Years Pennies

why do pennies of different years have different densities?

Basically there hasn’t been a 100% copper penny since 1857.
At various times, it’s contained copper with various amounts of nickel, tin, and zinc.
Some pennies haven’t contained any copper at all.
(There was a steel penny during WWII and an aluminum penny in 1974.)
In 1982 the penny changed from 95% copper and 5% zinc to
a zinc core that accounts for 97.5% of its mass with the remaining 2.5% due to a plating of copper, as caroline m noted.

So technically the two pennies are not made out of different materials, they’re both copper and zinc, but the proportions and arrangement were changed.

Look here for more detail:

http://en.wikipedia.org/wiki/Penny_(United_States_coin)

“1793–1857 100% copper
1857–1864 88% copper, 12% nickel (also known as NS-12)
1864–1942 1946–1962 bronze (94% copper, 6% tin and zinc)
1943 zinc-coated steel (also known as steel penny)
1944–1946 brass (94% copper, 6% zinc)
1974 Experimental aluminum variety
1962–1982 brass (95% copper, 5% zinc)
1982–present* 97.5% zinc core, 2.5% copper plating “

Minime @ 2011 Happy New You New years/ Pennies For Jesus giving a hand@ Project STL(Saving the LOU).