Why do atoms usually combine?

Most interactions among atoms take place in the outermost shell of each atom. The number of each electron in this shell determines how an atom combines with other atoms to form compounds. When atoms combine they gain, lose or share electrons in such a way that the outer shells become chemically complete.

Explanation:

the major pests we deal with are insects, weeds and fungal diseases. a pesticide is a product, whether organic or chemical, used to kill the target pest. there are other techniques gardeners should use that can prevent the severe infestations that make the use of pesticides necessary