Which elements have similar properties?

Answer 1

The periodic table was historically created and arranged to display elements with similar properties in columns. This still holds true. The reason for this is "valence electrons" - the electrons on the outermost shell of the atom. Valence electrons are the part of atoms that interact with the rest of the world. If two elements have the same arrangement of valence electrons, the world sees them similarly, and thus they have similar properties.