Here's the breakdown:

* Standard enthalpy of formation refers to the heat change associated with forming one mole of a substance from its elements in their standard states (usually at 298 K and 1 atm).

* Silicon gas (Si(g)) is silicon in its gaseous state.

The standard enthalpy of formation of silicon gas is +455.6 kJ/mol. This means that it takes 455.6 kJ of energy to form one mole of silicon gas from solid silicon in its standard state.

Important note: The standard enthalpy of formation for silicon gas is a positive value, indicating that the process is endothermic. This means that energy is absorbed during the formation of silicon gas.

Let me know if you have any other questions!