Define Octet Rule
The octet rule refers to an important chemical rule of thumb. Moreover, this rule means the observation that elements bond in a way such that every atom contains eight electrons in the valence shell.
Moreover, the octet rule is a rule which is not very rigid and sometimes may get broken. However, this applies to nitrogen, carbon, oxygen, the halogens, and most metals. Students can learn more information about the octet rule here.
Explanation of the Octet Rule
The octet rule refers to an important principle that the atoms which have bonded share eight outer electrons. This certainly means that the atom’s valence shell has a resemblance with a noble gas.
The octet rule states that the atoms like to have eight electrons only in their full outer shells. For achieving eight electrons in their outer shells, atoms would gain or lose the valence electrons.
Furthermore, the atom does this by bonding with each other. Moreover, these atoms can be the same element or with different elements.
Two notable exceptions to the octet rule are helium and hydrogen. This is because both are happy with two electrons belonging in the outer shells.
Octet Rule Example: Sodium Chloride
Ionic bonding is something that is between common between pairs of atoms. Here, one of the pair happens to be a metal of low electronegativity. A good example can be sodium.
The second is a nonmetal which has high electronegativity. A good example of this can be chlorine.
A chlorine atom in its outer electron shell has seven electrons. Here, the first and second shells have two and eight electrons respectively. The first electron affinity of chlorine stands at -328.8 kJ per mole of chlorine atoms.
In order to add a second electron to chlorine requires energy. Furthermore, energy recovery cannot take place by the formation of a chemical bond. What results is that chlorine forms a compound such that it has eight electrons in its outer shell.
A sodium atom certainly has a single electron in its outermost electron shell. Also, the first and second shells are full of two and eight electrons respectively. For the purpose of removing this outer electron requires just the first ionization energy.
Moreover, the first ionization energy is +495.8 kJ per mole of sodium atoms. This certainly is a small amount of energy.
The energy requirement for transferring an electron from a sodium atom to a chlorine atom is certainly small: +495.8 − 328.8 = +167 kJ mol−1. Moreover, this energy can be offset with ease by the lattice energy of sodium chloride: −787.3 kJ mol−1.
Limitations of Octet Rule
The main limitation of the rule happens to be hydrogen. Furthermore, hydrogen is at its lowest energy in case it has two electrons in its valence shell.
Helium (He) is similar in that respect too. Also, helium has room for only two electrons in its valence shell.
The second limitation is in relation to aluminum and boron, which can function in an efficient manner with six valence electrons. One must consider BF3. The boron shares its three electrons with three atoms of fluorine.
Then the fluorine atoms certainly follow the octet rule. However, there are only six electrons in boron. Also, most elements to the left of the carbon group are electron deficient.
Another limitation of the octet rule in Period 3 is that the elements on the right side of the periodic table have empty d orbitals. The d orbitals may sometimes accept the electrons and thus allowing elements to have more than an octet. Examples of such elements can be phosphorus and sulfur.
Consequently, the formation of compounds such as PCl5and SF6 may result. Most noteworthy, such compounds may have 10 and 12 electrons around their central atoms respectively.