Final answer:
The atomic symbol Ni represents the chemical element nickel, while U represents uranium. A nickel nucleus with 28 protons will have 31 neutrons to reach a mass number of 59, while a uranium nucleus has 92 protons and 146 neutrons to reach a mass number of 238.
Step-by-step explanation:
The atomic symbol Ni represents the chemical element nickel. Nickel has the atomic number 28, which indicates that it contains 28 protons in its nucleus. The mass number of an element is the sum of its protons and neutrons, and in the case of nickel, it is 59. Therefore, a nickel nucleus with 28 protons must have 31 neutrons to have a mass number of 59.
The hyphen and mass number can also be used to represent isotopes of an element. So, Nickel-59 or Ni-59 represents a nickel nucleus with 31 neutrons.
In addition to nickel, uranium is another chemical element. Its atomic symbol is U and it has the atomic number 92. This means that all uranium nuclei have 92 protons. The uranium nucleus mentioned in the question also has 146 neutrons, giving it a mass number of 238. Therefore, Uranium-238 or U-238 represents a uranium nucleus with 146 neutrons.