Which of the following is not an example of a chemical change?

a. gas burning on a stove
b. rust forming on an iron fence
c. salt dissolving in a glass of water
d. using electricity to break down water into hydrogen and oxygen

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.