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Theory:

Modern Periodic Law:
In $$1887$$, an English physicist, Henry Moseley, was born in Weymouth, Dorset. His father, Henry Nottidge Moseley, was a biologist and professor of anatomy and physiology at the University of Oxford. Moseley's mother, Amabel Gwyn Jeffreys' contribution to physics, justified the earlier empirical and chemical concept of the atomic number using physical rules.
Henry Gwyn Jeffreys Moseley ($$1887$$ - $$1915$$)

In $$1913$$, Henry Moseley showed that an element's atomic number (symbolised as $$Z$$) is more fundamental than its atomic mass. Therefore, He modified Mendeléev's periodic law, and the atomic number was adopted as the basis of the modern periodic table. The modern periodic law can be stated as follows:
The chemical and physical properties of the elements are the periodic functions of their atomic numbers.
Let us remember that the atomic number indicates how many protons are in the nucleus of an atom, and it increases by one as we proceed through the elements. When elements are arranged in the order of increasing atomic number, we get the modern periodic table.

Prediction of properties of elements could be made with more precision when elements were arranged based on increasing atomic numbers.

Activity 5.3

1. How did the positions of cobalt and nickel in the modern periodic table get resolved?

The atomic number is the basis for the modern periodic table. In the modern periodic table, cobalt ($$27$$) is placed before nickel ($$28$$).

2. How were the positions of isotopes of different elements decided in the modern periodic table?

The isotopes are positioned in the same group in the modern periodic table because they have the same atomic number.

3. Is it possible to put an element with an atomic number of $$1.5$$ between hydrogen and helium?

No, because an atomic number is a whole number, it is not possible.

4. Where do you think hydrogen should be placed in the modern periodic table?

The current position of hydrogen in the modern periodic table is correct.
Reference:
https://en.wikipedia.org/wiki/Henry_Moseley