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.

the number of valence electrons for each molecule or ion is shown beneath the structure. remember hydrogen will not have more than two electrons. this hydrogen is part of a covalent bond (sharing two electrons). a double bond here would cause hydrogen to share four electrons with phosphorus.