Ions are atoms or groups of atoms or molecules, non-neutral, that is, having electrical charges (one or more than one) positive and/or negative, due (respectively) to the loss or acquisition of an electron. The total charge of an ion is equal to the difference between the number of protons, minus the number of electrons. An ion is formed after taking one or more electric charges by means of loss (positive ions or cations) or acquisition (negative ions or anions) of one or more electrons.


where Z is the atomic number equal to the number of protons, X is the symbol of the chemical element considered, A is the mass number equal to protons+neutrons, \(p\) is the number of protons and \(e\) is the number of electrons. In other words, since an electron is negatively charged, when a neutral atom acquires an electron, this will form a negatively charged ion (anion, e.g. \(\textrm{Cl}^-\), \(\textrm{CO}_3^{2-}\)), vice versa, in case of a loss it will form a positively charged ion (cation, e.g. \(\textrm{Na}^+\), \(\textrm{NH}_4^+\)). When an atom transforms into an ion, the number of protons remains unchanged.

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