The most common exceptions to the octet rule are compounds or ions with central atoms having mor

The most common exceptions to the octet rule are compounds...

Key Concepts

Octet Rule

The octet rule is a chemical principle stating that atoms tend to form bonds in a way that gives them eight electrons in their valence shell, similar to the electron configuration of the noble gases. This guides the structure and stability of many molecules, although there are notable exceptions, particularly in molecules where the central atom possesses more than eight electrons.

Expanded Octet

The expanded octet concept refers to situations where a central atom accommodates more than eight electrons. This commonly occurs with elements in the third period and beyond because they have available d orbitals that can hold additional electrons. Examples of molecules with expanded octets include some phosphorus, sulfur, and halogen compounds.

Lewis Structures

Lewis structures are diagrams that represent the bonding between atoms in a molecule and the distribution of valence electrons, including lone pairs. They are used to predict molecular geometry and to illustrate exceptions to the octet rule, such as expanded octets seen in certain compounds.

Period 3 and Higher Elements

Elements from period 3 onward, such as phosphorus, sulfur, chlorine, and bromine, have access to d orbitals in their valence shell. This allows them to form more than four bonds or accommodate extra electrons beyond the octet, which is crucial for understanding how and why molecules with expanded octets can exist.

Formal Charge

Formal charge is a bookkeeping tool used in Lewis structures to verify that electrons are distributed in the most stable manner. Calculations of formal charge help determine the most plausible structure for a molecule by minimizing charge separation. It is especially important for molecules with expanded octets to ensure that the electron distribution is reasonable.

Transcript

00:02 In this video, we're going to be looking at some different, one as a very common exception to the octet rule.

00:11 Now, when looking at lewis structures, just a quick reminder, the octet rule, basically states that every molecule or every, sorry, atom in the compound, would need to have eight valence electrons that it has in its outer shell when it's completed, that are either shared or it has.

00:39 At its disposal there.

00:40 Okay.

00:41 But we're going to look at an exception here where it can have more than eight.

00:45 And this is usually with the central element, central atom in the compound itself.

00:53 So let's look at this one right here.

00:55 So pf5, so phosphorus and then five fluorine, fluorine, on this compound.

01:04 Okay.

01:05 And so the first thing we always want to do is just count how many valence electrons do we really even have here? so we know phosphorus has five valence electrons, and then fluorine has a total of seven in there, and so we're going to, and we have five of those, we're going times that by five.

01:24 And so seven times by five is 35 plus five makes a total of 40 valence electrons that we've got to put on this sucker.

01:32 So we have our phosphorus in the center, and then what we're we're going to do is we're going to have, and i'm going to try and space these out as much, one, two, three, four, five, as well as i can here.

01:45 Just know that these fluorines will be as far apart from each other as they possibly can.

01:53 You would have some equality as you're going through that.

01:57 So anyways, we have our slu of structure almost there.

02:02 And so what we've got to do is just subtract the bond right here.

02:04 So one, two, three, four, five, six, seven, eight, 9, 10.

02:07 So we're going to subtract 10 from our list.

02:10 So we still have 30 valence electrons.

02:12 We've got to add here.

02:13 And so what i'm going to do is to start adding these to fluorines.

02:16 So there's 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 17, 17, 18, 18, 19, 19, 19, 19, 20, 22, 24, 25, 26, 27, 28, 29, 30.

02:29 So there we go.

02:30 We've got our 30 valence electrons.