Dobereiner's Law of Triads Explanation

Johann Wolfgang Döbereiner proposed a classification system based on triads of elements. His Law of Triads states that when elements are arranged in order of increasing atomic weights, the arithmetic mean (average) of the atomic weights of the first and third elements in a triad is approximately equal to the atomic weight of the middle element.

Let's examine the atomic weights (approximate) for the elements in each triad provided:

• Calcium (Ca): 40.08 g/mol
• Strontium (Sr): 87.62 g/mol
• Barium (Ba): 137.33 g/mol
• Chlorine (Cl): 35.45 g/mol
• Bromine (Br): 79.90 g/mol
• Iodine (I): 126.90 g/mol
• Nitrogen (N): 14.01 g/mol
• Phosphorus (P): 30.97 g/mol
• Arsenic (As): 74.92 g/mol
• Beryllium (Be): 9.01 g/mol
• Magnesium (Mg): 24.31 g/mol
• Calcium (Ca): 40.08 g/mol

Analyzing Chemical Triads Using Atomic Weights

We will check if each triad follows Döbereiner's Law:

Triad 1: Ca, Sr, Ba

Average of Ca and Ba = $\frac{40.08 + 137.33}{2} = \frac{177.41}{2} = 88.705$ g/mol. This value is very close to the atomic weight of Sr (87.62 g/mol). Therefore, this is a Döbereiner triad.

Triad 2: Cl, Br, I

Average of Cl and I = $\frac{35.45 + 126.90}{2} = \frac{162.35}{2} = 81.175$ g/mol. This value is close to the atomic weight of Br (79.90 g/mol). Therefore, this is a Döbereiner triad.

Triad 3: N, P, As

Average of N and As = $\frac{14.01 + 74.92}{2} = \frac{88.93}{2} = 44.465$ g/mol. This value is significantly different from the atomic weight of P (30.97 g/mol). Therefore, this is NOT a Döbereiner triad.

Triad 4: Be, Mg, Ca

Average of Be and Ca = $\frac{9.01 + 40.08}{2} = \frac{49.09}{2} = 24.545$ g/mol. This value is close to the atomic weight of Mg (24.31 g/mol). Therefore, this is a Döbereiner triad.

Conclusion on Dobereiner's Triads

Based on the calculations, the triad consisting of Nitrogen (N), Phosphorus (P), and Arsenic (As) does not fit Döbereiner's Law of Triads, as the average atomic weight of Nitrogen and Arsenic is not approximately equal to the atomic weight of Phosphorus.

Therefore, the triad that was not given by Dobereiner is N, P, As.