What is happening when an oxygen molecule is formed from two separate oxygen atoms?

Nonmetal halogen family atmoic mass 35

How are evaporation and sublimation similar? a both involve the formation of a gas. b both release energy to the surroundings. c both take place throughout a solid. d both take place at the surface of a liquid.

Answer asap : ( a. how does mucus prevent the entry of pathogens? b. describe two ways white blood cells protect us from pathogens.