Selenium

Selenium is atom number 34 — every Selenium atom has exactly 34 protons in its center, and that number is what makes it Selenium and nothing else. Its symbol is Se. At room temperature it is a solid.

Selenium is a chemical element with symbol Se and atomic number 34. It is a nonmetal with properties that are intermediate between those of its periodic table column-adjacent chalcogen elements sulfur and tellurium. It rarely occurs in its elemental state in nature, or as pure ore compounds.

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

Selenium sits in period 4, group 16 of the p-block. Its 34 electrons fill shells of 2, 8, 18, 6 — written as [Ar] 3d10 4s2 4p4. Its electronegativity is 2.55 on the Pauling scale, and pulling the first electron off takes 941 kJ/mol. The outermost shell holds 6 electrons — the ones that do all the chemistry.

Full configuration: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4
Ionization energies (kJ/mol): 941, 2045, 2973.7, 4144, 6590, 7880, 14990
Electron affinity: 194.9587 kJ/mol · Electronegativity (Pauling): 2.55
Density: 4.81 g/cm³ · melts 494 K · boils 958 K

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