How do the valence electrons of an element determine how they will combine with other elements to produce a compound? Please help this is urgent :)

When a comet collides with earth, it adds material to our planet and causes great damage. therefore, a collision like this is a a. destructive force b. constructive force c. geologic process and event d. constructive and destructive force

Write the empirical chemical formula of calcium with a mass percent of 38.8, phosphorus with a mass percent of 20.0, and oxygen with a mass percent of 41.3.

How have the greenhouse gasses increased from the year 2000 to 2018