Why do the properties of elements change in a regular way across a period

Modern weaponry has increased the number of deaths in wars and violent conflicts.

Select all of the statements which are true. electrons are located in shells or orbits around the atom. electrons orbit slowly around the atom. electrons travel in one flat path around the nucleus of an atom. the valence of an atom is determined by the number of electrons in the atom's outermost shell.

What physical property of gold makes panning a useful way to get gold from streams?