It depends on whether it's made of standard copper-nickel alloy, or is one of the part-silver "war nickels" minted when nickel metal was needed for the war effort.
Please see the Related Question for details.
No such coin exists. All 1942-D nickels are made out of the standard 75% copper 25% nickel. Only 1942-S and 1942-P nickels were struck out of the "war nickel" alloy of 35% silver. A 1942-D nickel is worth about 7 cents if circulated and a couple of bucks if in better shape.
All U.S. nickels weigh 5 grams as issued.
There is no such thing as a Mercury nickel. See the related question below.
It depends, as there was actually a change in composition In late 1942.-- If the nickel has no mintmark, or a small mintmark to the right of Monticello on the back of the coin, it is 75% copper and 25% nickel.-- If the nickel has a large mintmark above the dome of Monticello on the back of the coin, it is 56% copper, 35% silver and 9% manganese.
It's the usual practice of this site to answer a single question at a time. Please see:"What is the value of a 1940 US nickel?""What is the value of a 1942 US nickel?""What is the value of a 1944 US nickel?"
75% copper, 25% nickel, same as every other nickel since 1866 (except for 1942-45).
Monticello is the building on the reverse of the coin.
If it's a regular nickel, 5 cents. If it's a silver war nickel (with a large mint mark on the back), $2.
All U.S. nickels (except for silver "war" nickels 1942-1945) are 75% copper and 25% nickel.
The last Buffalo nickel was made in 1938. A 1942 dated Jefferson that has a large mintmark on the reverse is 35% silver and worth about $1.00. If it does not have the large mintmark just spend it.
In 1942 silver nickels were only minted at Philadelphia and San Francisco.
The large "P" above the dome means it's a "war nickel" that is struck in 35% silver. The nickel was removed for the war effort and silver was used as a replacement, this was done from 1942 - 1945. Because it has a small amount of silver the value is about $1.00. NOTE: The 1942 war nickel is the first US coin to ever use a "P" Mintmark.