Final answer:
An atom of niobium with a mass number of 93 has 52 neutrons in its nucleus. An isotope of phosphorus with 15 protons and 15 neutrons in the nucleus has a mass of 30 units.
Step-by-step explanation:
In the case of niobium, the atom has a mass number of 93. The mass number is the sum of the number of protons and neutrons in the nucleus. Since the atomic number, which represents the number of protons, is not given, we can assume it is equal to the number of electrons, which is generally the case. The atomic number can be found on the periodic table, and for niobium, it is 41.
Therefore, to find the number of neutrons, we can subtract the atomic number from the mass number: 93 - 41 = 52 neutrons. For the isotope of phosphorus mentioned, the mass number is equal to the number of protons plus the number of neutrons. So, if it has 15 protons, then it also has 15 neutrons. Hence, the mass of this isotope is 15 + 15 = 30 units.