If the two members of a Dobereiner triad are chlorine and iodine, the third member of this triad isFluorineBromineSodiumCalcium

To determine the third member of a Dobereiner triad given chlorine and iodine, we can apply the concept of periodic trends and the properties of elements.

Understanding Dobereiner Triads

1. Definition: Dobereiner triads are groups of three elements where the atomic mass of the middle element is approximately equal to the average of the atomic masses of the other two elements. This was an early classification system used by chemist Johann Wolfgang Döbereiner in the early 19th century.

2. Elements Provided: We have chlorine (Cl) and iodine (I). Their atomic masses are:

   • Chlorine (Cl) = 35.5
   • Iodine (I) = 126.9

Finding the Average

3. Calculating the Average: To find the atomic mass of the third element (X) in the triad, we use the relationship: $\text{Atomic mass of X} = \frac{\text{Atomic mass of Cl} + \text{Atomic mass of I}}{2}$ Plugging in the values: $\text{Atomic mass of X} = \frac{35.5 + 126.9}{2} = \frac{162.4}{2} = 81.2$

Identifying the Element

4. Choosing the Closest Element: We need to identify an element whose atomic mass is closest to 81.2. From the given options:
   • Fluorine (F): Atomic mass = 19.0
   • Bromine (Br): Atomic mass = 79.9
   • Sodium (Na): Atomic mass = 23.0
   • Calcium (Ca): Atomic mass = 40.1

The element with the atomic mass closest to 81.2 is Bromine (Br).

Final Answer

The third member of the Dobereiner triad, alongside chlorine and iodine, is Bromine.