Which of the following equals one atomic mass unit as defined on the carbon 12 scale in modern chemistry?

Introduction / Context:
Atomic mass unit is a standard unit used to express atomic and molecular masses. Rather than using grams, chemists work with this small unit to compare atomic masses conveniently. This question asks for the correct definition of one atomic mass unit on the carbon 12 scale.

Given Data / Assumptions:
- The modern atomic mass unit is defined using the carbon 12 isotope.
- Carbon 12 has 6 protons and 6 neutrons in its nucleus and is assigned a mass of exactly 12 atomic mass units by definition.
- The atomic mass unit simplifies expressing masses of atoms relative to this standard.

Concept / Approach:
By international agreement, one atomic mass unit is defined as exactly one twelfth of the mass of an atom of carbon 12 in its ground state. This means that the mass of the carbon 12 isotope is exactly 12 atomic mass units. Nucleon masses (protons and neutrons) are close to one atomic mass unit each, but not exactly, because binding energy affects the total mass. The other options either describe incorrect fractions or refer to particles whose masses differ from one atomic mass unit.

Step-by-Step Solution:
Step 1: Recall the definition of atomic mass unit: it is based on the carbon 12 isotope. Step 2: Carbon 12 is assigned a mass of exactly 12 atomic mass units by definition. Step 3: Therefore, one atomic mass unit must be the mass of carbon 12 divided by 12. Step 4: Recognise that dividing the mass of a carbon 12 atom by 12 gives one twelfth of that mass. Step 5: Conclude that one atomic mass unit is one twelfth of the mass of one carbon 12 atom.

Verification / Alternative check:
If carbon 12 is exactly 12 atomic mass units, then one proton or neutron in that nucleus has a mass close to 1 atomic mass unit. Experimental values show that the mass of a proton or neutron is very close to 1 atomic mass unit, but not exactly, because nuclear binding energy slightly reduces the total mass. One electron has a much smaller mass, roughly one eighteen hundredth of an atomic mass unit. These experimental values agree with the formal definition and show that the dividing by 12 method using carbon 12 is the accepted standard.

Why Other Options Are Wrong:
- The mass of one electron: This is far smaller than 1 atomic mass unit and does not match the definition.
- One sixth of the mass of one helium 4 atom: Helium 4 has mass about 4 atomic mass units, so this fraction would not give exactly 1 atomic mass unit and is not used in the definition.
- The mass of one entire carbon 12 atom: That mass equals 12 atomic mass units, not 1 atomic mass unit.
- The mass of one neutron minus the mass of one proton: This describes a small difference, not a standard unit, and is not used to define atomic mass unit.

Common Pitfalls:
Students sometimes confuse atomic mass unit with the mass of a proton or neutron and think they are exactly equal. While they are close, the formal definition is tied specifically to carbon 12. Another mistake is to mix up historical definitions with the modern standard, but examinations focus on the current carbon 12 based definition.

Final Answer:
One atomic mass unit is defined as One twelfth of the mass of one carbon 12 atom.