One of the softest commonly encountered metallic elements is lead, with a Mohs hardness of 1.5. One of the hardest metallic elements is tungsten, with a Mohs hardness of 7.5. The Mohs scale of hardness ranks materials based on their scratch resistance on a scale of one to 10. A material with a higher Mohs hardness is able to scratch a material with a lower Mohs hardness. Diamond is the hardest known substance with a Mohs hardness of 10.
The alkali metals tend to rank low on the Mohs scale. Sodium metal, for example, can be cut with a knife and has a Mohs hardness of 0.5. Lithium, potassium and rubidium have hardness ratings of 0.6, 0.4 and 0.3, respectively. Caesium, which exists as a liquid at room temperature, has a hardness of 0.2. The noble metals tend to be soft as well. Gold and silver each have a hardness rating of 2.5.
In contrast, many transition metals rank exceptionally high on the Mohs scale. Titanium, niobium and rhodium each have a hardness rating of 6.0. Even higher still are tantalum, rhenium and osmium, with respective hardness ratings of 6.5, 7.0 and 7.0. Finally, certain metal alloys exceed the hardness of their parent element, such as tungsten carbide with a hardness of 8.5 to 9.0.