Let's check if Lithium (Li), Sodium (Na), and Potassium (K) form a Dobereiner Triad. Step 1: Find their atomic masses. Li = 6.9, Na = 23.0, K = 39.1. Step 2: Calculate the average of the first and third. Average = (Mass of Li + Mass of K) / 2. Step 3: Add the numbers: 6.9 + 39.1 = 46.0. Step 4: Divide by 2: 46.0 / 2 = 23.0. Step 5: Compare the result to the middle element. The average is 23.0, and the mass of Sodium is 23.0. Because they match, they form a perfect triad!