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listed twelve (Emsley's plus boron, carbon, silicon, selenium, bismuth, polonium, moscovium and livermorium).

On average, seven elements are included in such lists; individual classification arrangements tend to share common ground and vary in the ill-defined used three criteria to describe the six elements commonly recognised as metalloids: metalloids have ionization energies around 200 kcal/mol (837 k J/mol) and electronegativity values close to 2.0.

Five elements are less frequently so classified: carbon, aluminium, selenium, polonium, and astatine.

On a standard periodic table, all eleven are in a diagonal area in the p-block extending from boron at the upper left to astatine at lower right, along the dividing line between metals and nonmetals shown on some periodic tables. Most of their other physical and chemical properties are intermediate in nature.

The additional pull on outer electrons as nuclear charge increases generally outweighs the screening effect of having more electrons.

Going along a period, the nuclear charge increases with atomic number as do the number of electrons.The properties of form, appearance, and behaviour when mixed with metals are more like metals.Elasticity and general chemical behaviour are more like nonmetals.Going down a main group, the effect of increasing nuclear charge is generally outweighed by the effect of additional electrons being further away from the nucleus.Atoms generally become larger, ionization energy falls, and metallic character increases.