That statement is true
it was easy to organize 'all' the elements in a group of threes because back then they only knew about 15 - 20 elements
hope this helps
I think this one is the organelles.
1) Balanced chemical reaction: Cl₂ + 2NaI → 2NaCl + I₂.
Chlorine and iodine are diatomic molecules.
2) Balanced chemical reaction: 2NH₃ → N₂ + 3H₂.
Nitrogen and hydrogen are diatomic molecules.
3) Balanced chemical reaction: 4Na + O₂ → 2Na₂O.
Sodium in compounds has oxidation number +1 and oxygen -2.
