Ion and Ionization
An electrically neutral atom becomes an ion through the removal or addition of electrons. A negatively charged ion is called an anion and a positively charged ion is a cation. There are a variety of ways in which atoms can be ionized. Chemical reactions, heat, collision between particles, and radiation (such as X-rays and gamma rays) can alter the structure of an atom.
Ion can be manipulated by magnetic and electric fields, which affect their movement in different ways. Electricity causes ions to flow along the electric field. However, magnetism causes ions to spiral along the magnetic field lines. The aurora borealis and aurora australis (northern and southern lights) are caused by the sun's ions, which spiral along the lines of the earth's magnetic field causing the atmosphere to glow.
The study of ions began with Humphry Davy and his assistant Michael Faraday. Davy, a pioneer in electrochemistry, had passed an electric current through a variety of molten metals and liberated new metals in the process. It was Faraday, however, who used the terms ion, electrolysis (the process), electrolyte (the current-carrying solution), electrode, anode (positive electrode), and cathode (negative electrode). These names, suggested by British scholar William Whewell (1794-1866), remain in use.
Faraday suggested that ions, moving under the influence of electric current, were responsible for the flow of electricity through the electrolyte. The two laws of electrolysis, which established the relationship between electricity and chemistry, relate the mass, atomic weight, and valence (combining power) of the liberated substance to the quantity of electricity. Johann Wilhelm Hittorf (1824-1914) knew of Faraday's explanation of electrolysis, but in 1853 he suggested that the ions might travel at different speeds, carrying unequal amounts of current. Consequently, more ions could reach one electrode than the other, an idea that evolved into the electrochemical concept of the transport number.
In 1887 Swedish chemist Svante August Arrhenius set about to determine how electricity passes through electrolytes, comparing electrolytic substances (like salt) with non-electrolytic ones (such as sugar) and their effect on the freezing point of water.
He found that a gram of non-electrolytic glucose in a liter of water reduced the freezing point twice as much as an equal amount of sucrose, results he expected because the glucose molecule is only half the size of sucrose. But the amount of reduction by the electrolyte sodium chloride (salt) was twice what it should have been, and three times, in the case of barium chloride or sodium sulfate. When salt dissolved, each molecule dissociated into two particles, and barium chloride or sodium sulfate into three. Unlike the non-electrolytic compounds, though, electrolytic compounds released charged ions, allowing an electric current to flow.
The concept of electrically charged atoms was too radical for most chemists to accept. Arrhenius used the theory in his doctoral thesis and barely received a passing grade. However, he was vindicated in the 1890s after Joseph J. Thompson found evidence for the electron, and Antoine H. Becquerel discovered radioactivity. Thompson, one of the first to propose a theory of atomic structure, believed the electron was a universal component of nature. With the atomic theory, scientists could account for an ion's charge, seeing that a negatively-charged chlorine atom had gained an electron, while a positively-charged sodium atom had lost one. Irving Langmuir (1881-1957) used atomic theory to explain ionization and devised the electronic theory of valence, the basis for electronic bonding theory. In 1923 Peter J. Debye (1884-1966) suggested that each positive or negative ion was surrounded by a cloud of oppositely-charged ions, research that helped modern scientists better understand solutions.
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