Taking the example of an element of atomic number 16, explain how the electronic configuration of the atom of an element relates to its position in the modern periodic table and how valency of an element is calculated on the basis of its atomic number.

Atomic number of an element is 16.
To know its position in periodic table first we should write the electronic configuration of the element.
Electronic configuration of the element - 2 8 6.
Valence electrons:  The electrons, which are present in the outermost shell of an atom are called valence electrons.
This element contains 6 valence electrons hence, it belongs to VI A group.
In this element 3rd shell is the outermost shell (Valence shell) hence, it belongs to period 3.
Valency: The combining capacity of an element is known as valency.
The combining capacity of the atoms to form molecules either with same or different elements is defined as valency.
Atom contains less than four electrons in its outermost shell, the valency of an atom is equal to the number of electrons present in the valence shell.
Example: Sodium has one electron in its outermost shell, so the valency of sodium is 1.
Calcium has two electrons in its outermost shell, so the valency of calcium is 2.
Aluminum has three electrons in its outermost shell, so the valency of aluminum is 3.
If the outer shell has more than four electrons, the valency = 8 - the number of electrons in the outer shell.

Mg-26 and mg-24 are pair of isotopes. isotopes are defined as atoms of the same element having same atomic number but different mass number. i hope that was .