Final answer:
The neutron number is not unique to each element, as isotopes of the same element can have different numbers of neutrons.
Step-by-step explanation:
The question asks which option is NOT unique to each element. Let's consider the provided options:
- Proton number – This is unique to each element, as the number of protons (atomic number, Z) in the nucleus defines the element.
- Atomic number – This is synonymous with the proton number and is equally unique to each element.
- Neutron number – This can vary among atoms of the same element. These variants are called isotopes. Isotopes have the same atomic number but different numbers of neutrons.
- Chemical symbol – Each element is assigned a unique one-, two-, or three-letter symbol.
The neutron number is not unique to each element, as it can differ among isotopes of the same element without altering the chemical properties.
To illustrate, carbon (symbol C) has an atomic number of 6, which is unique to carbon. However, carbon has isotopes with differing neutron numbers, such as Carbon-12 and Carbon-13, which have six and seven neutrons respectively. Both isotopes are still carbon due to their atomic number of 6 protons.