What must be balanced on the left side of an equation with the atoms on the right side of the equation? total number of molecules total number of atoms total number of compounds free electrons

The total number of atoms must be the same on both sides in order for it to be a balanced chemical equation.

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.