Which of the following is the lightest element?

Appeared in: NCL (MP) Nursing Officer - 2019

Explanation

Helium is the correct answer as it has the lowest atomic mass among the choices.
An element's weight is determined by its atomic mass, which is the total number of protons and neutrons in the nucleus.
Helium has an atomic number of 2 and an atomic mass of approximately 4 atomic mass units (amu).
The other options—Titanium (≈48 amu), Uranium (≈238 amu), and Plutonium (≈244 amu)—are significantly heavier metals.

Option A: Titanium is a metal with an atomic mass of approximately 48 amu. While it is considered a 'lightweight' metal for its strength, it is much heavier than the gas Helium.
Option B: Uranium is one of the heaviest naturally occurring elements, with an atomic mass of approximately 238 amu. It is a dense, radioactive metal.
Option C: Plutonium is a synthetic, radioactive element that is even heavier than Uranium, with an atomic mass of approximately 244 amu.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Atomic Structure and Properties of Elements as background academic context rather than a clinical decision trigger.
While a general science question, the properties of these elements have medical and scientific relevance.
Helium's low density and inertness make it useful in breathing mixtures for respiratory conditions and as a cryogenic coolant for MRI (Magnetic Resonance Imaging) machines.
Titanium is highly biocompatible and corrosion-resistant, making it a preferred material for surgical implants like joint replacements and dental implants.

How to Approach the Question

First, understand what the question is asking. 'Lightest element' refers to the element with the lowest atomic mass.
Recall the basic properties of the elements listed in the options. You might remember that Helium is a very light gas used in balloons.
The other three options—Titanium, Uranium, and Plutonium—are all metals. Metals, especially those with high atomic numbers like Uranium and Plutonium, are generally dense and heavy.
Compare the atomic masses if you know them, or use their position in the periodic table as a guide. Elements at the top of the periodic table are lighter than those at the bottom.
Helium (atomic number 2) is at the very top of the table, while the others are much lower down. This confirms that Helium is the lightest among the choices.

Concept Tested & Keywords

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E6 Textbook of Biochemistry for medical StudentsDM Vasudevan Part 2 — Subpart B (pp 240-463 of 478) p. 176-178

E6 Harper's Illustrated Biochemistry2023 (pp 26-793 of 813) p. 81-83

Practise the full NCL (MP) Nursing Officer - 2019

Attempt every question from this paper in a timed mock, then review the full solution for each one.