Which sequence represents a correct order of historical developments leading to the modern model of the atom?
(1) the atom is a hard sphere => most of the atom is empty space=> electrons exist in orbitals outside the nucleus
(2) the atom is a hard sphere => electrons exist in orbitals outside the nucleus => most of the atom is empty space
(3) most of the atom is empty space => electrons exist in orbitals outside the nucleus => the atom is a hard sphere
(4) most of the atom is empty space=> the atom is a hard sphere => electrons exist in orbitals outside the nucleus