All atoms tend to adopt the stable structure of noble gases. This can be done for instance - by catching one or more electrons in order to complete the octet of the last layer : The names of the anions are identical to the names of the corresponding atoms in compounds, f.i: bromide, oxide, nitride ion. An atom doesn't catch more than 3 electrons, because the growing negative charge makes it more and more difficult to acquire some further electron (repulsion between charge - and -). Catching electrons is therefore only permitted for pnictogens, chalcogens and halogens. - by expelling the electrons of the last layer: So the second last layer will become the last and it will be complete with the structure of a noble gas! As the number of protons in the nucleus did not change, these ions will have less electrons than protons. They will become positive ions (cations), for instance The names of cations are the same as the names of the corresponding atoms, f.i.: sodium ion, baryum ion, aluminium ion. An atom doesn' lose more than 3 electrons, because the growing positive charge makes it more and more difficult to loose an electron (attraction between charges + and -). Losing electrons is therefore only permitted for alkaline metals, alkaline earth metals and the boron group.