Ions are generally categorized into the following groups based on mobility values and dimensions.
(1) Free-floating electron An free-floating electron exists by itself and weights only about 1/1800 of the hydrogen atom. Its mobility values are as large as beta rays generated by cathode rays or radiant substances. It is generally found at high altitudes where the air is rarefied, or in highly purified nitrogen, helium and argon. (2) Ionized atom An atom, which has lost an electron, is a positively ionized atom. An electronically neutral atom, which has obtained an electron, is a negatively ionized atom. Both types of ions along with electrons exist only in the upper layers of the atmosphere. (3) Small ion Most ions found in the atmosphere belong to this group (also known as Lightweight or Normal ion). As soon as an electron or ionized atom shows up in the atmosphere, it attracts gaseous molecules and combines with them to form a small ion molecule while positioning itself in the center. A small ion molecule consists of 2 to 30 molecules. Generally, positive ions weigh more than negatively charged ions, and mobility values are larger than 0.4-0.8 (cm2/Vs). (4) Large ion A large ion (also known as Heavy ion) is a negative or positive small ion (molecule) absorbed by dust, mist or another tiny particle. While having the same structure as small ions, it can weight 1,000 times more. Mobility values range from 0.0005 to 0.01 (cm2/Vs). Many exist in polluted air. (5) Middle ion This group of ions was discovered by Pollock and exists only in low humidity conditions, and does not exist near the earth's surface. Mobility values range from 0.01 to 0.1 (cm2/Vs).
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