Cesium

Cesium is atom number 55 — every Cesium atom has exactly 55 protons in its center, and that number is what makes it Cesium and nothing else. Its symbol is Cs. At room temperature it is a solid.

Caesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C (82 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium is an alkali metal and has physical and chemical properties similar to those of rubidium and potassium.

Summary from the source dataset, derived from Wikipedia (CC BY-SA 3.0) · original article

Cesium sits in period 6, group 1 of the s-block. Its 55 electrons fill shells of 2, 8, 18, 18, 8, 1 — written as [Xe] 6s1. Its electronegativity is 0.79 on the Pauling scale, and pulling the first electron off takes 375.7 kJ/mol. The outermost shell holds 1 electron — the ones that do all the chemistry.

Full configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s1
Ionization energies (kJ/mol): 375.7, 2234.3, 3400
Electron affinity: 45.505 kJ/mol · Electronegativity (Pauling): 0.79
Density: 1.93 g/cm³ · melts 301.7 K · boils 944 K

Values exactly as published — see where the data comes from.