Uranium

The heaviest element found in nature in quantity, the fuel of nuclear power, and the element in which radioactivity was discovered.

At a glance

Symbol: U
Atomic number: 92
Average atomic mass: 238.03
Group: none; it sits in the actinide row below the table
Period: 7
Family: actinide
Electron configuration: [Rn] 5f³ 6d¹ 7s²
Electronegativity: 1.38
Common ions: +6 most commonly, as in the uranyl ion UO₂²⁺; also +4
Isotopes: ²³⁸U (99.3%), with a half-life of 4.5 billion years, and ²³⁵U (0.7%), the isotope that sustains a chain reaction

On the table

Period 7, in the actinide row set below the main table. The other actinides are tinted, and every cell opens its own article.

Electrons and bonding

Uranium has 92 electrons, one for every proton, so the atom carries no charge. Below are the same 92 electrons written two ways.

Simple form: electrons per shell

shell 12shell 28shell 318shell 432shell 521shell 69shell 72

Counting outward from the nucleus: 2, 8, 18, 32, 21, 9, 2. The outer shell is picked out.

Orbital form: electrons per subshell

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 5f³ 6s² 6p⁶ 6d¹ 7s²

Short form: [Rn] 5f³ 6d¹ 7s², where [Rn] stands for the 86 electrons radon already has.

That list runs by shell. In the order the subshells fill it reads [Rn] 7s² 5f³ 6d¹, because a subshell fills by its energy, not by its shell number.

5f³↑↑↑

6d¹↑

7s²↑↓

The boxes hold the electrons beyond [Rn]. Each box is one orbital and takes two electrons at most, spinning opposite ways. They spread out singly before they pair up.

How to read the two

The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 4 here is 4s² 4p⁶ 4d¹⁰ 4f¹⁴, and 2 + 6 + 10 + 14 is 32.

Uranium sits in period 7 and has 7 shells. The period number counts the shells.

What that means for bonding

The outer shell holds 2 electrons. Those are the ones uranium can lose, gain or share, so they are the ones that decide what it bonds with.

Radioactive, like every actinide. It gives up electrons to form positive ions.

Properties

State at 25 °C: solid
What it looks like: dense, silvery-gray metal that tarnishes in air
Melting point: 1135 °C
Boiling point: 4131 °C
Density: 19.0 g/cm³

Where it is found

More common in the crust than silver or tin. It is mined as uraninite, also called pitchblende, and other ores, mostly in Kazakhstan, Canada, Namibia and Australia. About 2 billion years ago, uranium ore at Oklo in Gabon ran as a natural nuclear reactor.

Discovery and name

Martin Heinrich Klaproth found it in pitchblende in 1789 and named it after the planet Uranus, discovered eight years earlier. Eugène-Melchior Péligot isolated the metal in 1841, and in 1896 Henri Becquerel discovered radioactivity when uranium salts fogged a wrapped photographic plate.

What it is used for

Nuclear power plants run on uranium enriched to a few percent uranium-235 and supply about a tenth of the world's electricity; uranium also fuels naval reactors and nuclear weapons. Depleted uranium, left over from enrichment, is dense enough for armor-piercing ammunition, tank armor and aircraft counterweights. Uranium glass, which glows green under ultraviolet light, was popular in the early 1900s.

In compounds

UO₂uranium dioxide, reactor fuelUF₆uranium hexafluoride, for enrichmentU₃O₈yellowcakeUO₂(NO₃)₂uranyl nitrate

Handle with care

Only weakly radioactive, but chemically toxic to the kidneys. Enriched uranium and the products of its fission are far more dangerous.