Atomic mass: why nickel is abnormal
One element breaks the pattern
1. General properties: mass, radius and metallic property
2. Lighter but later?
Nickel (Z = 28) comes after cobalt (Z = 27). Yet the book's table gives nickel the smaller atomic mass.
Pick the right answer
Predict: why is nickel lighter than cobalt?
- A. It is a misprint in the table
- B. Its atomic mass is an average over its isotopes
- C. Nickel has fewer protons than cobalt
الشرح
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.
3. The atomic mass column
| Element (Sc → Cu) | Atomic mass (u) |
|---|---|
| Sc | 45 |
| Ti | 47.9 |
| V | 51 |
| Cr | 52 |
| Mn | 54.9 |
| Fe | 55.9 |
| Co | 58.9 |
| Ni | 58.7 |
| Cu | 63.5 |
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.
| Element | Z | Stable isotopes | Atomic mass |
|---|---|---|---|
| Cobalt | 27 | one (all cobalt-59) | 58.9 u |
| Nickel | 28 | five (mostly nickel-58) | 58.7 u |
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 50 %
Average atomic mass of the mix 59 u
Cobalt (all cobalt-59) 58.9 u
6. A new pair
Pick the right answer
Outside the transition series: argon (Z = 18) has atomic mass 39.9, potassium (Z = 19) only 39.1. Which statement explains it?
- A. Atomic mass depends on the mix of isotopes, not only on the number of protons
- B. It must be a misprint: the larger Z is always heavier
- C. Potassium has fewer protons than argon
الشرح
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.
True or false?
Atomic mass always increases with atomic number.
الشرح
Atomic number counts protons. Atomic mass is the average over the isotopes, so the two need not rise together.
7. Write the book's sentence
Fill in the blanks
The atomic mass _____ gradually with increasing atomic number. But nickel is abnormal: it has five stable _____ with average mass 58.7 u.
الشرح
Book point 1, p.12: mass increases gradually; nickel's five stable isotopes average 58.7 u.
Need a hint?
- Look at the general direction of the mass column.
- Nickel atoms differ in their number of neutrons.
- Same atomic number, different mass numbers: you met this word with chlorine.
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.
Back to the hook: can you explain why nickel is lighter than cobalt, though it has one more proton?
Reveal
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.