How does the electronic configuration of an element relate to its position in the modern periodic table? 

Explanation

Electronic configuration determines the group number and period number and hence the exact position of an element in the modern periodic table. Period number: The number of electron shells gives period of an element.
Group number: The number of valence electrons decides the group of an element. For elements with 1 or 2 valence electrons, their group number is same as the number of valence electrons. For elements with valence electrons > 2, group number = 10 + number of valence electrons.
  Examples:
1) Sodium has the atomic number 11. Its electronic configuration is (2, 8, 1). This means it has two electrons in K shell, 8 electrons in L shell, and I electron in M shell. Since it has three shells, its period is 3. Since it has valence electron, its group is 1.
2]) Carbon has the atomic number 6. Its electronic configuration is (2, 4). This means it has 2 electrons in K shell and 4 electrons in L shell. Since it has two shells, its period is 2. Since it has 4 valence electrons, its group is 4 + 10 = 14.