* 1s
* 2s
* 2p
The first period of the periodic table consists of only two elements, hydrogen and helium. Hydrogen has one electron in the 1s orbital. Helium has two electrons in the 1s orbital.
The second period of the periodic table consists of eight elements: lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon. Lithium has one electron in the 2s orbital. Beryllium has two electrons in the 2s orbital. Boron has two electrons in the 2s orbital and one electron in the 2p orbital. Carbon has two electrons in the 2s orbital and two electrons in the 2p orbital. Nitrogen has two electrons in the 2s orbital and three electrons in the 2p orbital. Oxygen has two electrons in the 2s orbital and four electrons in the 2p orbital. Fluorine has two electrons in the 2s orbital and five electrons in the 2p orbital. Neon has two electrons in the 2s orbital and six electrons in the 2p orbital.
The third period of the periodic table consists of eight elements: sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, and argon. Sodium has one electron in the 3s orbital. Magnesium has two electrons in the 3s orbital. Aluminum has two electrons in the 3s orbital and one electron in the 3p orbital. Silicon has two electrons in the 3s orbital and two electrons in the 3p orbital. Phosphorus has two electrons in the 3s orbital and three electrons in the 3p orbital. Sulfur has two electrons in the 3s orbital and four electrons in the 3p orbital. Chlorine has two electrons in the 3s orbital and five electrons in the 3p orbital. Argon has two electrons in the 3s orbital and six electrons in the 3p orbital.
And so on.
