CBSE Class 10 Science 2022 · Paper 31(B)/5 · Question 2

Two properties of three elements A, B and C that form a Döbereiner's triad: 1. They have similar chemical properties; they behave alike chemically, as lithium, sodium and potassium do. 2. When they are arranged in order of increasing atomic mass (say A, B, C), the atomic mass of the middle element B is roughly the average (arithmetic mean) of the atomic masses of A and C: $m_B\approx \frac{m_A+m_C}{2}$. For example, $\frac{6.9+39.0}{2}=22.95$, close to $23.0$, the atomic mass of sodium.