Caesium is a very reactive metal.
Uses and research
A transition in caesium-133 defines the SI second.
A family of reactive metals
Alkali metals occupy group 1, apart from hydrogen, which is not an alkali metal. The metals react readily and occur naturally in compounds. An element in a mineral or salt is not the same substance as a piece of its pure metal. Distinguishing the substance from the element is a useful first step.
What the atomic number tells us
Caesium has atomic number 55. This is the number of protons in its nucleus. A neutral atom also has 55 electrons. Losing or gaining electrons makes an ion; it does not change the element unless the proton count changes.
The model's rings help count electrons by shell. They are not actual circular paths. A model of one atom and the properties of a whole material are different levels of description.
Temperature and state
25°C: Solid
Illustrative particle motion: sizes, distances and speeds are not to scale. Particles represent atoms or molecules.
← CoolHeat →
Melting point 28.44°C / Boiling point 670.85°C
Pure element at 1 atmosphere. Image conditions are explained in captions. Pictures do not change with temperature.