around each carbon atom is linear. This is due to the arrangement of the hybrid orbitals; since there are two regions of electron density (one for the bond with hydrogen and one for the bond with the other carbon), the bond angle is 180 degrees, resulting in a straight line configuration.