In basic atomic structure, the atomic weight (approximately the mass number) of an element is given by which combination of subatomic particles?

Introduction / Context:
Atomic weight, often approximated by the mass number for a given isotope, is a key idea in basic chemistry. It appears in periodic table data, stoichiometry calculations and nuclear chemistry. An exam question like this checks whether you understand which subatomic particles contribute most to the mass of an atom and therefore determine its atomic weight. This understanding is essential for balancing equations, using moles and interpreting isotopic notation.

Given Data / Assumptions:

• The question asks what atomic weight (approximately the mass number) is based on.
• We consider protons, neutrons and electrons as the basic subatomic particles.
• We assume electron mass is negligible compared to proton and neutron mass at school level.
• We focus on the sum of whole numbers of particles, as used when defining mass number.

Concept / Approach:
Protons and neutrons reside in the nucleus and each has a mass close to 1 atomic mass unit. Electrons are much lighter, with a mass around 1 over 1836 of a proton, and their contribution to total atomic mass is usually neglected in introductory chemistry. The mass number, written as a whole number above the element symbol, is defined as the sum of protons and neutrons. Atomic weight listed on the periodic table is a weighted average of these mass numbers across natural isotopes, but in simple questions it is often approximated by the mass number. Thus, the sum of protons and neutrons in the nucleus is what gives the atomic weight in this simplified sense.

Step-by-Step Solution:
Step 1: Recall that the mass number A is defined as A = Z + N, where Z is the number of protons and N is the number of neutrons.Step 2: Recognise that each proton and neutron contributes roughly 1 unit of mass, while electrons contribute very little.Step 3: Understand that atomic weight on the periodic table is close to the mass number for each isotope, averaged over natural abundance.Step 4: Compare the options and look for the combination that matches Z + N.Step 5: Option b states that atomic weight is given by the sum of neutrons and protons, which is exactly the definition of mass number.Step 6: Therefore, select the sum of neutrons and protons as the correct basis for atomic weight in this context.

Verification / Alternative check:
Take a familiar example like carbon 12. Its atomic number is 6, meaning 6 protons, and its mass number is 12, so it must have 6 neutrons. The sum of protons and neutrons is 6 + 6 = 12, which matches the mass number and approximate atomic weight. For oxygen 16, there are 8 protons and 8 neutrons, again giving 16 as the mass number. In both cases, adding protons and neutrons yields the mass number, confirming the concept used in the question.

Why Other Options Are Wrong:
Option a, sum of neutrons and electrons, ignores the mass of protons and includes electrons, which are very light. Option c, only neutrons, clearly cannot be correct because it would give zero mass number for hydrogen 1, which has one proton and no neutrons. Option d, only protons, would give the same number for all isotopes of an element, which is not true in reality. Option e, sum of protons and electrons, again ignores neutrons, which contribute significantly to mass, and wrongly counts electrons that contribute very little to atomic weight.

Common Pitfalls:
Students sometimes confuse atomic number with atomic weight. Atomic number counts only protons, while atomic weight and mass number count both protons and neutrons. Another pitfall is to assume that because electrons are part of the atom they must contribute significantly to atomic weight, which is not accurate at this level. To avoid these errors, always remember that nucleus mass comes from protons plus neutrons, and that is what approximates atomic weight.

Final Answer:
In basic chemistry, the atomic weight or mass number of an element is approximately the sum of protons and neutrons in its nucleus.