Thallium

Thallium is atom number 81 — every Thallium atom has exactly 81 protons in its center, and that number is what makes it Thallium and nothing else. Its symbol is Tl. At room temperature it is a solid.

Thallium is a chemical element with symbol Tl and atomic number 81. This soft gray post-transition metal is not found free in nature. When isolated, it resembles tin, but discolors when exposed to air.

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

Thallium sits in period 6, group 13 of the p-block. Its 81 electrons fill shells of 2, 8, 18, 32, 18, 3 — written as [Xe] 4f14 5d10 6s2 6p1. Its electronegativity is 1.62 on the Pauling scale, and pulling the first electron off takes 589.4 kJ/mol. The outermost shell holds 3 electrons — the ones that do all the chemistry.

Full configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p1
Ionization energies (kJ/mol): 589.4, 1971, 2878
Electron affinity: 36.4 kJ/mol · Electronegativity (Pauling): 1.62
Density: 11.85 g/cm³ · melts 577 K · boils 1746 K

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