1.
Consider the following changes:

M(s)→M(g)\text{M(s)} \rightarrow \text{M(g)} ... (1)
M(g)→M2+(g)+2e−\text{M(g)} \rightarrow \text{M}^{2+}\text{(g)} + 2e^- ... (2)
M(g)→M+(g)+e−\text{M(g)} \rightarrow \text{M}^+\text{(g)} + e^- ... (3)
M+(g)→M2+(g)+e−\text{M}^+\text{(g)} \rightarrow \text{M}^{2+}\text{(g)} + e^- ... (4)
M(g)→M2+(g)+2e−\text{M(g)} \rightarrow \text{M}^{2+}\text{(g)} + 2e^- ... (5)

The second ionisation energy of M could be determined from the energy values associated with