Acarbene is a highly reactive intermediate in which a carbon atom bears a lone pair and no formal charge: how many hydrogen atoms are attached to the central carbon atom above?

Carbene  is :CH₂ (i.e, methylene) which contains one neutral divalent carbon atom with valency of two  and two unshared valence electrons. Carbene is of two types: Singlet carbene and Triplet carbene. Carbenes are very short lived and unstable. It is highly reactive intermediate. General carbene which refers to specific species methylene ( :CH₂) contains two hydrogen atoms. Carbene is one of the very important intermediate of various organic synthesis.

it takes on an electronical energy

atomic mass = 39

number of neutrons = 20

number of protons= 19

electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s1

the number of electron in first shell = 2

number of electrons in n=3 shell = 8