Chemistry of Metals and Alloys

Stainless steel is primarily iron alloyed with chromium (minimum 10.5%). Nickel is often added for corrosion resistance and strength, but iron is the base metal. Zinc is not used in stainless steel.

Mercury is unique among metals in that it is liquid at room temperature. Its melting point is -38.83°C (-37.89°F), so it exists as a liquid under normal conditions. Mercury is not the hardest metal (that would be diamond-like materials), is not highly radioactive, and is not used in aircraft construction.

Plastic is significantly lighter than gold. Gold is a very dense metal with a density of about 19.3 g/cm³, while most plastics have densities between 0.9-1.4 g/cm³. This means a given volume of gold would be about 15-20 times heavier than the same volume of plastic. Silver and copper are also metals denser than plastic.

Olympic gold medals are primarily made of silver (at least 92.5%) and are only plated with a thin layer of gold (minimum 6 grams since 1912). The last solid gold medals were awarded in 1912.

Sodium has the highest thermal conductivity among liquid metals at approximately 142 W/(m·K), which is why it's used as a coolant in nuclear reactors. Mercury has relatively low thermal conductivity (~8.3 W/(m·K)), and both potassium and lead are also lower than sodium.

The Indus Valley Civilization (c. 3300-1300 BCE) knew and used copper, bronze (alloy of copper and tin), gold, and silver. Ironworking came to India later, after the decline of the Indus Valley Civilization, making iron the correct answer as it was not known to them.

Uranium is the primary fuel used in nuclear power plants for atomic power generation. Its unstable isotopes undergo controlled fission reactions, releasing large amounts of heat energy. Nickel, palladium, and molybdenum have important industrial uses but are not nuclear fuels.

Lithium has the lowest atomic mass (6.94 u) among all metals, making it the lightest metal in the periodic table. Sodium (23 u) and potassium (39 u) are alkali metals like lithium but much heavier, while calcium (40 u) and magnesium (24 u) are alkaline earth metals with significantly higher atomic masses.

Sodium is an alkali metal that is extremely soft - so soft it can be easily cut with a knife. Potassium is even softer, but among the given options, sodium is correctly identified as a soft metal. Tin is harder, while arsenic is actually a metalloid, not a pure metal.

Mercury is unique among metals because it exists as a liquid at room temperature (melting point -38.8°C). It's a heavy, silvery metal that was commonly used in thermometers and other scientific instruments before its toxicity was fully understood. It is not the hardest metal, is not highly radioactive, and is not used in aircraft construction due to its liquid state and toxicity.