Hafnium

Hafnium is atom number 72 — every Hafnium atom has exactly 72 protons in its center, and that number is what makes it Hafnium and nothing else. Its symbol is Hf. At room temperature it is a solid.

Hafnium is a chemical element with symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869, though it was not identified until 1923, making it the penultimate stable element to be discovered (rhenium was identified two years later).

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

Hafnium sits in period 6, group 4 of the d-block. Its 72 electrons fill shells of 2, 8, 18, 32, 10, 2 — written as [Xe] 4f14 5d2 6s2. Its electronegativity is 1.3 on the Pauling scale, and pulling the first electron off takes 658.5 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 5d2
Ionization energies (kJ/mol): 658.5, 1440, 2250, 3216
Electron affinity: 17.18 kJ/mol · Electronegativity (Pauling): 1.3
Density: 13.31 g/cm³ · melts 2506 K · boils 4876 K

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