igure 2-41. A sodium atom (Na) willingly loses one electron to become a much more stable positively charged sodium ion (Na+) with the electron configuration of neon (Ne).
Negatively charged ions are called anions
. Ions bearing a positive charge are called cations
. If it is not entirely clear why the fluoride anion is negatively charged and the sodium cation is positively charged, recall that atoms are always electroneutral and that an electron has a negative charge of -1. Adding an electron to an electroneutral atom (or molecule) gives rise to an anion. Removing an electron from an electroneutral atom (or molecule) produces a cation.
Formulas of anions and cations are written exactly like those of non-charged atoms and molecules, except the charge value and its sign are added as a superscript after the formula of the ion. If the charge of an ion is +1 or -1, the "1" is omitted. For example, formulas of the cations produced upon removal of one electron from Li, Na, and K are written as Li+
, and K+
. For the dications
derived from Mg, Ca, and Zn, the formulas are presented as Mg+2
, and Zn+2
, respectively. The formula of the triply charged aluminum cation is Al+3
. Cations are easy to name: just say the name of the element and add "ion" or "cation" afterwards. For instance, Na+
is "sodium ion" or "sodium cation" and Al3+
is "aluminum ion" or "aluminum cation".
Monoatomic anions are written in a similar manner (for example, F-
), but are named slightly differently. To name a monoatomic anion, just add the suffix "ide" to the stem of the name of the element. Some examples follow. Fluor
ine (F) – fluoride
ine (Cl) – chloride
gen (O) – oxide
ur (S) – sulfide
ogen (N) – nitride
As we know now, the enhanced stability of Na+
vs. Na, F-
vs. F, Ca2+
vs. Ca, etc. comes from the exceptionally stable n
electron configuration of the nth
outermost shell. Note that this most stable configuration involves a total of eight
s and six
p), and is consequently often referred to as the octet
(from German oktett
and Italian ottetto
= 8). From this number takes its name the octet rule, stating that atoms tend to pursue, by participation in chemical transformations, the stable eight-electron configuration of the outermost shell.
There is one element that the octet rule does not apply to. This element is hydrogen, of course, because the 1st
electron shell can house only up to two electrons, not eight. Sometimes a hydrogen atom accepts one electron to become H-
, the hydride ion
, which has the electron configuration of helium. Much more often, however, a hydrogen atom loses its only electron to become just a proton. Just a proton? Yes, because an atom of hydrogen (protium, 1
H) is made up of only one electron and one proton in the nucleus. Remove the electron from it, and what is left is just the nucleus, which consists of just one proton. That is why chemists prefer to call the H+
"proton" rather than the longer names "hydrogen cation" or "hydrogen ion". 2.4.3. Ionic Bond: "One Man's Trash is Another Man's Treasure".
As shown in Figure 2-41, a sodium atom willingly discards its sole electron in the 3s orbital to attain the stable octet configuration 2s2
. Where does this electron go after it leaves the sodium atom? As the electron cannot just disappear completely, it has to go somewhere to find a new home.
Our sodium atom wants to get rid of one electron (Figure 2-41). Meanwhile, a fluorine atom somewhere seeks exactly the opposite, an opportunity to acquire an extra electron in order to gain stability (Figure 2-40). The electron that a sodium atom wants to give away, its "trash", is a "treasure" for the fluorine atom (Figure 2-42). Unsurprisingly, the two enter into a symbiosis, a chemical reaction involving the transfer of one electron from the Na to the F. This reaction produces two highly stable ions, Na+
, which, being oppositely charged, experience mutual electrostatic attraction. This electrostatic attraction holds the ions together, thereby constituting an ionic bond
between the Na+
. The process shown in Figure 2-42 is also illustrated in this animated cartoon
, which you are encouraged to watch.