Anion means that an atom obtains one or several electrons due to external action, so that the outermost electron number reaches a stable structure. The smaller the atomic radius, the stronger the ability to obtain electrons and the weaker the metallicity. Anions are negatively charged ions, and nuclear charge number = proton number.

The smaller the radius, the stronger the atom's ability to absorb electrons, the easier it is to form anions, and the stronger the nonmetal. The most nonmetallic element is fluorine. Electrons with more than 4 electrons in the outermost layer of an atom form anions (nonmetallic substances show negative valence, and anions are represented by the symbol "-"). )

Extended data:

Experimental principle

Common anions are 13: SO42-, SiO2-, PO43-, CO32-, SO32-, S2O32-, S2-, Cl-, Br-, I-, NO3-, NO2- and Ac-

Among anions, some are easily decomposed in the presence of acid, and some are redox with each other and cannot exist. Therefore, the analysis of anions has the following two characteristics:

Anions are easy to change in the analysis process, and it is not easy to carry out complex system analysis.

There are few opportunities for anions to exist with each other, and there are many available special effects, which can be analyzed separately.

In the analysis of anions, separation analysis method is mainly used, and only when some anions interfere with each other can separation method be properly used.

However, the method of separation and analysis is not to test all the ions studied one by one, but to exclude the anions that definitely do not exist through preliminary experiments, and then to determine the possible anions one by one.

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