The most common type of exception to the octet rule are...

The most common type of exception to the octet rule are compounds or ions with central atoms having more than eight electrons around them. $\mathrm{PF}_{5}, \mathrm{SF}_{4}, \mathrm{ClF}_{3}$ and $\mathrm{Br}_{3}^{-}$ are examples of this type of exception. Draw the Lewis structure for these compounds on ions. Which elements, when they have to, can have more than eight electrons around them? How is this rationalized?

Key Concepts

Availability of d Orbitals

Atoms in the third period and below (such as phosphorus, sulfur, and chlorine) possess empty 3d orbitals that can be used to hold additional electrons beyond the octet. This ability to utilize d orbitals is the key to rationalizing why these atoms can form compounds with more than eight electrons around the central atom.

Octet Rule

The octet rule is a chemical principle stating that atoms tend to form bonds until they are surrounded by eight valence electrons, mimicking the electron configuration of a noble gas. This rule is a guiding principle for many stable molecules but has several well?known exceptions mainly involving atoms in the third period or beyond.

Expanded Octet

An expanded octet is the phenomenon where a central atom, typically from the third period or lower in the periodic table, accommodates more than eight electrons in its valence shell. This occurs because these atoms have d orbitals available in their valence shell, which can participate in bonding, thereby allowing the formation of hypervalent compounds.

Lewis Structures for Hypervalent Compounds

Drawing Lewis structures for hypervalent molecules involves placing the central atom with more than four bonds, which leads to more than eight electrons around it. The validity of these structures is supported by the concept of an expanded octet for atoms in lower rows of the periodic table. However, while Lewis structures serve as important models, they do not fully capture the nuances of orbital interactions in such molecules.

Transcript

00:01 Problem 91 says that some molecules can disobey the octet rule when central atoms can have more than eight electrons.

00:11 So these are sort of a special case, and they follow the same general rules of drawing lewis structures, but keep in mind that the central atom can, in fact, sort of be an exception to the octet rule.

00:25 So for pf5 here, phosphorus has five valence electrons and fluorine has seven each, so making the total valence contribution from fluorine 35.

00:46 In that case, we're looking for a total of 40 valence electrons.

00:52 So we can draw the central atom here, phosphorus, and n.

00:58 To that we'll draw five fluorines.

01:09 So already we have two, four, six, eight, ten electrons.

01:16 We can start by filling in the valence electrons for fluorine to account for the rest.

01:32 And so right now, because we have five fluorines bound to the central phosphorus, each of them being surrounded by eight electrons, we have a total of 40, which is what we wanted.

01:45 But here you'll see that the central phosphorus is surrounded not by 8, but by 10 electrons.

01:54 Now the question asks us why some of the atoms on the periodic table can do this.

02:00 And the answer is, if you look at where phosphorus falls on the periodic table, you'll see that at that point of the table and below, d -orbital's open.

02:16 And so what can happen with elements like phosphorus is even though, you know, in theory, they should only have eight electrons surrounding them, the extra electrons that come from making more bonds than you might expect can go to these empty d orbitals that are relatively close in energy to s &p orbitals.

02:38 Smaller atoms like, you know, say oxygen can't do this.

02:49 So we can move on to sf4.

02:52 Again, we have a central sulfur down to four fluorines.

03:00 So for sulfur, we are in general looking for six valence electrons, and with four fluorines, that's 28...

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