At 25 degrees Celsius and 1 atmosphere, select the liquid elemental sample from these cards: mercury with 80 protons, sodium with 11, copper with 29, and magnesium with 12. Enter the proton count on the selected card.

Explication

Elemental mercury is liquid under the stated conditions, so the matching card supplies the proton count 80. Calling it solid merely because it is a metal, or gaseous merely because it is volatile, would use an invalid shortcut.