Argon

Argon is atom number 18 — every Argon atom has exactly 18 protons in its center, and that number is what makes it Argon and nothing else. Its symbol is Ar. At room temperature it is a gas.

Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most common gas in the Earth's atmosphere, at 0.934% (9,340 ppmv), making it over twice as abundant as the next most common atmospheric gas, water vapor (which averages about 4000 ppmv, but varies greatly), and 23 times as abundant as the next most common non-condensing atmospheric gas, carbon dioxide (400 ppmv), and more than 500 times as abundant as the next most common noble gas, neon (18 ppmv).

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

Argon sits in period 3, group 18 of the p-block. Its 18 electrons fill shells of 2, 8, 8 — written as [Ne] 3s2 3p6. Its electronegativity is — on the Pauling scale, and pulling the first electron off takes 1520.6 kJ/mol. The outermost shell holds 8 electrons — the ones that do all the chemistry.

Full configuration: 1s2 2s2 2p6 3s2 3p6
Ionization energies (kJ/mol): 1520.6, 2665.8, 3931, 5771, 7238, 8781, 11995, 13842 … (18 published)
Electron affinity: -96 kJ/mol · Electronegativity (Pauling): —
Density: 1.784 g/L · melts 83.81 K · boils 87.302 K

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