He thought elements that haven't been discovered belonged in the place of the gap. He could also use the atomic mass of the missing elements
atomic number = number of protons
number of protons = number of electrons
So, number of electrons = 5
So, electron configuration = 2, 3
So, its valency = 3
The ph level depends on temperature.
10
subtract the atomic mass from the atomic number
The valence electrons are easily delocalized, I believe