How do you know when a chemical change has occurred?

The five conditions of chemical change: color change, formation of a precipitate, formation of a gas, odor change, temperature change

Hope that helps

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.