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A simple example of a polyatomic ion is the hydroxide ion, which consists of one oxygen atom and one hydrogen atom, jointly carrying a net charge of −1; its chemical formula is O H −. In contrast, an ammonium ion consists of one nitrogen atom and four hydrogen atoms, with a charge of +1; its chemical formula is N H + 4.
Ions consisting of only a single atom are termed atomic or monatomic ions, while two or more atoms form molecular ions or polyatomic ions. In the case of physical ionization in a fluid (gas or liquid), "ion pairs" are created by spontaneous molecule collisions, where each generated pair consists of a free electron and a positive ion. [5]
Ionic radius, r ion, is the radius of a monatomic ion in an ionic crystal structure. Although neither atoms nor ions have sharp boundaries, they are treated as if they were hard spheres with radii such that the sum of ionic radii of the cation and anion gives the distance between the ions in a crystal lattice.
Each ion can be either monatomic (termed simple ion), such as sodium (Na +) and chloride (Cl −) in sodium chloride, or polyatomic, such as ammonium (NH + 4) and carbonate (CO 2− 3) ions in ammonium carbonate. Salts containing basic ions hydroxide (OH −) or oxide (O 2−) are classified as bases, such as sodium hydroxide and potassium oxide.
Monoatomic (composed of one atom). Examples include He , Ne , Ar , and Kr . All noble gases are monoatomic. Diatomic (composed of two atoms). Examples include H 2 , N 2 , O 2 , F 2 , and Cl 2 . Halogens are usually diatomic. Triatomic (composed of three atoms). Examples include O 3 .
2 ion, found in mercury(I) (mercurous) compounds. The existence of the metal–metal bond in Hg(I) compounds was established using X-ray studies in 1927 [ 2 ] [ page needed ] and Raman spectroscopy in 1934 [ 3 ] making it one of the earliest, if not the first, metal–metal covalent bonds to be characterised.
It is usually applied to gases: a monatomic gas is a gas in which atoms are not bound to each other. Examples at standard conditions of temperature and pressure include all the noble gases ( helium , neon , argon , krypton , xenon , and radon ), though all chemical elements will be monatomic in the gas phase at sufficiently high temperature (or ...
However, multiple ionizations occur always in practice. Free electrons at the cathode surface are created by the impacting ions. The problem is that the number of thereby created electrons strongly depends on the material of the cathode, its surface ( roughness , impurities) and the environmental conditions (temperature, humidity etc.).