Standard enthalpy of formation requires steps from elemental solid state

Steps:

• Define the process: ΔH_f[Na+(g)] is the enthalpy change for Na(s) → Na+(g).
• Apply Hess's law: Break into Na(s) → Na(g) and Na(g) → Na+(g).
• Identify ΔH for Na(s) → Na(g) as atomisation enthalpy.
• Note ΔH for Na(g) → Na+(g) as first ionisation energy; both sum to total ΔH_f.

Why B is correct:

• Enthalpy change of atomisation is ΔH for Na(s) → Na(g), the initial step from standard state to gaseous atom, required by the definition of standard enthalpy of formation.

Why the others are wrong:

• A: First ionisation energy applies only after atomisation; insufficient alone for the full process from solid.
• C: Enthalpy change of formation of sodium (Na(s)) is zero by convention for elements in standard state; provides no useful value.

Not enough information for option D.

Final answer: B