The mass of an atom is due to the masses of protons and neutrons in the nucleus. The relative masses of protons and neutrons are almost equal to one. Therefore, the atomic mass of an element should be nearly a whole number. But in many cases the atomic masses are fractional. The main reason for these fractional atomic masses is that these elements occur in nature as a mixture of several isotopes. The atomic mass of an element is the average of the atomic masses of these isotopes in the ratio of their proportion of occurrence.
Chlorine occurs in nature in the form of two isotopes with atomic masses 35 u and 37 u in the ratio of 3 : 1 respectively. Atomic mass of chlorine is

Options

A. 35.5 u
B. 34.5 u
C. 35 u
D. 36 u

Explanation

To find the atomic mass of chlorine based on its isotopes, we can follow these steps:

### Step 1: Identify the isotopes and their atomic masses
Chlorine has two isotopes:
- Isotope 1: Atomic mass = 35 u
- Isotope 2: Atomic mass = 37 u

### Step 2: Determine the ratio of occurrence of the isotopes
The isotopes occur in the ratio of 3:1. This means:
- Isotope 1 (35 u) occurs 3 parts
- Isotope 2 (37 u) occurs 1 part

### Step 3: Calculate the total number of parts
Total parts = 3 (for 35 u) + 1 (for 37 u) = 4 parts

### Step 4: Calculate the weighted average atomic mass
To find the average atomic mass, we use the formula:

\[ \text{Average Atomic Mass} = \frac{(mass_1 \times ratio_1) + (mass_2 \times ratio_2)}{total \, parts} \]

Substituting the values:

\[ \text{Average Atomic Mass} = \frac{(35 \, \text{u} \times 3) + (37 \, \text{u} \times 1)}{4} \]

Calculating the numerator:

\[ = \frac{(105 \, \text{u}) + (37 \, \text{u})}{4} = \frac{142 \, \text{u}}{4} \]

Now, dividing:

\[ = 35.5 \, \text{u} \]

### Step 5: Conclusion
The atomic mass of chlorine is approximately 35.5 u.

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