Electronic Structures of the First 20 Elements

The electronic structure of an atom represents the distribution of electrons in the atomic shells.
Electrons occupy energy levels around the nucleus, known as shells or orbitals.
The first energy level (nearest the nucleus) can hold up to two electrons.
The second and third energy levels can hold up to eight electrons each.
The electronic structure is normally written as numbers, separated by commas. Each number represents the number of electrons in each shell, from innermost to outermost.
The first 20 elements, with their electronic structures, are as follows:
- Hydrogen: 1
- Helium: 2
- Lithium: 2,1
- Beryllium: 2,2
- Boron: 2,3
- Carbon: 2,4
- Nitrogen: 2,5
- Oxygen: 2,6
- Fluorine: 2,7
- Neon: 2,8
- Sodium: 2,8,1
- Magnesium: 2,8,2
- Aluminum: 2,8,3
- Silicon: 2,8,4
- Phosphorus: 2,8,5
- Sulfur: 2,8,6
- Chlorine: 2,8,7
- Argon: 2,8,8
- Potassium: 2,8,8,1
- Calcium: 2,8,8,2
Elements in the same group (column) of the periodic table have the same number of electrons in their outer shell. This outer shell is also known as the valence shell.
The number of valence electrons determines many of the element’s properties, including its reactivity. Elements with full outer shells (such as helium, neon, and argon) are stable and unreactive, while those with nearly full or nearly empty outer shells are often very reactive.