What is an octet rule? What are the limitations of octet rule?
Answers
Answer:
The octet rule is a chemical rule of thumb that reflects the observation that main group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas.
Explanation:
One limitation of the octet rule is that it cannot be applied to the nonmetals after silicon in the Periodic Table. These elements can “expand their octet” and have more than eight valence electrons around the central atom. ... Molecules with an odd number of electrons such as NO and NO2 cannot satisfy the octet rule.
Compounds like Li, Be and B have 1,2 and 3 valence electrons. The octet rule is not satisfied for all atoms in molecules with an odd number of electrons such as nitric oxide, NO and nitrogen dioxide, NO2. The rule of the octet is evidently based on the chemical inertness of the noble gases.
Octet rule deals with the stability of an atom. According to this rule, an atom is stable if its outermost shell have total of 8 e–. It is a special case of the stability rule of atoms which suggests that the outermost shell should be completely filled for an atom to be stable. The other case of stability rule is duplet rule. Although, the octet rule is not universal, it is useful in understanding the structures of most of the organic compounds. The octet rule mainly applies to the second and third period elements of the modern periodic table.
Answer:
Explanation:
The octet rule is a chemical rule of thumb that reflects the observation that main group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium.
The valence electrons can be counted using a Lewis electron dot diagram as shown at the right for carbon dioxide. The electrons shared by the two atoms in a covalent bond are counted twice, once for each atom. In carbon dioxide each oxygen shares four electrons with the central carbon, two (shown in red) from the oxygen itself and two (shown in black) from the carbon. All four of these electrons are counted in both the carbon octet and the oxygen octet, so that both atoms are considered to obey the octet rule.
The octet rule is a chemical rule of thumb that reflects the observation that main group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The rule is especially applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium.
The valence electrons can be counted using a Lewis electron dot diagram as shown at the right for carbon dioxide. The electrons shared by the two atoms in a covalent bond are counted twice, once for each atom. In carbon dioxide each oxygen shares four electrons with the central carbon, two (shown in red) from the oxygen itself and two (shown in black) from the carbon. All four of these electrons are counted in both the carbon octet and the oxygen octet, so that both atoms are considered to obey the octet rule.
Ionic bonding is common between pairs of atoms, where one of the pair is a metal of low electronegativity (such as sodium) and the second a nonmetal of high electronegativity (such as chlorine).
A chlorine atom has seven electrons in its third and outer electron shell, the first and second shells being filled with two and eight electrons respectively. The first electron affinity of chlorine (the energy release when chlorine gains an electron to form Cl-) is 349 kJ per mole of chlorine atoms.[1] Adding a second electron to form a hypothetical Cl2- would require energy, energy that cannot be recovered by the formation of a chemical bond. The result is that chlorine will very often form a compound in which it has eight electrons in its outer shell (a complete octet), as in Cl-.
A sodium atom has a single electron in its outermost electron shell, the first and second shells again being full with two and eight electrons respectively. To remove this outer electron requires only the first ionization energy, which is +495.8 kJ per mole of sodium atoms, a small amount of energy. By contrast, the second electron resides in the deeper second electron shell, and the second ionization energy required for its removal is much larger: +4562 kJ per mole.[2] Thus sodium will, in most cases, form a compound in which it has lost a single electron and have a full outer shell of eight electrons, or octet.
In the late 19th century, it was known that coordination compounds (formerly called “molecular compounds”) were formed by the combination of atoms or molecules in such a manner that the valencies of the atoms involved apparently became satisfied. In 1893, Alfred Werner showed that the number of atoms or groups associated with a central atom (the “coordination number”) is often 4 or 6; other coordination numbers up to a maximum of 8 were known, but less frequent.[8] In 1904, Richard Abegg was one of the first to extend the concept of coordination number to a concept of valence in which he distinguished atoms as electron donors or acceptors, leading to positive and negative valence states that greatly resemble the modern concept of oxidation states. Abegg noted that the difference between the maximum positive and negative valences of an element under his model is frequently eight.[9] In 1916, Gilbert N. Lewis referred to this insight as Abegg's rule and used it to help formulate his cubical atom model and the "rule of eight", which began to distinguish between valence and valence electrons.[10] In 1919, Irving Langmuir refined these concepts further and renamed them the "cubical octet atom" and "octet theory".[11] The "octet theory" evolved into what is now known as the "octet rule".