What is electron configuration? Orbital filling rules and how to write it

What is electron configuration?

An electron configuration is a notation that shows how many electrons sit in each shell and subshell of an atom. The configuration of sodium is written 1s²2s²2p⁶3s¹. The first number is the shell number, the letter is the subshell, and the superscript is the number of electrons there. The sum of the superscripts (2 + 2 + 6 + 1 = 11) equals the total number of electrons in the atom.

This simulator shows the subshells from 1s to 4s. In the first 20 elements, no electrons go into the d and f subshells.

Three rules for filling orbitals

1. Aufbau principle: electrons fill the lowest-energy orbitals first: 1s, 2s, 2p, 3s, 3p, 4s, 3d.
2. Pauli exclusion principle: one orbital holds at most 2 electrons, and they have opposite spins (↑↓).
3. Hund's rule: in orbitals of equal energy, electrons first go in one at a time, and then they pair up.

In potassium and calcium, the 19th and 20th electrons go into 4s, not 3d, because 4s has lower energy than 3d.

In noble-gas notation, the symbol of the previous noble gas replaces the inner electrons: sodium is [Ne]3s¹, and potassium is [Ar]4s¹.

How do you use the simulator?

1. The simulator opens on sodium. Use the − and + buttons in the ELEMENT section of the bottom row to choose an element with a number from 1 to 20.
2. In the diagram on the left, each square box is one orbital, and each arrow is one electron. Energy increases as you go up the ENERGY axis.
3. The right panel shows the name of the element, the number of electrons, the full notation, and the NOBLE-GAS NOTATION.
4. On a phone, scroll down the panel to the ELECTRONS PER SHELL section to see the number of electrons in each shell. For sodium, it is 2, 8, 1.
5. The yellow button in the header returns to hydrogen.

Tasks to try in the simulator

• Use the + button to go from nitrogen to oxygen. What changes in 2p? Answer: Nitrogen has one ↑ in each of the three boxes, and oxygen adds a ↓ in the first box to form a pair.
• Go from argon to potassium. Where does the 19th electron go? Answer: Into 4s, and the noble-gas notation is [Ar]4s¹.
• Compare the noble-gas notation of fluorine and chlorine. Answer: [He]2s²2p⁵ and [Ne]3s²3p⁵. Both have 7 electrons in the outer shell, so they are in the same group.

Common mistakes

• Forgetting Hund's rule. Writing the 2p⁴ of oxygen as ↑↓ ↑↓ is wrong. The correct form is ↑↓ ↑ ↑.
• Not checking the superscripts. When you finish, add the superscripts and compare the sum with the atomic number.
• Taking the wrong noble gas. Use the noble gas that comes before the element: [Ne] for chlorine and [Ar] for potassium.

The number of electrons in the outer shell tells you the position of an element. To see this connection, read the periods and groups guide.

Sources

• Chemistry LibreTexts: Electron Configurations, How Electrons Occupy Orbitals
• Chemistry LibreTexts: Quantum-Mechanical Orbitals and Electron Configurations