A detailed mechanistic investigation of Si-Me formation over the silicon (100)-2 X 1 surface using the Si9H12 cluster model has been performed using various reagents, based on two basic mechanisms: dissociation and substitution. The reagents CH4, CH3Cl for dissociation and CH3Li, CH3MgBr for substitution mechanism are used to explore the methylation process on the silicon surface at the M062X/6-311-G(2d, p) level of theory. The associated potential energy surfaces explored here are aimed to unveil the most favored pathway of methylation with appropriate reagents. Dissociation of methane forms a monomethylated product (D1) through an energetically unfavorable pathway. All the adsorption modes of CH3Cl over the silicon surface are also detected and analyzed. Methyl chloride dissociates to form another monomethylated product D2 and its derivative D3 in the entrance channel, while, in the next step, bridged compounds I1 (Cl-bridged) and I2 (H-bridged) are produced from them, respectively. The C-Cl dissociation leads to the formation of D2 having a lower activation barrier. With a comparably high activation barrier in the C-H dissociation, producing D3, very interestingly carbene intermediate has been detected in the reaction pathway. Detection of energetically unfavored conversions from D2 to I1 and D3 to 12 ensured that the methylation process will not be hampered through these interconversions. For substitution, HCl- and Cl-2-passivated Si surfaces are taken, where chlorine is to be substituted by the methyl group of both of the methylating agents. With both substituents, HCl-passivated Si9H12 gives D1. The substitution process on Cl-2-passivated Si9H12 leads to the formation of D2 in the first step and dimethylated product (Si) in the final step. In all the above substitution processes, methyl lithium proved to be the better substituent for the formations of D1, D2, and Si on HCl- or Cl-2-passivated surfaces. The present work not only demonstrated methyl lithium as one of the best methylating agents but also revealed the interrelation among the dissociative adsorption modes of CH3Cl, reported earlier, in a single potential energy surface with a remarkable detection of carbene intermediate formed in the pathway of C-H dissociation.