Group 4: the titanium group

Four outer electrons and the +4 state

Each member has two d electrons and two s electrons outside its core: titanium is [Ar] 3d² 4s², zirconium [Kr] 4d² 5s² and hafnium [Xe] 4f¹⁴ 5d² 6s². Using all four gives the +4 state found in their oxides, TiO₂, ZrO₂ and HfO₂. Titanium can also be +2 or +3, but zirconium and hafnium nearly always stay at +4.

These metals react readily with oxygen, yet the thin oxide film that forms seals the surface, so in everyday conditions they barely corrode.

Down the group: heavier and harder to melt

Unlike the metals of groups 1 and 2, group 4 metals melt at higher temperatures as you go down: 1941 K for titanium, 2128 K for zirconium and 2506 K for hafnium. Electronegativity edges down, from 1.54 to 1.33 and 1.3.

Density shows a striking jump. Zirconium is 6.52 g/cm³, but hafnium is 13.31 g/cm³, about twice as dense. The lanthanide series that comes just before hafnium shrinks its atoms, so a hafnium atom is almost the same size as a zirconium atom while carrying nearly twice the mass. The two are such close twins that hafnium hid unnoticed in zirconium ores until 1923.

Where you meet group 4

Titanium dioxide is the bright white pigment in paint, paper, toothpaste and many sunscreens. Titanium metal is strong yet light, 4.506 g/cm³ against 7.874 g/cm³ for iron, so it goes into aircraft, bike frames and hip and dental implants that the body accepts well. Zirconium alloys wrap the fuel in nuclear reactors because zirconium lets neutrons pass, while hafnium soaks up neutrons and is used in control rods. Cubic zirconia, a low-cost diamond look-alike, is zirconium dioxide.

Rutherfordium

Rutherfordium (104), named after Ernest Rutherford, is radioactive and has been made only a few atoms at a time, so its melting point and density have never been measured. Brief experiments suggest it forms compounds much like those of zirconium and hafnium.