# Aufbau principle: the order of filling

> Why 4s fills before 3d


## Objectives

- State the Aufbau principle and the filling order 1s 2s 2p 3s 3p 4s 3d 4p.
- Use K and Ca as evidence that 4s is filled before 3d.
- Use the filling order to share an atom's electrons between 4s, 3d and 4p.

## 1. Aufbau principle (order of filling): from zero




## 2. Where do electrons 19 and 20 go?

Predict:

- Into 3d
- Into 4p
- Into 4s ✓

Electrons 19 and 20 go into 4s. By the Aufbau principle, 4s fills before 3d, so calcium is $[\text{Ar}]\ 4s^2$; 3d starts filling at scandium.

[ar-EG] الـ electrons رقم 19 و 20 بيروحوا الـ 4s. حسب الـ Aufbau principle، الـ 4s بيتملي قبل الـ 3d، فالـ calcium هو $[\text{Ar}]\ 4s^2$؛ والـ 3d بيبدأ يتملي عند الـ scandium.
## 3. The Aufbau principle

Aufbau principle (building-up principle) Electrons fill the sublevels in order, starting with the lowest-energy sublevel, 1s. The filling order up to 4p Follow the arrows one after another, each from top right to bottom left. This gives the filling order.


## 4. Build the atoms from Ar to V

[Ar] means argon's 18 electrons: $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6$. Change the atomic number from Ar to V and watch where each new electron goes. Atomic number (Z) $_{18}\text{Ar}$: $1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6$ $_{19}\text{K}$: $[\text{Ar}]\ 4s^1$ $_{20}\text{Ca}$: $[\text{Ar}]\ 4s^2$ $_{21}\text{Sc}$: $[\text{Ar}]\ 4s^2\ 3d^1$ $_{22}\text{Ti}$: $[\text{Ar}]\ 4s^2\ 3d^2$ $_{23}\text{V}$: $[\text{Ar}]\ 4s^2\ 3d^3$ From Sc to V, each new electron goes into 3d. Beyond V, Cr and Cu break this pattern; you will meet them in Ch.1. 3d fills before 4s, because 3 is less than 4.


## 5. Use the order

Put these sublevels in the order they are filled. Gallium (Z = 31) has 13 electrons after [Ar]. Write how many go into each sublevel: 4s ___, 3d ___, 4p ___.


## 6. Summary

Aufbau: electrons fill the sublevels in order, starting with 1s. Order: 1s 2s 2p 3s 3p **4s 3d** 4p. Back to the hook: electrons 19 and 20 go into 4s; 3d starts at Sc. 4s fills first (K, Ca); 3d starts at Sc After calcium $[\text{Ar}]\ 4s^2$, the sublevel (3d) is filled successively (Ch.1 p.4). Coming up in Ch.1 When a transition metal forms an ion, it loses its 4s electrons first, because 4s is in the outermost level, n = 4 (Ch.1 p.9). Using calcium, can you explain why '3 comes before 4' is the wrong way to find the filling order?



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-atom.aufbau