# Atomic mass: why nickel is abnormal

> One element breaks the pattern


## Objectives

- Read the atomic mass column of the book's table (for illustration only).
- State that atomic mass increases gradually with increasing atomic number.
- Explain why nickel (58.7 u) is abnormal: it has five stable isotopes with average mass 58.7 u.

## 1. General properties: mass, radius and metallic property




## 2. Lighter but later?

Predict: why is nickel lighter than cobalt?

- Nickel has fewer protons than cobalt
- Its atomic mass is an average over its isotopes ✓
- It is a misprint in the table

Atomic mass is the average mass of an element's isotopes. Nickel has five stable isotopes with average mass 58.7 u, lower than cobalt's, even though nickel has one more proton.

[ar-EG] الـ atomic mass هي متوسط كتلة الـ isotopes بتاعة العنصر. الـ nickel عنده خمس stable isotopes متوسط كتلتهم 58.7 u، وده أقل من الـ cobalt، مع إن الـ nickel عنده proton زيادة.
## 3. The atomic mass column

Atomic mass from Sc to Cu (book's table, p.12, for illustration only) Book point 1: the atomic mass **increases gradually with increasing atomic number**. One element breaks the pattern: nickel, 58.7 u, after cobalt, 58.9 u.


## 4. Five isotopes, one average

The book's reason: nickel has **five stable isotopes**. Its atomic mass, 58.7 u, is their average. Most nickel atoms are the light isotope nickel-58. So the average stays below cobalt's 58.9 u. One isotope against a mix of five Nickel's 58.7 is a misprint.


## 5. Mix nickel's isotopes

A simplified model: keep only nickel's two most common isotopes, nickel-58 and nickel-60. Change the share of nickel-58. share of nickel-58 Average atomic mass of the mix Cobalt (all cobalt-59) 58.7: the book's value. Mostly light nickel-58 keeps the average below cobalt's 58.9.


## 6. A new pair

Outside the transition series: argon (Z = 18) has atomic mass 39.9, potassium (Z = 19) only 39.1. Which statement explains it?

- It must be a misprint: the larger Z is always heavier
- Potassium has fewer protons than argon
- Atomic mass depends on the mix of isotopes, not only on the number of protons ✓

Atomic mass is an average over an element's isotopes, so it depends on the isotope mix, not on the number of protons alone. Most argon atoms are argon-40, which lifts argon's average above potassium's.

[ar-EG] الـ atomic mass متوسط على الـ isotopes بتاعة العنصر، فهي بتعتمد على خليط الـ isotopes، مش على عدد الـ protons لوحده. أغلب ذرات الـ argon هي argon-40، وده بيرفع متوسط الـ argon فوق الـ potassium.
## 7. Write the book's sentence

The atomic mass ___ gradually with increasing atomic number. But nickel is abnormal: it has five stable ___ with average mass 58.7 u.


## 8. Summary

Atomic mass increases gradually with increasing atomic number from Sc to Cu. Nickel is abnormal: 58.7 u, less than cobalt's 58.9 u. Reason: nickel has five stable isotopes; 58.7 u is their average, and light nickel-58 is the most common. Give reasons: the atomic mass of nickel is less than that of cobalt Nickel is abnormal because it has **five stable isotopes** with **average mass 58.7 u**. Back to the hook: can you explain why nickel is lighter than cobalt, though it has one more proton? Nickel has five stable isotopes, and its atomic mass, 58.7 u, is their average. Most nickel atoms are the light isotope nickel-58, so the average stays below cobalt's 58.9 u, even with one more proton.



Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-te.atomic-mass-trend