Imagine a set of jars with a different combination of blue (protons) red (nuetrons) and yellow (electrons) balls
Put them in order on a line
You could have chosen total number, num of red num of blue num of yellow or any combination in between.
The number of blue is what was chosen.
Aa such if i have 4 blue 5 blue 7 blue, then the 6 blue is missing.
The perodic table also arranges elements in groups that behave similarly for our current understanding of chemistry and physics. It is just what made sense at the time and continues to make sense.
New chemistry or physics, quamtum physics could come along and give us a new layout,and that layout is missing elements as well that are defined by the new definition.
There were gaps for undiscovered atoms at some point. If we called 1 apple and Adam, 2 apples a Beth, 3 apples a Charlie. We haven’t discovered 4 apples yet, but we call groups of 5 apples an Eve, and so on. Then look back, of course the names correspond to continuous numbers.
The definition of hydrogen is having 1 proton (it can have isotopes with different numbers of neutron). If we discover a new thing with 1 proton and 1337 neutron, it will be an isotope of hydrogen.
Because the gaps in the periodic table give us clues about what properties the atom has and therefore where to find it.
Initially the periodic table did have a lot of gaps but because of its predictive power it made it much easier to fill in these gaps over time.
Look up a history of the periodic table to get more info
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