Classification of elements and periodicity in their properties

For example, he identified the triad of Lithium (Li), Sodium (Na), and Potassium (K).
* Atomic mass of Li ≈ 7
* Atomic mass of K ≈ 39
* Average = (7 + 39) / 2 = 23. This is close to the atomic mass of Na ≈ 23.

This was an early attempt but only a few elements could be grouped this way.

For example:
| Li | Be | B | C | N | O | F |
|----|----|---|---|---|---|---|
| Na | Mg | Al| Si| P | S | Cl|

The Modern Periodic Table is different from earlier versions (like Mendeleev's) because it's based on atomic number (Z), not atomic mass. The atomic number is the number of protons in an atom's nucleus, and for a neutral atom, it's also the number of electrons. This makes sense because electrons are what primarily determine an element's chemical properties.