Predict the formula of protactinium(V) oxide.
Pa5+ with O2- crosses to Pa2O5 — the tantalum pattern (Ta2O5) one row down, confirmed on gram-scale samples that represent a meaningful fraction of all protactinium ever isolated.
What Is the Charge of Protactinium?
Protactinium commonly forms Pa4+ Pa5+ in introductory ionic compounds.
Protactinium favors +5 (Pa2O5, PaCl5) with an accessible +4 (PaO2) — tantalum-like chemistry from uranium's obscure neighbor. Perhaps the least-studied element before plutonium: the world's usable stockpile traces back largely to one 1960s British extraction of 125 grams.
| Element | Protactinium |
|---|---|
| Symbol | Pa |
| Atomic number | 91 |
| Common ionic charge | +4, +5 |
| Common ions | Pa4+, Pa5+ |
Visual reference
Protactinium element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
The electron story behind Protactinium's charge
Protactinium is [Rn]5f2 6d1 7s2. Its five outer electrons can all engage (+5, leaving an empty f shell), or one f electron can stay home (+4). This is the actinide series mid-transition: thorium behind it is fixed at +4, uranium ahead adds +6 — protactinium is the hinge.
This is a classroom reference range, not an exhaustive list of all oxidation states.
The assumption that trips people up on Protactinium
Guessing 'proto-actinium' means it comes before actinium chemically. The name records decay ancestry: Pa-231 decays INTO actinium. In the table it sits after Ac and Th — element 91, parent by nucleus, descendant by position.
Worked example: Pa in a formula problem
Pa4+ and Pa5+ in real compounds
PaCl4
Protactinium chloride
Pa4+ balances chloride ions.
Pa2O3
Protactinium oxide
Protactinium can form oxides where total positive and negative charge balances.
Questions students ask about protactinium
What is protactinium actually used for?
Almost nothing applied — but Pa-231 measurements date ocean sediments and validate thorium-reactor physics. Its scarcity is the application's ceiling: demand exists; grams do not.
Why was isolating it such an ordeal?
Pa5+ hydrolyzes viciously, gluing itself to any surface and forming colloids — a radiochemist's nightmare ion. The 1961 UK campaign processed 60 tonnes of ore residue for its 125 grams.
Where does protactinium sit in the thorium fuel cycle?
Dead center: Th-232 + neutron → Pa-233 → (27 days) → U-233. Reactor designers must manage that month-long Pa4+/5+ inventory carefully — protactinium parked in the salt is future fuel in escrow.
How do I use this charge in formulas?
Combine positive and negative ions so the total charge is zero — or let the ionic compound calculator do the crossing for you. The full chart lives on the periodic table with charges.
Where this data comes from
Charges and element facts on this page are checked against the primary references below. Found an error? Email hello@periodictablewithcharges.com — corrections ship in the next content update. See how this site is edited.