Which one of the following metallic elements is known to be the poorest conductor of heat among the choices given?

Introduction / Context:
This question comes from basic physics and chemistry of metals. It asks you to identify which metal among the listed options is the poorest conductor of heat, a property that influences how materials are used in cookware, wiring and thermal insulation.

Given Data / Assumptions:

• The options are the metals sodium, lead, zinc, mercury and aluminium.
• We are comparing thermal conductivity, not electrical conductivity.
• We assume typical textbook comparisons among common metals.
• We are interested in the relatively poorest conductor of heat among the options given.

Concept / Approach:
Metals in general are good conductors of heat due to the presence of free electrons and closely packed atoms. However, different metals have different thermal conductivities. Light metals like copper and aluminium conduct heat very well, while some heavy metals like lead conduct heat relatively poorly compared with other common metals. In many basic exam questions, lead is cited as the poorest conductor of heat among typical structural metals.

Step-by-Step Solution:
1) Aluminium is widely used in utensils and heat sinks because it is a very good conductor of heat, so it cannot be the poorest.2) Sodium is a highly reactive alkali metal and, as a metal, has good thermal conductivity, though it is not usually used structurally.3) Zinc also conducts heat reasonably well and is used in alloys and galvanisation rather than as a thermal insulator.4) Mercury is a liquid metal at room temperature and has decent thermal conductivity, but in many secondary level comparisons lead is highlighted as the poorest heat conducting metal among common solids.5) Therefore, lead is regarded as the poorest conductor of heat among the given metallic options in typical exam contexts.

Verification / Alternative check:
Standard data tables in many textbooks and reference books list approximate thermal conductivities and note that lead has much lower thermal conductivity than metals like copper, aluminium and zinc. This is why lead is sometimes used where heat conduction needs to be reduced, such as in some shielding or protective applications. Although mercury is a liquid and has its own special properties, school level questions traditionally identify lead as the answer to this comparison.

Why Other Options Are Wrong:
Sodium: Being an alkali metal with free electrons, it conducts heat well compared to lead.
Zinc: It has better thermal conductivity and is not usually cited as the poorest conductor among metals.
Mercury: While a metal, it is not typically given as the standard example in such questions; lead is more commonly identified as the poorer solid metal conductor.
Aluminium: Known for high thermal conductivity and used for cooking utensils and heat exchangers, so it is clearly not the poorest.

Common Pitfalls:
Students sometimes overthink this question by focusing on the fact that mercury is a liquid and guessing it might conduct heat poorly. However, thermal conductivity depends on atomic structure and free electrons, not simply on physical state. For most standard competitive exams, the expected answer is lead. Remembering that copper and aluminium are excellent conductors and that lead is the typical poor conductor among metals helps avoid confusion.

Final Answer:
The poorest conductor of heat among the given metallic elements is lead.