The number of

is different for each element and is known as the

number,

Electrons, Atomic Number

Explanation:

The total number of electrons is different for each and every element of the periodic table.

It is known as the atomic number of an element.

Mendeleev's Periodic Table's principle:

The properties of the elements is a periodic function of their atomic mass.

But, this was true only for the lighter elements.

So, the principle of the Long Form of the Periodic Table is:

The properties of the elements is a periodic function of their atomic numbers.