Atomic Number
In the modern periodic table, the elements are actually arranged in order of
increasing atomic number--that's the number of protons in one atom of a
particular element. An undisturbed atom is electrically neutral, so the
number of electrons in it is the same as its atomic number.
Atomic weight almost always increases with
atomic number, so Mendeleev's sequence of elements was almost exactly the
same as the one used today, though there are a couple of weird exceptions.
In general, it's correct to think of atoms getting heavier as you go down a
column or to the right across a row.
