About this experiment
Why do sodium and potassium behave so alike, and why does the periodic table have its odd shape? The answer is how electrons are arranged around the nucleus. Electrons fill energy levels and orbitals in a set order, two to an orbital with opposite spins, and they spread out across equal orbitals before pairing up. Build atoms from hydrogen to krypton one proton at a time, compare the shell picture with the orbital picture, and make ions to see which electrons leave first.
Learning goals
- Write the electron configuration of elements 1 to 36 in full and noble gas shorthand
- Apply the aufbau principle, the Pauli exclusion principle and Hund's rule
- Link the shell arrangement 2, 8, 8 to the full subshell picture
- Explain how the s, p and d blocks of the periodic table follow from the filling order
- Write configurations of ions, including transition metal ions
How to use it
- 1Press Build up and watch atoms from hydrogen to krypton. Which subshell fills after 3p, and why is it not 3d?
- 2Stop at nitrogen. How are the three 2p electrons arranged in the boxes? How many are unpaired? Compare with oxygen.
- 3Compare the configurations of lithium, sodium and potassium. What do they have in common, and how does it explain their chemistry?
- 4Look at the table at the bottom. Which block is the current element in, and how does that match the last subshell filled?
- 5Go to chromium and copper. How do they differ from the simple pattern? Read the note.
- 6Make Na⁺, Cl⁻ and O²⁻. Which noble gas configuration does each reach?
- 7Make Fe²⁺ and Fe³⁺. Which electrons leave first? Why is Fe³⁺ especially stable?
- 8Switch between shell order and filling order. Which convention does your exam board use?
The model
Assumptions
- Ground states of free gas-phase atoms and ions. Only elements 1 to 36 are covered.
- The neutral atom follows the n + l filling order except chromium (3d⁵ 4s¹) and copper (3d¹⁰ 4s¹), the only exceptions in this range.
- Positive ions remove electrons from the highest shell first, and within a shell from p before s. Negative ions add electrons by the filling order.
- The orbital boxes are a bookkeeping picture: arrows show spin up and spin down, not a real motion.
Try this in class
- Q1Write the full configuration of sulfur and of S²⁻.
- Q2An element has the configuration [Ar] 3d³ 4s². Name it and give its group.
- Q3How many unpaired electrons does a manganese atom have? And Mn²⁺?
- Q4Why does 4s fill before 3d in potassium and calcium, but 4s electrons are lost first when scandium forms Sc³⁺?
- Q5Why do elements in the same group have similar chemical properties?
Frequently asked questions
Should 4s be written before or after 3d?
Both are seen. Writing subshells in shell order ([Ar] 3d⁶ 4s²) is the IUPAC style and is used by many exam boards. Writing them in filling order ([Ar] 4s² 3d⁶) shows the order they were added. Check which one your course uses; the lab shows both.
Why is the 2, 8, 8 rule not the whole story?
The third shell can hold 18 electrons, but 4s is lower in energy than 3d, so the fourth shell starts at potassium before the third shell is full. The simple 2, 8, 8 pattern only works up to calcium.
Are there more exceptions further down the table?
Yes. Molybdenum, silver, gold and several others also break the simple order, and for heavy elements the energies are so close that the configurations are hard to predict without calculation.