Among the following, which is the lightest metal under normal conditions?

Introduction / Context:
The concept of density and relative lightness of elements is important in both physics and chemistry. Metals differ significantly in density and atomic mass. This question asks which of the listed metals is the lightest, a common point in general knowledge and periodic table awareness. It also tests whether you can distinguish between metals and non metals in the given options.

Given Data / Assumptions:

• Options include hydrogen gas, lithium, sodium, potassium, and aluminium.
• We are asked about the lightest metal, not the lightest element in general.
• Standard conditions of temperature and pressure are assumed.
• Hydrogen is present as a distractor, since it is often mentioned as the lightest element.

Concept / Approach:
Hydrogen is the lightest element, but at standard conditions it is a non metallic gas. The question specifically focuses on metals. Among the metals in the periodic table, lithium has the lowest density and one of the lowest atomic masses. Sodium and potassium are also light alkali metals, but heavier than lithium. Aluminium is relatively light among structural metals but is still denser than lithium. Therefore, lithium is the lightest metal in the options given.

Step-by-Step Solution:
Step 1: Identify which options are metals. Hydrogen gas is not a metal at room temperature; it is a non metal. Step 2: Lithium, sodium, and potassium are alkali metals in Group 1 of the periodic table. Step 3: Aluminium is a metal in Group 13 with higher density than alkali metals. Step 4: Compare alkali metals. Lithium has the smallest atomic number and mass among lithium, sodium, and potassium. Step 5: Lithium has density close to half that of water, while sodium and potassium are also low density but still heavier than lithium. Step 6: Thus, lithium is the lightest metal among the options provided.

Verification / Alternative check:
You can verify using periodic table data. Atomic numbers increase as you move down Group 1, and so does atomic mass: lithium (3), sodium (11), potassium (19). Generally, the higher the atomic mass, the heavier the element. Density tables also confirm that lithium has the lowest density among these metals. Aluminium, although light compared to many structural metals like iron or copper, still has much higher density than lithium. Recognising hydrogen as a non metal at room temperature confirms that it cannot be the correct answer to a question specifically about metals.

Why Other Options Are Wrong:
Hydrogen gas: It is the lightest element but not a metal under normal conditions; the question asks for the lightest metal.
Sodium: An alkali metal but heavier and denser than lithium, so not the lightest metal.
Potassium: Another alkali metal, heavier than both lithium and sodium.
Aluminium: A light structural metal but far denser than lithium and other alkali metals.

Common Pitfalls:
Students often see hydrogen in the list and quickly select it because they remember that hydrogen is the lightest element. However, the keyword metal is crucial and changes the answer. Another pitfall is to assume aluminium is the lightest because it is known as a light metal used in aircraft and packaging, but alkali metals are much lighter. Carefully reading the wording of the question and relating it to the periodic table avoids these mistakes.

Final Answer:
Lithium is the lightest metal among the given options.