Iridium

Iridium is atom number 77 — every Iridium atom has exactly 77 protons in its center, and that number is what makes it Iridium and nothing else. Its symbol is Ir. At room temperature it is a solid.

Iridium is a chemical element with symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, iridium is generally credited with being the second densest element (after osmium) based on measured density, although calculations involving the space lattices of the elements show that iridium is denser. It is also the most corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.

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

Iridium sits in period 6, group 9 of the d-block. Its 77 electrons fill shells of 2, 8, 18, 32, 15, 2 — written as [Xe] 4f14 5d7 6s2. Its electronegativity is 2.2 on the Pauling scale, and pulling the first electron off takes 880 kJ/mol. The outermost shell holds 2 electrons — the ones that do all the chemistry.

Full configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d7
Ionization energies (kJ/mol): 880, 1600
Electron affinity: 150.94 kJ/mol · Electronegativity (Pauling): 2.2
Density: 22.56 g/cm³ · melts 2719 K · boils 4403 K

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