To determine the number of neutrons in a single \( \text{Fe}^{2+} \) atom with a mass number of 56, follow these steps:
1. Understand the terms:
- Mass number (A): This is the total number of protons and neutrons in the atomic nucleus. For \( \text{Fe}^{2+} \), the mass number is given as 56.
- Atomic number (Z): This is the number of protons in the nucleus of the atom. For iron (Fe), the atomic number is always 26.
2. Calculate the number of neutrons:
- The number of neutrons is found by subtracting the atomic number from the mass number.
- Formula: Number of neutrons = Mass number - Atomic number
3. Substitute the values:
- Mass number (A) = 56
- Atomic number (Z) = 26
So, the number of neutrons = 56 - 26
4. Perform the subtraction:
- Number of neutrons = 56 - 26 = 30
Therefore, a single \( \text{Fe}^{2+} \) atom has 30 neutrons.
The correct answer is:
30
