Which one of the following state- ment, is not correct ?

The statement in option 2 is incorrect because it describes isotopes, not isobars. Isobars are defined as atoms of different chemical elements that possess the same mass number (A) but different atomic numbers (Z). Conversely, isotopes are atoms of the same element that have the same atomic number but different mass numbers due to varying neutron counts [3]. The other statements are scientifically accurate: elements are indeed defined by their proton count (atomic number) [3]; the mass number is the sum of protons and neutrons (nucleons) in the nucleus [2]; and valency is correctly defined as the combining capacity of an atom, which is determined by its ability to gain, lose, or share electrons to achieve stability.

Sources

1. [1] https://www.energy.gov/science/doe-explainsisotopes
2. [3] https://isotopes.gov/isotope-basics
3. [2] https://www.khanacademy.org/science/biology/history-of-life-on-earth/radiometric-dating/a/atomic-number-atomic-mass-and-isotopes-article