2. which of the following is unique for any given element?
a its mass number
b the number of its neutrons
c its atomic number

C its atomic number

Explanation:

Every element has a unique atomic number. The atomic number is always the same for a given element and does not change.

The number of protons in an atom is the atomic number. Protons are located in the nucleus. Being nuclei particles, they do not have a variable number. Their number is fixed and we can easily identify an atom with the number of protons, which is the atomic number.

pure substance: iron,copper

impure substance: water,charcoal