Physics 2000 Science Trek The Periodic Table

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.

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