Actinium

Actinium is atom number 89 — every Actinium atom has exactly 89 protons in its center, and that number is what makes it Actinium and nothing else. Its symbol is Ac. At room temperature it is a solid.

Actinium is a radioactive chemical element with symbol Ac (not to be confused with the abbreviation for an acetyl group) and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902.

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

Actinium sits in period 7, group 3 of the f-block. Its 89 electrons fill shells of 2, 8, 18, 32, 18, 9, 2 — written as [Rn] 6d1 7s2. Its electronegativity is 1.1 on the Pauling scale, and pulling the first electron off takes 499 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 5d10 6p6 7s2 6d1
Ionization energies (kJ/mol): 499, 1170
Electron affinity: 33.77 kJ/mol · Electronegativity (Pauling): 1.1
Density: 10 g/cm³ · melts 1500 K · boils 3500 K

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