The atomic mass of an element is determined primarily by the total number of which subatomic particles present in its nucleus?

Introduction / Context:
Atomic mass is a fundamental concept in chemistry and physics that helps us compare the relative masses of different atoms. It is closely related to the structure of the atom, which contains protons, neutrons and electrons. Understanding which subatomic particles contribute significantly to atomic mass is important for interpreting mass numbers, isotopes and nuclear reactions. This question asks which particles determine the atomic mass of an element.

Given Data / Assumptions:
- Atoms consist of a nucleus that contains protons and neutrons, surrounded by electrons in various energy levels.
- Protons and neutrons each have a mass close to one atomic mass unit, while electrons have a much smaller mass.
- The mass number of an atom is defined as the total number of protons plus neutrons in its nucleus.
- We focus on the main contributors to atomic mass in basic chemistry, not on very small corrections from electrons or binding energy.

Concept / Approach:
The atomic mass of an element is largely determined by the mass of its nucleus, because electrons are about one thousand eight hundred times lighter than protons or neutrons. The nucleus contains protons and neutrons, which are collectively called nucleons. The mass number of an isotope is the total number of these nucleons, and it is this total that largely determines the atomic mass in atomic mass units. Therefore, both protons and neutrons together are responsible for an element atomic mass, while electrons contribute only a very small fraction and can be neglected in simple calculations.

Step-by-Step Solution:
Step 1: Recall that protons carry a positive charge and have a mass close to one atomic mass unit. Step 2: Recall that neutrons are electrically neutral and also have a mass close to one atomic mass unit. Step 3: Understand that electrons are negatively charged but have a very small mass compared with protons and neutrons. Step 4: Note that the mass number of an atom is defined as the sum of the number of protons and the number of neutrons, which together account for almost all the mass of the atom. Step 5: Conclude that the atomic mass depends primarily on the combined number of protons and neutrons in the nucleus, as stated in option D.

Verification / Alternative check:
When we look at isotopes of a given element, such as carbon 12 and carbon 14, we see that they have the same number of protons but different numbers of neutrons. Their different atomic masses reflect the change in neutron count, while the electron count stays the same. Standard nuclide notation writes mass number as a superscript representing protons plus neutrons, which reinforces the idea that these nucleons determine atomic mass. Periodic table entries often list average atomic masses that reflect the weighted averages of isotope mass numbers, again based on proton and neutron counts. These common references support the view that protons and neutrons are the main contributors to atomic mass.

Why Other Options Are Wrong:
Option A is incorrect because electrons have very small mass and do not primarily determine the atomic mass. Option B mentions only protons, but ignoring neutrons would fail to account for differences in isotope mass. Option C considers only neutrons and similarly neglects the contribution of protons. Option E refers vaguely to charged particles outside the nucleus, which are just electrons and not the main source of atomic mass. Only option D correctly identifies both protons and neutrons together as the primary contributors to atomic mass.

Common Pitfalls:
Students sometimes confuse atomic number, which is the number of protons, with mass number, which is protons plus neutrons. Another pitfall is to assume that electrons play a larger role in atomic mass because they participate in chemical bonding and are often discussed in detail. Remembering that the nucleus holds almost all the mass and that both protons and neutrons have similar masses helps clarify these distinctions. Keeping track of the difference between atomic number and mass number is also essential for correctly answering questions about atomic structure.

Final Answer:
The atomic mass of an element depends primarily on Both protons and neutrons together in the nucleus.