In the nineteenth century, what was known about atoms?

a. the atom determines the chemical property of an element. b. the exact structure of all atoms c. the half-life of all atoms d. the number of atoms that exist in the world is increasing.

 A. The atom determines the chemical property of an element.

Explanation:

     A. The atom determines the chemical property of an element.

Explanation:

An atom is made up protons, electrons and neutrons. The atoms valance electrons determine its chemical properties. For example, Na has 1 valance electron. Na just like all other alkali elements lose that one outermost electron to form a cation thus gaining an octet. Because of the one valance electron, alkali elements show metallic property and other chemical properties. If an atom has an unstable nucleus then it becomes radioactive.

8. a. 7

   b. 7(2x+4y+3)

9. 3(5a-2b+12)

  -3(-5a+2b-12)