Which of the following is not Dobereiner triad?

He, Na and Ar

The correct answer is option 2. He, Na, and Ar.

Johann Wolfgang Döbereiner attempted to group elements with similar properties into sets of three, known as Dobereiner triads. In these triads, the atomic weight of the middle element was roughly the average of the atomic weights of the other two elements.

Now, let's examine the given triad:

He (Helium)

Na (Sodium)

Ar (Argon)

The atomic weights of these elements are:

He (Helium): ~4.00

Na (Sodium): ~22.99

Ar (Argon): ~39.95

If we compare the atomic weights, the average of Helium and Argon is approximately (4.00 + 39.95)/2 = 21.98, which is significantly different from the atomic weight of Sodium (22.99). In a true Dobereiner triad, the atomic weight of Sodium should be close to the average of Helium and Argon, but in this case, it does not fit the expected pattern.

Therefore, He, Na, and Ar do not form a Dobereiner triad based on the criteria proposed by Döbereiner.