Unlocking the Periodic Table

Of the 118 elements listed in the Periodic Table, more than 90 are classed as metals. 

Only nine of these were known to ancient cultures: iron, copper, silver, gold, platinum, zinc and mercury, tin and lead. Most metals we know today have been identified, and brought into use since the 1700s – many are now proven contributors to our modern, high-technology society.

As a general definition, metals are those elements that are: shiny; good conductors of heat and electricity; malleable, that is they can be beaten or pressed into a shape without cracking; and they are ductile, they can be drawn into a thin wire or sheet.

Not all metals are created equal. The differences between categories of metal are greater than what they have in common. The Periodic Table shows up the differences very clearly, into:

  • The chemically-reactive alkali metals and alkaline earth metals in the left two columns;
  • Transition metals, which cover most of the things we think of as metals;
  • Other metals and “metalloids” on the right hand side, which tend to be soft, poisonous or quirky, though often useful;
  • The middle-weight lanthanides, all of which are “rare earth elements”; and
  • The heavy actinides, which are radioactive, and most of which must be produced synthetically.

Alkali metals and alkaline earth metals

Put a lump of pure calcium metal into water and it will fizz and bubble, and vanish into a white powder in suspension of calcium oxide (lime) and hydrogen gas. Magnesium can be set on fire – it burns with a bright white flame, as any fireworks fan will know. Adding water to Mg will only make it burn brighter. These metals are driven to react; they are highly unstable in their pure form.

These metals are best known to us as their salts – table salt is sodium chloride (NaCl); baking soda is sodium bicarbonate (NaHCO3); limestone is calcium carbonate (CaCO3); saltpetre is potassium nitrate (KNO3), used in gunpowder and fertiliser; calcium phosphate minerals are the source of Ca in cow’s milk; calcium sulphate (CaSO4) is made into plaster of paris, cement plaster and gib board.     

Transition metals

Civilisations were built on iron, gold, copper (often alloyed with tin or zinc), and silver. But the general characteristic of the transition metals is their natural occurrence in chemically stable compounds, making them difficult physically (e.g., heat), and chemically to smelt or extract from their ore.

When combined or alloyed with iron/steel, selected transition metals help produce high-performance metals - light, tough, resilient to wear and tear, resistant to heat, anti-rust. Such metals include titanium, vanadium, chromium, nickel, zirconium, molybdenum, and tungsten.

Among the transition metals, elements that are close to each other along the rows of the Periodic Table have similar chemical and physical properties, as do the metals grouped in columns. This property has given rise to a number of sub-groups within the transition metals, noting significant overlaps for the last three groups:

  • Refractory metals – niobium, molybdenum, tantalum, tungsten, rhenium – extremely resistant to heat and wear
  • Noble metals – ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold – resistant to corrosion and oxidation (rusting)
  • Precious metals - palladium, silver, platinum, gold – naturally occurring, high lustre, highly resistant to corrosion
  • Platinum group metals – ruthenium, rhodium, palladium, osmium, iridium and platinum – similar properties, often found together in mineral deposits 

On the far left of the transition metals: scandium and yttrium are two of the 17 rare earth elements, which include the lanthanides. The REEs are among the technology metals that are essential to modern society.

On the far right of the transition metals: zinc, cadmium and mercury lean towards the category of other metals and metalloids, in being chemically reactive, soft (or liquid), and heavy.                         

Other metals and metalloids

Arsenic and antimony, tin and lead, silicon and aluminium would be familiar to most people, and for different reasons. Less well known members of this disparate group include: gallium, germanium and tellurium, used in electronics; indium, used in digital touchscreen technology; bismuth, having some use in medicine; the extremely poisonous thallium (the use of which is increasingly avoided); and radioactive polonium.

By “metalloid” is meant an element that strays towards acid-forming in its chemical behaviour, and to being soft, non-shiny, brittle, having a low melting point, and being less conductive of electricity.        

Rare earth metals

The 15 lanthanide elements are among the 17 “rare earth elements” (along with the transition metals, scandium and yttrium). The REEs have gained celebrity status among metals in recent years for their importance in new technologies, and their limited abundance. Sandwiched between barium to the left in the Periodic Table, and the heavier transition metals such as tungsten, platinum and gold, the lanthanides occur naturally as oxides and other dull-coloured minerals (earths), similar to their alkaline earth neighbours in the Periodic Table. 

Radioactive metals

Among the heaviest of metals are the actinides, the best known of which are thorium, uranium and plutonium – for their use in nuclear energy. These metals are radioactive, i.e., they emit naturally various types of sub-atomic particle, and in doing so decay into lighter elements.

All of the elements heavier than uranium, the trans-uranium metals, must be produced in the laboratory under extreme conditions. With the exception of plutonium, these have no practical application other than in high-energy atomic physics.