Element A is soft and shiny, has a melting point of 68 °C and conducts electricity well in both its solid and liquid states. Its atomic number is one greater than that of a monoatomic gaseous element, present to the extent of 1% in the earth's atmosphere. Element B has an atomic number one less than that of a monoatornic gaseous element. It exists as a brown liquid at 25 °C and as a brown diatomic gas at temperatures above 58 °C. Identify A and B.
