Final answer:
The atom X with 20 protons and 20 neutrons that forms a cation X²⁺ is calcium (Ca). The notation is Ca²⁺, where Ca is the symbol for calcium and the 2+ indicates it is a cation with two fewer electrons than protons.
Step-by-step explanation:
- To write the notation for an atom X that contains 20 protons and 20 neutrons and forms a cation X²⁺, we first need to identify element X. The element with an atomic number of 20 (the number of protons) is calcium (Ca).
- The nuclide notation for this atom would be written as Ca-40, where the atomic number (number of protons) and the mass number (number of protons and neutrons combined) are written.
- Since the mass number is the sum of the protons and neutrons, it would be 20 + 20 = 40. The cation with a 2+ charge is denoted by a superscript, so the complete notation for this ion is Ca²⁺.
- Therefore, the atom X is calcium, and the notation for a calcium atom with 20 protons and 20 neutrons that forms a cation with a +2 charge is Ca²⁺.