Erbium

Erbium is atom number 68 — every Erbium atom has exactly 68 protons in its center, and that number is what makes it Erbium and nothing else. Its symbol is Er. At room temperature it is a solid.

Erbium is a chemical element in the lanthanide series, with symbol Er and atomic number 68. A silvery-white solid metal when artificially isolated, natural erbium is always found in chemical combination with other elements on Earth. As such, it is a rare earth element which is associated with several other rare elements in the mineral gadolinite from Ytterby in Sweden, where yttrium, ytterbium, and terbium were discovered.

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

Erbium sits in period 6, group 3 of the f-block. Its 68 electrons fill shells of 2, 8, 18, 30, 8, 2 — written as [Xe] 4f12 6s2. Its electronegativity is 1.24 on the Pauling scale, and pulling the first electron off takes 589.3 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 4f12
Ionization energies (kJ/mol): 589.3, 1150, 2194, 4120
Electron affinity: 30.1 kJ/mol · Electronegativity (Pauling): 1.24
Density: 9.066 g/cm³ · melts 1802 K · boils 3141 K

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