Protactinium (Pa)
Protactinium is element 91, an actinide in the actinide series (period 7). A neutral atom has 91 protons, 140 neutrons and 91 electrons, 2 of them valence electrons.
Actinide- Atomic number
- 91
- Atomic mass
- 231.04 u
- Valence electrons
- 2*
- Protons · neutrons
- 91 · 140
- Electrons per shell
- 2, 8, 18, 32, 20, 9, 2
- At room temperature
- solid
Bohr model of protactinium
The Bohr model draws protactinium as a nucleus of 91 protons and 140 neutrons, with its 91 electrons on 7 circular shells: 2 in shell 1, 8 in shell 2, 18 in shell 3, 32 in shell 4, 20 in shell 5, 9 in shell 6, 2 in shell 7. Press Build the atom to watch the shells fill from the inside out. The outer ring (the Q shell) is dashed: the electrons there are the ones that take part in chemistry.
Real electrons don't travel in neat circles. They live in fuzzy orbitals, shown below, but the Bohr picture is the quickest way to count shells and valence electrons.
Electron configuration of protactinium
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f² 6d¹
Short form (noble gas notation): [Rn] 7s² 5f² 6d¹. The [Rn] stands for the electrons of radon, the noble gas before it.
Protactinium breaks the usual filling order. The Aufbau rule predicts 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f³, but experiments show the configuration above. The energy levels of the subshells are so close here that a different arrangement wins.
How many valence electrons does protactinium have?
Protactinium has 2 valence electrons.
Protactinium is a actinide, so its valence electrons are less clear-cut than for main-group elements. It has 2 electrons in its outermost shell (shell 7), and electrons in its inner f and d subshell can also take part in bonding.
Protons, neutrons and electrons
| Protons | 91 | equal to the atomic number |
|---|---|---|
| Electrons | 91 | equal to the protons in a neutral atom |
| Neutrons | 140 | mass number 231 − 91 protons |
Atoms of the same element can have different numbers of neutrons (isotopes). The count above uses the rounded atomic mass, the way it's taught in class.
Protactinium properties
| Symbol | Pa |
|---|---|
| Category | Actinide |
| Group · period · block | — · 7 · f |
| Atomic mass | 231.04 u |
| State at 25 °C | solid |
| Melting point | 1,845 K (1,570 °C, 2,860 °F) |
| Boiling point | — |
| Density | 15.37 g/cm³ |
| Electronegativity (Pauling) | 1.5 |
| Atomic radius | 243 pm |
| First ionization energy | 5.89 eV |
| Electron affinity | — |
| Oxidation states | +5, +4 |
| Common ion charge | +5, +4 |
| Discovered | 1913 by William Crookes |
Where protactinium sits on the periodic table
Actinides: Actinium, Thorium, Uranium, Neptunium, Plutonium, Americium, Curium, Berkelium.
Questions about protactinium
How many valence electrons does protactinium have?
Protactinium has 2 valence electrons in its outer shell; inner d or f electrons can also bond.
How many protons, neutrons and electrons does protactinium have?
A neutral protactinium atom has 91 protons and 91 electrons. Its most common form has about 140 neutrons (mass number 231 − 91 protons).
What is the electron configuration of protactinium?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f² 6d¹, written in short form as [Rn] 7s² 5f² 6d¹.
What does the Bohr model of protactinium look like?
A nucleus of 91 protons and 140 neutrons, circled by 7 electron shells holding 2, 8, 18, 32, 20, 9, 2 electrons.
What is the atomic number of protactinium?
91. The atomic number is the number of protons in the nucleus, and it is what makes an atom protactinium.