Chlorophenols impart unpleasant taste and odor to drinking water at concentrations as low as 5 mg/m3. They are formed when the chlorine dis- infection process is applied to phenol-containing waters. What is the threshold for unpleasant taste and odor in units of (a) mg/L, (b) mg/L, (c) ppmm, and (d) ppbm

you need to take a look at the periodic system in order to sort these elements into pairs that would most likely exhibit similar chemical properties. first, you need to find elements that are found in the same group (groups are vertical in the periodic system), and then to see whether they have the same valence number. if they meet these criteria, they will have similar properties.

the first pair: ne (neon) and kr (krypton), are noble gases found in group 8

the second pair: li (lithium) and k (potassium), found in group 1

the third pair: s (sulfur) and te (tellurium), found in group 6

the answer is na (sodium)