8.1. Ions http://www.ck12.org
Cations
As you have seen before, cations are the positive ions formed when an atom loses one or more electrons. The
cations most commonly formed by the representative elements are those that involve the loss of all valence electrons.
Consider the alkali metal, sodium (Na). It has one valence electron in the third principal energy level. Upon losing
that electron, the sodium ion now has an octet of electrons from the second principal energy level. The equation
below illustrates this process.
Na → Na++e−
1 s^22 s^22 p^63 s^11 s^22 s^22 p^6 (octet)The electron configuration of the sodium ion is now the same as that of the noble gas, neon. The termisoelectronic
is used to describe two atoms or ions that have the same electron configuration. The sodium ion is isoelectronic
with the neon atom. Consider the analogous processes for magnesium and aluminum:
Mg → Mg^2 ++ 2 e−
1 s^22 s^22 p^63 s^21 s^22 s^22 p^6 (octet)Al → Al^3 ++ 3 e−
1 s^22 s^22 p^63 s^23 p^11 s^22 s^22 p^6 (octet)Both of these atoms form ions by losing all of their valence electrons, two in the case of magnesium, and three in
the case of aluminum. The same noble gas configuration is achieved by all of these ions. In other words, the Mg^2 +
ion, the Al^3 +ion, the Na+ion, and the Ne atom are all isoelectronic. Under typical conditions, the representative
elements form cations by losing a maximum of three electrons.
We can also show the loss of valence electron(s) with an electron dot diagram.
Na•→Na++e−Anions
Anions are the negative ions formed by gaining one or more electrons. When nonmetal atoms gain electrons, they
often do so until they reach an octet of valence electrons in their outermost principal energy level. This process is
illustrated below for the elements fluorine, oxygen, and nitrogen.
F+e− → F−
1 s^22 s^22 p^51 s^22 s^22 p^6 (octet)O+ 2 e− → O^2 −
1 s^22 s^22 p^41 s^22 s^22 p^6 (octet)