Identify the number of neutrons present in a pair of elements with atomic numbers 20 and 13, if their mass numbers are 40 and 27, respectively.

Options

A. 20 and 13
B. 20 and 13
C. 17 and 20
D. 20 and 14

Explanation

To find the number of neutrons in the elements with atomic numbers 20 and 13 and mass numbers 40 and 27 respectively, follow these steps:

### Step 1: Understand the relationship between mass number, atomic number, and neutrons.
- The mass number (A) of an element is the total number of protons (Z) and neutrons (N) in the nucleus.
- The formula is:
\[ A = Z + N \]
- Rearranging this gives us:
\[ N = A - Z \]

### Step 2: Calculate the number of neutrons for the first element (atomic number 20, mass number 40).
- For the first element:
- Atomic number (Z) = 20
- Mass number (A) = 40
- Using the formula:
\[ N = A - Z = 40 - 20 = 20 \]
- Therefore, the number of neutrons in the first element is **20**.

### Step 3: Calculate the number of neutrons for the second element (atomic number 13, mass number 27).
- For the second element:
- Atomic number (Z) = 13
- Mass number (A) = 27
- Using the formula:
\[ N = A - Z = 27 - 13 = 14 \]
- Therefore, the number of neutrons in the second element is **14**.

### Final Answer:
- The number of neutrons in the first element (atomic number 20, mass number 40) is **20**.
- The number of neutrons in the second element (atomic number 13, mass number 27) is **14**.