Rubidium

Rubidium is atom number 37 — every Rubidium atom has exactly 37 protons in its center, and that number is what makes it Rubidium and nothing else. Its symbol is Rb. At room temperature it is a solid.

Rubidium is a chemical element with symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group, with an atomic mass of 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other alkali metals, such as very rapid oxidation in air.

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

Rubidium sits in period 5, group 1 of the s-block. Its 37 electrons fill shells of 2, 8, 18, 8, 1 — written as [Kr] 5s1. Its electronegativity is 0.82 on the Pauling scale, and pulling the first electron off takes 403 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 5s1
Ionization energies (kJ/mol): 403, 2633, 3860, 5080, 6850, 8140, 9570, 13120 … (10 published)
Electron affinity: 46.884 kJ/mol · Electronegativity (Pauling): 0.82
Density: 1.532 g/cm³ · melts 312.45 K · boils 961 K

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