In the most common isotope of hydrogen (protium), the nucleus of a hydrogen atom consists of which fundamental particle or particles?

Introduction / Context:
Hydrogen is the simplest and lightest element in the periodic table and is often used as a starting point for understanding atomic structure. The structure of its nucleus and surrounding electron cloud illustrates basic ideas about protons, neutrons, and electrons. This question asks about the composition of the nucleus of a hydrogen atom, specifically the most common isotope, which is essential background knowledge for both chemistry and physics.

Given Data / Assumptions:

• The question refers to hydrogen atom nucleus, usually taken to mean the most abundant isotope, protium.
• Protium has atomic number 1 and mass number 1.
• Fundamental particles considered are protons, neutrons, and electrons.
• Electrons are located outside the nucleus in the atomic model.

Concept / Approach:
In the basic Bohr model and in modern quantum descriptions, the nucleus of an atom contains protons and neutrons, while electrons occupy regions around the nucleus. Hydrogen, with atomic number 1, has one proton in its nucleus. The most common isotope, protium, has mass number 1, which means it has one proton and zero neutrons in the nucleus. Deuterium and tritium are heavier isotopes of hydrogen that contain one and two neutrons respectively, but they are less abundant. The question focuses on the common hydrogen atom, whose nucleus contains just one proton.

Step-by-Step Solution:
Step 1: Recall that atomic number Z defines the number of protons in the nucleus. For hydrogen, Z = 1. Step 2: For the most abundant isotope of hydrogen, protium, the mass number A is also 1. Step 3: Since mass number A equals the sum of protons and neutrons, and A = 1 with Z = 1, the number of neutrons must be zero. Step 4: Therefore, the nucleus of the common hydrogen atom contains one proton and no neutrons. Step 5: Electrons are not part of the nucleus; the single electron in hydrogen orbits outside the nucleus, so it is not counted in the nuclear composition.

Verification / Alternative check:
Consider the different isotopes of hydrogen. Protium, written as hydrogen 1, has one proton and zero neutrons. Deuterium, hydrogen 2, has one proton and one neutron. Tritium, hydrogen 3, has one proton and two neutrons. When exam questions simply say "hydrogen atom nucleus" without specifying the isotope, they nearly always refer to protium because it accounts for the vast majority of hydrogen atoms in the universe. This confirms that the nucleus consists of one proton only in the usual context.

Why Other Options Are Wrong:
Two protons would correspond to an element with atomic number 2, namely helium, not hydrogen. Two neutrons only would give a neutral particle with no protons, which is not an atomic nucleus in the conventional sense. One electron only is incorrect because electrons do not reside in the nucleus; they occupy the surrounding space. One proton and one neutron describes deuterium, an isotope of hydrogen, but not the most common hydrogen atom referred to in basic questions. Therefore, these options do not match the nucleus of ordinary hydrogen.

Common Pitfalls:
Learners sometimes confuse isotopes and think that all hydrogen nuclei must contain both protons and neutrons, leading them to choose an option with both particles. Others may mistakenly include electrons in the nucleus. To avoid these mistakes, remember that the simplest and most abundant form of hydrogen has only one proton in its nucleus and that the nucleus by definition does not include electrons, which are found in orbitals outside the nucleus.

Final Answer:
The nucleus of the most common hydrogen atom (protium) consists of one proton only.