Which metal conducts less heat

Answer 1

The least conductive elemental metal is Neptunium (atomic number 93) with a conductivity of 0.063 W/cm·K. Plutonium (atomic number 94) is only slightly better, with a conductivity of 0.0674 W/cm·K. Note that both Neptunium and Plutonium are radioactive and members of the Actinoids.

Next in conductivity is Manganese with a conductivity of 0.0782 W/cm·K. Few people will encounter Neptunium or Plutonium, but manganese is a much more common metal and non-radioactive.

Note that sometimes some of metalloids are included with metals since they may form alloys with metals. The least conductive metalloid is Telurium with a conductivity of just 0.0235 W/cm·K. A metalloid is a chemical element with properties that are in-between or a mixture of those of metals and nonmetals, and which is considered to be difficult to classify unambiguously as either a metal or a nonmetal. There is no standard definition of a metalloid nor is there agreement as to which elements are appropriately classified as such.

Mixtures of metals, aka alloys, have conductivities somewhere in between the most and least conductive included elements, so the least conductive metals will be elemental - as will the most conductive metals.