Among the following metals, which one is the lightest metal in terms of density?

Introduction / Context:
Density is defined as mass per unit volume and is an important physical property that helps compare how heavy materials feel for a given size. Metals vary widely in density; some are very heavy, like lead, while others are extremely light, which makes them useful in batteries and lightweight alloys. This question asks you to identify which metal from the list is the lightest in terms of density, not total mass, meaning it has the smallest mass per unit volume.

Given Data / Assumptions:

• The metals listed are lithium, silver, mercury, lead and sodium.
• All densities are considered at room temperature and normal pressure.
• Lithium and sodium are alkali metals known to be relatively light.
• Silver and lead are much denser, and mercury is a dense liquid metal.

Concept / Approach:
Among the known metals, alkali metals such as lithium, sodium and potassium have some of the lowest densities. Lithium is the lightest metal in terms of density; it is lighter even than sodium. Heavy metals like silver and lead have much higher densities, and mercury, although liquid at room temperature, is also quite dense. Therefore, the approach is to recall that lithium is at the top of the alkali metal group and is famous for being the lightest metal, widely used in lightweight batteries and specialised alloys.

Step-by-Step Solution:
Step 1: Recognise that lithium and sodium are alkali metals, which are known to be softer and less dense than most other metals. Step 2: Among these, lithium occupies the highest position in the group and has the lowest atomic mass and lowest density. Step 3: Sodium is also light but is denser than lithium, so it cannot be the lightest metal in the list. Step 4: Silver is a transition metal used in jewellery and electronics and is much denser than alkali metals. Step 5: Lead is known as a heavy metal and has a very high density, so it is far from being the lightest. Step 6: Mercury is unique because it is a liquid metal at room temperature, but in terms of density it is actually very heavy and not a light metal. Step 7: Therefore, lithium is the lightest metal among the options given.

Verification / Alternative check:
Typical density values support this conclusion. The density of lithium is about 0.53 grams per cubic centimetre, which is less than that of water, so lithium pieces can actually float on some hydrocarbon liquids. Sodium has a density around 0.97 grams per cubic centimetre, still low but higher than lithium. Silver has a density above 10 grams per cubic centimetre, lead is around 11 grams per cubic centimetre and mercury is approximately 13.6 grams per cubic centimetre. These numerical values clearly demonstrate that lithium is the least dense metal in the list and, in fact, the lightest metal known.

Why Other Options Are Wrong:
Option B, silver, is a dense precious metal and is much heavier than lithium for the same volume. Option C, mercury, is a liquid at room temperature but has a high density and is used in devices such as barometers partly for this reason. Option D, lead, is a classic heavy metal and is clearly not light. Option E, sodium, is relatively light but has a higher density than lithium. Only option A, lithium, accurately identifies the lightest metal by density among the options provided.

Common Pitfalls:
A common mistake is confusing physical state with density, for example believing that because mercury is liquid it must be light, when in fact it is quite dense. Another error is thinking that all metals used in small devices, such as silver in electronics, are light, ignoring their actual densities. To avoid these mistakes, remember that lithium is the standard example of the lightest metal and that alkali metals in general have low densities compared with transition metals and heavy metals like lead. Linking these properties to their uses, such as lithium in batteries, helps reinforce the concept.

Final Answer:
The lightest metal in terms of density among the options given is Lithium.