Atomic number and mass number
What the 26 in ₂₆Fe and the 60 in cobalt 60 mean
1. Atomic number and mass number: from zero
2. Two numbers, two meanings
The book writes ₂₆Fe and talks about cobalt 60. What do the 26 and the 60 count?
Remember: the nucleus holds protons (+) and neutrons (no charge). The electrons (−) are outside it.
Pick the right answer
Predict: in ₂₆Fe, what does 26 count?
- A. The protons in the nucleus
- B. The protons and the neutrons together
- C. The protons and the electrons together
الشرح
In ₂₆Fe, 26 is the atomic number: the number of protons in the nucleus. Protons and neutrons together give the mass number.
3. Atomic number (Z)
Atomic number
The number of protons in the nucleus of an atom.
gatheredatomic number (Z)\\ = number of protonsgathered
Each element has its own atomic number. Scandium is 21 and zinc is 30.
4. Mass number
Mass number
The sum of the numbers of protons and neutrons in the nucleus.
A = Z + N N = A - Z
Worked example: cobalt 60
⁶⁰₂₇Co
- Z = 27, so 27 protons.
- A neutral atom, so 27 electrons.
- Neutrons = 60 − 27 = 33.
5. Build a nucleus
Change the protons and the neutrons. Watch what changes.
Protons (atomic number Z) 26
Neutrons 30
Z = 26: this is iron, Fe.
Mass number (A) 56
Electrons in the neutral atom 26
Change only the neutrons. Does the element change?
No. The element is fixed by Z, the number of protons. Changing the neutrons changes only the mass number.
6. Count the particles
Fill in the blanks
An atom of ⁵⁸₂₈Ni has _____ protons, _____ electrons and _____ neutrons.
الشرح
Protons = Z = 28; electrons = 28 (neutral atom); neutrons = 58 − 28 = 30.
Pick the right answer
A copper atom has 29 protons and 34 neutrons. What is its mass number?
- A. 29
- B. 63
- C. 92
- D. 5
الشرح
Mass number = protons + neutrons = 29 + 34 = 63. The electrons are not counted, because they are too light.
7. Summary
- Atomic number (Z) = protons = electrons in a neutral atom: ₂₆Fe has 26.
- Mass number (A) = protons + neutrons: cobalt 60 has A = 60.
- Neutrons = A − Z: 60 − 27 = 33.
Can you explain why Fe²⁺ has 24 electrons but is still iron?
Reveal
Fe²⁺ has lost 2 electrons, so it has 24 electrons. It still has 26 protons, and the element is fixed by Z, the number of protons.