Which one of the following is not an isotope of hydrogen but instead belongs to a completely different element?

Introduction / Context:
Hydrogen is famous for having three well known isotopes that differ in the number of neutrons but share the same atomic number. These isotopes are important in chemistry, nuclear physics and applications such as heavy water reactors and radiotracers. This question asks you to identify which listed species is not an isotope of hydrogen and instead belongs to a different chemical element. Recognising the names and mass numbers of hydrogen isotopes is basic but very useful knowledge.

Given Data / Assumptions:

• Hydrogen isotopes include protium, deuterium and tritium.
• The options include three known names along with helium 4 and a general statement option.
• We assume understanding that hydrogen has atomic number 1 and helium has atomic number 2.
• Mass numbers are indicated by the number following the element name, such as hydrogen 1 or helium 4.

Concept / Approach:
Isotopes of an element must share the same atomic number, meaning the same number of protons, and differ only in the number of neutrons. Hydrogen has one proton in the nucleus. Protium, deuterium and tritium all have one proton but differ in neutron count. Helium, however, has two protons and therefore is a different element. A helium nucleus with two protons and two neutrons is helium 4. Since helium has a different atomic number, none of its nuclei can be classified as isotopes of hydrogen. The approach is to check which option corresponds to a nucleus with a proton count other than one.

Step-by-Step Solution:
Step 1: Recall that protium, hydrogen 1, has 1 proton and 0 neutrons and is an isotope of hydrogen. Step 2: Deuterium, hydrogen 2, has 1 proton and 1 neutron, so it is also an isotope of hydrogen. Step 3: Tritium, hydrogen 3, has 1 proton and 2 neutrons, again clearly an isotope of hydrogen. Step 4: Helium 4 has 2 protons and 2 neutrons, giving it atomic number 2, so it is a helium nucleus and not hydrogen at all. Step 5: Therefore, the species that is not an isotope of hydrogen is helium 4.

Verification / Alternative check:
Standard nuclear notation writes these isotopes as 1H1 for protium, 1H2 for deuterium and 1H3 for tritium, where the lower left number is the atomic number 1, and the upper left number is the mass number. Helium 4 is written as 2He4, clearly showing atomic number 2. Checking a periodic table confirms that hydrogen and helium occupy different positions and have different proton counts. Since isotopes must share atomic number, helium 4 cannot be considered an isotope of hydrogen. This reinforces helium 4 as the correct answer.

Why Other Options Are Wrong:
Option A, protium, is the most abundant natural isotope of hydrogen, making up the majority of hydrogen atoms in the universe. Option B, deuterium, is a stable hydrogen isotope found in heavy water and used in nuclear reactors and tracer studies. Option C, tritium, is a radioactive hydrogen isotope used in some luminous devices and fusion research. Option E, which states that all of the listed options are not isotopes of hydrogen, is clearly incorrect because the first three options are indeed hydrogen isotopes. Only option D, helium 4, is not an isotope of hydrogen, since it has a different atomic number and is a completely different element.

Common Pitfalls:
Learners sometimes focus only on mass numbers and assume that any nucleus with a small mass number might be hydrogen related. It is crucial to remember that the defining property of an element is its atomic number, which equals the number of protons. Another pitfall is misreading abbreviations such as D and T for deuterium and tritium. Keeping track of both the element symbol and the mass number helps avoid confusion. Always check the proton count to decide whether two nuclei are isotopes of the same element or belong to different elements altogether.

Final Answer:
The species that is not an isotope of hydrogen is Helium 4, because it has two protons and belongs to the element helium, not hydrogen.