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Periodic table · chemistry · physics
Element story

Caesium

Atomic number 55 means that every Caesium nucleus contains 55 protons. That proton count is the element's identity: change it and you no longer have Caesium.

Start with the nucleus

Atomic number 55 means that every Caesium nucleus contains 55 protons. That proton count is the element's identity: change it and you no longer have Caesium.

Caesium (Cs) is chemical element 55. It belongs to the alkali metals, and its reference state at room temperature is a solid. Those basic facts are a useful starting point for its chemistry and applications.

In practice, it turns up most often in atomic clocks, drilling fluids and scientific research.

See the atom

Conceptual visual: electron shells are a learning aid, not a literal picture of quantum orbitals.

What this number means

Atomic number 55 means that every Caesium nucleus contains 55 protons. That proton count is the element's identity: change it and you no longer have Caesium.

Keep exploring

→ Caesium · Atomic structure · Periodic trends

55CsCaesium
Caesium · Cs · Z = 55

Caesium (Cs) is chemical element 55. It belongs to the alkali metals, and its reference state at room temperature is a solid. Those basic facts are a useful starting point for its chemistry and applications.

Understanding the element

Caesium (Cs) is chemical element 55. It belongs to the alkali metals, and its reference state at room temperature is a solid. Those basic facts are a useful starting point for its chemistry and applications.

Where do we encounter it?

In practice, it turns up most often in atomic clocks, drilling fluids and scientific research.

What its position in the table tells us

Caesium sits in period 6, group 1 and the S block. That location is useful because neighbouring elements often share trends in valence electrons, oxidation states and bonding behaviour.