Electronic Structures of the First 20 Elements

The first energy level (also known as shell) can hold a maximum of 2 electrons.
The second energy level can hold a maximum of 8 electrons.
The third energy level can hold up to 8 electrons.

Elements 1-10 (Hydrogen to Neon)

Hydrogen (H) and Helium (He) have electrons in the first energy level only.
Beginning with Lithium (Li), electrons start filling the second energy level.
Beryllium (Be) has 4 electrons: 2 in the first level and 2 in the second level.
Boron (B) to Neon (Ne) - have full second energy levels of 8 electrons.

Elements 11-20 (Sodium to Calcium)

Sodium (Na) to Argon (Ar) have full second energy levels and start filling the third level.
Potassium (K) and Calcium (Ca) begin to fill a fourth energy level.

Remember, an atom’s electron configuration can be represented by a distribution of electrons among energy levels which is denoted by number and type of sublevel and number of electrons in that sublevel e.g., 2.8.8 for Argon.

An understanding of the electronic structures is critical to understand how elements interact and bond with each other to form compounds.

Pro Tip: The position of an element in the Periodic Table gives a clue about its electronic structure. Element’s Group number (except Transition metals) is equal to the number of electrons in its outermost shell, and the Period number is equal to the number of shells used by that element.