Predict the formula of cesium sulfide.
Cs+ with S2- takes two cesiums: Cs2S. Every alkali metal writes the same equation; cesium just gets there with the most enthusiasm.
What Is the Charge of Cesium?
Cesium commonly forms Cs+ in introductory ionic compounds.
Cesium forms Cs+ — Group 1's +1, delivered explosively. CsCl even names a crystal-structure type. Its outer electron is the loosest of any stable element, which is why cesium defines both violent water reactions and the world's time standard.
| Element | Cesium |
|---|---|
| Symbol | Cs |
| Atomic number | 55 |
| Common ionic charge | +1 |
| Common ions | Cs+ |
Visual reference
Cesium element image and atomic model
Images are used as visual references; charge rules still depend on the compound.
Why Cesium gives up exactly one electron
Cesium is [Xe]6s1, and that 6s electron is held by just 376 kJ/mol — the lowest ionization energy of any stable element. It leaves at the slightest provocation (even dim light ejects it in photocells), and the xenon core beneath permits nothing further. Hence Cs+, instantly and only.
The formula slip students make with Cesium
Confusing chemical cesium with radioactive cesium-137. The fallout isotope behaves chemically as ordinary Cs+ — which is exactly the danger: the body files it alongside K+ into muscle. The charge chemistry is textbook; the nucleus is the hazard.
Worked example: Cs in a formula problem
Cs+ in real compounds
CsCl
Cesium chloride
Cs+ balances chloride ions.
Cs2O
Cesium oxide
Cesium can form oxides where total positive and negative charge balances.
Questions students ask about cesium
How does cesium's single electron define the second?
Atomic clocks count 9,192,631,770 microwave cycles of a spin-flip in neutral cesium-133's outer electron. The SI second is literally a property of that one 6s electron — the same electron whose loss makes Cs+.
Is cesium more reactive than francium below it?
Practically, cesium wears the crown: francium is so radioactive (half-life 22 minutes) that gram quantities cannot exist. Relativistic effects also tighten francium's electron slightly — cesium likely remains the true reactivity champion.
What is CsCl structure, and why do teachers mention it?
Equal-sized large ions pack with each Cs+ touching eight Cl- — a cubic arrangement distinct from NaCl's six-neighbor pattern. It is the standard example that ion size ratio, not charge, picks the crystal blueprint.
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.