An atom has 7 electrons in its M-shell and contains 18 neutrons in its nucleus .What is its mass number ?

Options

A. 25
B. 27
C. 35
D. 43

Explanation

To find the mass number of the atom, we can follow these steps:

### Step 1: Understand the relationship between protons, neutrons, and mass number
The mass number (A) of an atom is calculated using the formula:
\[ A = \text{Number of protons} + \text{Number of neutrons} \]

### Step 2: Determine the number of protons
We know that the number of electrons in a neutral atom is equal to the number of protons. The problem states that there are 7 electrons in the M-shell. However, we must consider the total number of electrons in all shells.

- K shell can hold a maximum of 2 electrons.
- L shell can hold a maximum of 8 electrons.
- M shell has 7 electrons as given.

Thus, the total number of electrons is:
\[ \text{Total electrons} = 2 (\text{K shell}) + 8 (\text{L shell}) + 7 (\text{M shell}) = 17 \]

Since the number of protons is equal to the number of electrons in a neutral atom:
\[ \text{Number of protons} = 17 \]

### Step 3: Use the number of neutrons
The problem states that there are 18 neutrons in the nucleus.

### Step 4: Calculate the mass number
Now we can substitute the values into the mass number formula:
\[ A = \text{Number of protons} + \text{Number of neutrons} \]
\[ A = 17 + 18 \]
\[ A = 35 \]

### Conclusion
The mass number of the atom is 35.

### Final Answer
The correct option is C: 35.

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