Protoactinium (Pa): Bohr Model, Protons, Neutrons & Electrons
Protoactinium is element number 91, an actinide in the f-block (the actinide row drawn below the table), period 7. Its atom has 91 protons, 140 neutrons and 91 electrons arranged in 7 shells: 2, 8, 18, 32, 20, 9, 2.
- 91 protons
- 140 neutrons
- 91 electrons
Protons, neutrons and electrons in protactinium
Le atomic number of protactinium is 91, so every protactinium atom has 91 protons. A neutral atom has the same number of electrons, 91. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For protactinium-231, that is 231 − 91 = 140 neutrons. Every isotope of protactinium is radioactive; 231 is the mass number of its most stable one.
How to draw the Bohr model of protactinium
- Draw the nucleus and write 91 p⁺ et 140 n⁰ inside it.
- Draw the first shell and put 2 electrons on it (the first shell holds at most 2).
- Draw the second shell and put 8 electrons on it.
- Draw the third shell and put 18 electrons on it.
- Draw the fourth shell and put 32 electrons on it.
- Draw the fifth shell and put 20 electrons on it.
- Draw the sixth shell and put 9 electrons on it.
- Draw the seventh shell and put 2 electrons on it.
- Check: 2 + 8 + 18 + 32 + 20 + 9 + 2 = 91 electrons, the same as the number of protons.
From potassium onwards the shells don’t simply fill as 2, 8, 8: the fourth shell starts before the third is full, so the real arrangement above (2, 8, 18, 32, 20, 9, 2) is the one to draw.
Electron configuration
The electron configuration of protactinium is [Rn] 7s² 5f² 6d¹. As an actinide it fills an inner f-subshell, and its common ion is Pa⁴⁺.
Key facts about protactinium
| Symbol | Pa |
|---|---|
| Atomic number | 91 |
| Relative atomic mass | 231.04 |
| Group · period · block | f-block · period 7 |
| Type | Actinide |
| State at 25 °C | solid |
| Couches électroniques | 2, 8, 18, 32, 20, 9, 2 |
| Electron configuration | [Rn] 7s² 5f² 6d¹ |
| Électrons de valence | varies (outer shell: 2) |
| Common ion | Pa⁴⁺ |
| Electronegativity (Pauling) | 1.50 |
| First ionisation energy | 5.89 eV (568 kJ/mol) |
| Atomic radius (empirical) | 180 pm |
Où protactinium sits on the periodic table
Foire aux questions
How many protons, neutrons and electrons does protactinium have?
Protactinium has 91 protons and 91 electrons (atomic number 91). Its most common isotope, protactinium-231, has 231 − 91 = 140 neutrons.
What is the electron configuration of protactinium?
[Rn] 7s² 5f² 6d¹. In shells that is 2, 8, 18, 32, 20, 9, 2.
How many valence electrons does protactinium have?
Protactinium is an actinide, so its valence electrons are spread over two shells; its outermost shell holds 2.
Where is protactinium on the periodic table?
In the f-block (the actinide row drawn below the table), period 7, in the f-block. It is an actinide.
