Post 1984 pennies are composed of a zinc core and a copper shell. Why is this best?

Most of the metals we use are alloys. This is certainly true for coinage. Clearly, the alloys used for coinage must be: (i) durable; (ii) resistant to chemical and physical damage; and (iii) easy to machine. This particular alloy evidently meets these properties. If there is a coated zinc core, presumably the less reactive copper coating hinders corrosion of the zinc.

See this old [answer for more examples of alloys.](https://socratic.org/questions/what-is-an-example-of-an-alloy-how-does-its-properties-may-make-it-more-useful-t)

The alloys we use for currency must also be cheap and inexpensive. We are long past the days of #"gold sovereigns"# and #"silver florins"# where currency had a significant precious metal content and an intrinsic value. Or are we? You can still buy #"Krugerrands"#, which are 90% gold; I don't recommend that you use them to buy your groceries. These are investment items used to speculate on the price of gold. On the other hand, I own a half dozen old sterling silver sixpences, which I put in the Christmas Pudding each year.