What Determines the Hardness of Metallic Elements?
Every element on the periodic table has certain measurements that make it unique as a material. One of these measurements is the hardness, commonly thought of as how strong that particular material is. While there are a variety of different methods to test hardness based on the type of material being tested, any solid material's hardness is caused by the same chemical reactions.
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Metallic Elements
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There is some debate as to the definition of what is and isn't a metal, but metals all share certain characteristics. A metal has luster (it's shiny), good thermal and electrical conductivity and it can be permanently shaped or deformed at room temperature. Some examples of metals include silver, platinum, lead, iron and gold, all of which share the properties of metals.
Hardness
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A metal's hardness is how well it resists being scratched or deformed by indentation. It may sometimes be used to refer to the tensile strength or the amount of power that you need to put into cutting a metal as well. In general, though, hardness is thought of as a composite measurement of all of these resistance traits in a metal.
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Cause
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Hardness is a direct result of the bonding between the atoms that make up a particular metal. Simple bonds among the atoms result in weaker bonds, whereas more complicated bonds form a mesh that increase the hardness of the metal. This does not appear to have a correlation to the density of a metal. Just because lead is soft and has a high density, that density doesn't affect the lead's strength.
Atomic Number and Weight
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According to Vince Calder at Ask a Scientist!, the atomic number of an element also plays into the hardness. The higher the atomic number, the larger the atomic weight of that particular material is. This is a minor contributor to the hardness, however, as the major strength of a metal comes from the strength of the bonds holding it together on the atomic level.
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References
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