The weight of an atom might be a measure of its complexity. If so, it must hide some internal structure, some way the atom is physically put together, which generates those properties.

What came to be known as the atomic age began in 1897, when J. J. Thomson in Cambridge discovered the electron.

The atom has constituent parts; it is not indivisible, as its Greek name had implied. The electron is a tiny part of its mass or weight, but a real part, and it carries a single electric charge. Each element is characterised by the number of electrons in its atoms. And their number is exactly equal to the number of the place in Mendeleev's table of elements that element occupies when hydrogen and helium are included in first and second place. That is, lithium has three electrons, beryllium has four electrons, boron has five, and so on steadily all through the table. The place in the table that an element occupies is called its atomic number, and now that turned out to stand for a physical reality within its atom - the number of electrons there. The picture has shifted from atomic weight to atomic number, and that means, essentially, to atomic structure.

That is the intellectual breakthrough with which modern physics begins.  Thompson's discovery led inevitably to efforts to split the atom into its component parts, which revealed a human capacity to produce sustainable energy and global destruction as the outcomes of releasing nuclear power.