Here's how to determine which metal is most difficult to oxidize:

Understanding Oxidation

Oxidation is the loss of electrons. Metals are oxidized when they lose electrons to form positive ions. The more easily a metal loses electrons, the more easily it is oxidized.

Using the Activity Series

The activity series of metals helps us compare their tendency to lose electrons (i.e., their reactivity). Metals higher on the activity series are more reactive and thus easier to oxidize.

The Activity Series (Partial)

* Li > K > Ba > Sr > Ca > Na > Mg > Al > Mn > Zn > Cr > Fe > Cd > Co > Ni > Sn > Pb > H > Cu > Ag > Au > Pt**

Analysis

* Pb (Lead): Lead is lower on the activity series than the other metals listed.

* Hg (Mercury): Mercury is also lower on the activity series than the other metals listed.

* Na (Sodium): Sodium is higher on the activity series than both lead and mercury.

* Ca (Calcium): Calcium is even higher on the activity series than sodium.

Conclusion

Since lead and mercury are lower on the activity series, they are less reactive and therefore more difficult to oxidize than sodium and calcium.

Between lead and mercury, lead is slightly more reactive than mercury, making mercury the most difficult to oxidize out of the four metals listed.