Find the number of neutrons in an atom represented by 4521X.

The number 45₂₁X represents an atom of an element with atomic number 21. The atomic number of an element represents the number of protons in its nucleus. Since the atom is neutral, the number of protons in its nucleus is equal to the number of electrons surrounding the nucleus. To determine the number of neutrons, we can use the formula: number of neutrons = mass number - atomic number The mass number of the atom is not given, so we cannot directly calculate the number of neutrons. However, we know that for a particular element, the number of protons plus the number of neutrons is equal to the mass number. Since the atomic number of the element is 21, we can look up its atomic mass on the periodic table, which is approximately 45. Therefore, the number of neutrons in the atom is: number of neutrons = mass number - atomic number number of neutrons = 45 - 21 number of neutrons = 24 Therefore, the correct answer is 24.