Because CH_4CH4 is the formula of the methane molecule.......i.e. a carbon nucleus can form up to 4xxC-H4×C−H bonds......
Explanation:
"Methane"Methane is the first of a series of the homologous alkanes, general formula, C_nH_(2n+2)CnH2n+2. The next member is "ethane"ethane, C_2H_6C2H6, then "propane"propane, C_3H_8C3H8........