Atomic theory had been pretty well established well before the turn of the previous century, but no one knew much about the nature of atoms other than that the atoms of different elements had different masses (later advances revealed that they have different numbers of protons), and were composed of a positive nucleus and negative electrons, with positive and negative charges being equal in number so that the net electrical charge was zero.
Electrons were definitely established by J. J. Thomson in 1897, and he postulated that atoms were more of less continuous lumps of matter matter with positive charge in which lumps of negative charge (electrons) were embedded, although they had some freedom of movement.