Which one of the following statements is not correct?

The statement that all atoms of an element will always have the same number of neutrons is incorrect. While the number of protons (atomic number) is constant for all atoms of a specific element [1], the number of neutrons can vary. Atoms of the same element with different numbers of neutrons are known as isotopes. For example, hydrogen exists as protium (0 neutrons), deuterium (1 neutron), and tritium (2 neutrons) [1]. Conversely, atoms of different elements can have the same mass number (the sum of protons and neutrons), which are called isobars, such as Argon-40 and Calcium-40. Additionally, because isotopes have different neutron counts, atoms of the same element can indeed have different mass numbers. Therefore, while the proton count defines the element, the neutron count is not fixed.

Sources

1. [1] https://isotopes.gov/isotope-basics