Question

how would you justify the presence of 18 elements in fifth period?( please don't copy answer from text)​

Answer 1

We can justify this using the Aufbau Principle.

When we fill the 5th shell, according to the Aufbau Principle, 5s, 5p, and 4d shells are filled.

So, we can add the number of electrons.

2 + 6 + 10 =  18 electrons or elements in 5th shell or period

Therefore it is justified that there are 18 elements in 5th period.