S2.2.1 The octet rule

Summary

This video explores the octet rule, which explains how atoms bond to achieve a full valence shell of eight electrons, mirroring noble gases' electron configurations. It distinguishes between covalent and ionic bonding, illustrating how atoms share or transfer electrons to reach stability. The script also addresses exceptions to the rule, such as hydrogen and helium with two electrons, and elements like boron and sulfur that deviate from the standard octet, including expanded octets in period three elements.

Takeaways

• 🌌 The Octet Rule states that atoms bond to achieve a full valence shell of eight electrons, mirroring the electron configuration of noble gases.
• 🔬 Noble gases like helium, neon, argon, and krypton have full valence shells, which contribute to their stability and lack of reactivity.
• 💠 Helium is an exception to the octet rule, having only two electrons in its valence shell, as it is in period one of the periodic table.
• 🔄 Atoms can achieve a full valence shell through covalent bonding by sharing electrons or ionic bonding by transferring electrons.
• 🌐 Covalent bonds, like those in Br2 and O2, involve the sharing of electrons to allow each atom to reach a full valence shell.
• ⚡ Ionic bonds occur when electrons are transferred between atoms, forming ions with full valence shells, like in the case of sodium chloride.
• 🚫 Noble gases are unreactive because they already have full valence shells, making them stable and unlikely to form compounds.
• 🚀 Exceptions to the octet rule include hydrogen and helium, which are stable with two electrons, and elements like beryllium and boron, which can be stable with fewer than eight electrons.
• 🌀 In compounds like BF3 and BeCl2, the central atoms have less than an octet, which is an exception to the typical bonding rules.
• 🌍 Period three elements, such as sulfur in SF6, can exceed the octet rule with an expanded valence shell of more than eight electrons.
• 🔬 The expanded octet is common for non-metal elements in period 3 and beyond, allowing them to have more than eight electrons in their valence shell.

Q & A

What is the Octet Rule?

The Octet Rule states that atoms bond together to achieve a full valence shell containing eight electrons, which gives them the same electron configuration as a noble gas.

Why are noble gases stable?

Noble gases are stable because they have full valence shells, which means they have achieved the same electron configuration as the nearest noble gas, making them unlikely to form compounds.

How many electrons are in the valence shell of helium and neon?

Helium has two electrons in its 1s sublevel, and neon has eight electrons in its valence shell, with two in the 2s sublevel and six in the 2p sublevel.

What are the two types of bonding that allow atoms to achieve a full valence shell?

The two types of bonding are covalent bonding, where atoms share electrons, and ionic bonding, where electrons are transferred between atoms to form ions.

What is the difference between covalent and ionic bonds?

Covalent bonds involve the sharing of electrons between atoms, while ionic bonds result from the transfer of electrons, leading to the formation of positively and negatively charged ions.

How many valence electrons does a bromine atom have, and how does it achieve a full valence shell?

A bromine atom has seven valence electrons. It achieves a full valence shell by sharing two electrons through a single covalent bond with another bromine atom.

What is the electron configuration of an oxygen molecule (O2) and how does it achieve a full valence shell?

An oxygen molecule (O2) has a double covalent bond, which consists of four electrons. Each oxygen atom shares these four electrons to achieve a full valence shell.

How does a sodium atom transfer its electron to form an ionic bond with a chlorine atom?

A sodium atom transfers one of its valence electrons to a chlorine atom, resulting in a positively charged sodium ion and a negatively charged chloride ion, each with a full valence shell.

What are the exceptions to the Octet Rule mentioned in the script?

The exceptions to the Octet Rule mentioned are hydrogen and helium, which are stable with two electrons in their valence shells, beryllium and boron, which can be stable with four and six electrons respectively, and period three elements like sulfur and phosphorus, which can have more than eight electrons in their valence shells (expanded octet).

Why does boron form an exception to the Octet Rule in BF3?

In BF3, the boron atom has only six electrons in its valence shell, which is an exception to the Octet Rule because boron is stable with fewer than eight electrons in its valence shell.

What is an expanded octet, and which elements are known to exhibit this phenomenon?

An expanded octet refers to atoms having more than eight electrons in their valence shell, which is often seen in non-metal elements from period 3 onwards, such as sulfur and phosphorus.

Outlines

00:00

🌌 The Octet Rule and Noble Gases

The video begins by explaining the Octet Rule, which states that atoms tend to bond to achieve a full valence shell of eight electrons, mirroring the electron configuration of noble gases. The script introduces the first four noble gases—Helium, Neon, Argon, and Krypton—and their electron configurations, emphasizing that all but Helium have eight electrons in their valence shells. It highlights the stability of noble gases due to their full valence shells and introduces the concepts of covalent and ionic bonding as methods for atoms to achieve a full valence shell.

05:01

🔬 Covalent and Ionic Bonding Mechanisms

This paragraph delves into the specifics of covalent and ionic bonding. Covalent bonding is described as the sharing of electrons between atoms, exemplified by the Lewis structures of bromine and molecular oxygen, where atoms share electrons to achieve a full valence shell. Ionic bonding, in contrast, involves the transfer of electrons, leading to the formation of ions with full valence shells, as illustrated by the reaction between sodium and chlorine atoms. The paragraph concludes with a summary that ionic bonding results in the formation of ions, each with a noble gas electron configuration.

🚫 Exceptions to the Octet Rule

The final paragraph addresses exceptions to the Octet Rule. It starts by discussing hydrogen and helium, which are stable with only two electrons in their valence shells due to their position in the periodic table. The script then moves on to compounds like boron trifluoride (BF3) and beryllium chloride (BeCl2), where the central atoms have fewer than eight electrons in their valence shells. The video also mentions sulfur hexafluoride (SF6), where sulfur exceeds the octet with twelve electrons due to an expanded octet, a phenomenon observed in non-metal elements from period three onwards. The summary concludes by listing the exceptions: hydrogen and helium with two electrons, beryllium and boron with four and six electrons, respectively, and period three elements capable of an expanded octet.

Keywords

💡Octet Rule

The Octet Rule is a chemical principle stating that atoms tend to bond in such a way that each atom has eight electrons in its valence shell, achieving a stable electron configuration similar to that of noble gases. This rule is central to the video's theme, as it explains the driving force behind chemical bonding. For example, the script mentions that atoms bond to achieve the electron configuration of noble gases like helium, neon, argon, and krypton, which all have full valence shells.

💡Valence Shell

The valence shell is the outermost electron shell of an atom, which is involved in chemical bonding. The video emphasizes the importance of the valence shell in the context of the Octet Rule, as atoms aim to fill this shell to achieve stability. The script provides examples of noble gases with full valence shells, illustrating the stability achieved when the valence shell is complete.

💡Noble Gases

Noble gases, such as helium, neon, argon, and krypton, are a group of chemical elements that are stable due to their full valence electron shells. The video uses noble gases as a reference point for the Octet Rule, showing that atoms strive for the electron configuration of these gases to be stable. The script lists the electron configurations of the first four noble gases to highlight their full valence shells.

💡Covalent Bonding

Covalent bonding is a type of chemical bond formed by the sharing of electron pairs between atoms. The video explains that this type of bonding allows atoms to achieve a full valence shell, as seen in the Lewis structures of bromine and molecular oxygen. Covalent bonds are central to the video's discussion on how atoms can satisfy the Octet Rule through electron sharing.

💡Ionic Bonding

Ionic bonding involves the transfer of electrons from one atom to another, resulting in the formation of oppositely charged ions that are attracted to each other. The video contrasts ionic bonding with covalent bonding, using the example of sodium and chloride ions to illustrate how atoms can achieve a full valence shell by transferring electrons and forming ions with noble gas electron configurations.

💡Lewis Structure

A Lewis structure is a graphical representation of the valence electrons of an atom or molecule, showing how these electrons are arranged to form chemical bonds. The video uses Lewis structures to illustrate the bonding in molecules like methane (CH4) and to explain the electron sharing or transfer in covalent and ionic bonds, respectively.

💡Electron Configuration

Electron configuration refers to the distribution of electrons in an atom's orbitals. The video discusses how atoms achieve stable electron configurations by bonding, either by sharing or transferring electrons to match the configurations of noble gases. The script provides examples of noble gases and ions formed through ionic bonding to demonstrate stable electron configurations.

💡Helium

Helium is a noble gas with a unique electron configuration, having two electrons in its 1s sublevel. The video notes that helium, along with hydrogen, is an exception to the Octet Rule, as it is stable with only two electrons in its valence shell. This is an important point in the video, as it highlights the variability of the Octet Rule.

💡Boron Trifluoride (BF3)

Boron trifluoride (BF3) is a compound where the boron atom has six electrons in its valence shell, which is an exception to the Octet Rule. The video uses BF3 to illustrate that some atoms can be stable with fewer than eight electrons in their valence shells, challenging the traditional application of the Octet Rule.

💡Expanded Octet

An expanded octet refers to a situation where an atom has more than eight electrons in its valence shell, typically seen with elements in period 3 and beyond. The video mentions sulfur hexafluoride (SF6) as an example, where sulfur has twelve electrons in its valence shell, demonstrating the flexibility of the Octet Rule in certain cases.

💡Exceptions to the Octet Rule

The video discusses several exceptions to the Octet Rule, including hydrogen and helium, which are stable with two electrons, and elements like beryllium and boron, which can be stable with four and six valence electrons, respectively. It also mentions period three elements, such as sulfur and phosphorus, that can have an expanded octet. These exceptions highlight the limitations and variability of the Octet Rule.

Highlights

The Octet Rule states that atoms bond to achieve a full valence shell of eight electrons, mirroring noble gas electron configurations.

Noble gases like helium, neon, argon, and krypton have full valence shells, contributing to their stability.

Helium, with two electrons, is the exception among noble gases, as it achieves stability with a duet instead of an octet.

Atoms can gain a full valence shell through covalent or ionic bonding.

Covalent bonding involves sharing electrons, as seen in the Lewis structures of Br2 and O2.

Ionic bonding occurs through electron transfer, forming ions with full valence shells, like in the case of Na+ and Cl-.

Sodium ions achieve the electron configuration of neon after ionic bonding, while chloride ions resemble argon.

Hydrogen and helium are exceptions to the Octet Rule, with hydrogen forming bonds with only two electrons in its valence shell.

Boron trifluoride (BF3) is an exception with boron having only six electrons in its valence shell, stable despite not fulfilling the octet.

Beryllium chloride (BeCl2) also defies the Octet Rule, with beryllium having just four electrons in its valence shell.

Sulfur hexafluoride (SF6) exemplifies an expanded octet with sulfur having 12 electrons in its valence shell.

Period three elements like sulfur and phosphorus can exceed the octet rule, having more than eight electrons in their valence shells.

The video provides a detailed explanation of the Octet Rule and its significance in chemical bonding.

The electron configurations of noble gases are used as a reference point for understanding the Octet Rule.

Covalent and ionic bonding mechanisms are explained with examples of their Lewis structures.

The video clarifies the concept of full valence shells in ionic bonding and how it relates to noble gas configurations.

Exceptions to the Octet Rule are highlighted, emphasizing the unique stability of certain elements with incomplete octets.

The video concludes with a summary of the exceptions to the Octet Rule, providing a comprehensive understanding of their electron configurations.

Transcripts

this is msj chem in this video i'll be

looking at the octet rule the octet rule

states that atoms bond together in order

to achieve a full valence shell

containing eight electrons and by doing

so they achieve the same electron

configuration as a noble gas here we can

see the symbols of the first four noble

gases they are helium neon argon and

krypton

if we look at the electron

configurations we can see that each

noble gas has a full valence shell

for helium that's the two electrons in

the 1s sub level for neon that's the two

electrons in the 2s sublevel and the 6

electrons in the 2p sublevel for argon

that's the two electrons in the 3s

sublevel and the 6 electrons in the 3p

sublevel and for krypton it's the two

electrons in the 4s sublevel and the 6

electrons in the 4p sublevel

so as we can see all noble gases with

the exception of helium have eight

electrons in their valence shells

an important point to note is that noble

gases are stable in that they don't form

compounds because they have four valence

shells

so next we'll have an introduction to

two types of bonding atoms can gain a

full valence shell by either shearing

electrons which is covalent bonding or

by transferring electrons which is ionic

bonding

on the left we have the lewis structure

of ch4 which is methane the atoms in

methane are held together by covalent

bonds on the right we have an example of

ionic bonding ionic bonding occurs

between oppositely charged ions

so next we'll look at each type of

bonding in a bit more detail

so we'll start by looking at covalent

bonding which is sharing of electrons

here we can see two lewis structures we

have bromine on the left and molecular

oxygen on the right a molecule of

bromine consists of two bromine atoms

bonded by a single covalent bond a

single covalent bond is composed of two

electrons each bromine atom has seven

valence electrons

so by sharing these two electrons each

bromine atom can achieve a full valence

shell next we look at our second example

which is o2 an oxygen atom has six

electrons in its valence shell a double

covalent bond is composed of four

electrons

so by sharing these four electrons each

oxygen atom can achieve a full valence

shell

so to summarize in covalent bonding

atoms shear electrons to achieve a full

valence shell

next we look at ionic bonding which is

the transfer of electrons

on the left we have a sodium ion and on

the right we have a chloride ion during

the formation of an ionic bond a sodium

atom transfers an electron to a chlorine

atom this results in the formation of a

sodium ion and a chloride ion the sodium

ion now has a full valence shell and as

we can see it has the same electron

configuration as the noble gas neon the

chloride ion has also achieved a full

valence shell and now has the same

electron configuration as the noble gas

argon

so to summarize ionic bonding results in

the formation of ions each ion has a

full valence shell and the same electron

configuration as a noble gas

so we'll end the video by looking at the

exceptions to the octet rule

the first two are hydrogen and helium

both atoms are in period one of the

periodic table

therefore they can only hold two

electrons in their valence shells so

when hydrogen forms a bond with another

atom it can only hold two electrons in

its valence shell helium being a noble

gas does not form compounds

it exists as a monoatomic element with

two electrons in its valence shell

next we have bf3 which is boron

trifluoride

if we look at the lewis structure we can

see that the boron atom only has six

electrons in its valence shell

boron is an exception to the octet rule

in that it is stable with less than

eight electrons in its valence shell the

same is true for becl2 which is

beryllium chloride

if we look at the lewis structure we can

see that the beryllium atom only has

four electrons in its valence shell once

again this is an exception to the octet

rule our last example is sf6 which is

sulfur hexafluoride if we look at the

lewis structure we can see that the

sulfur atom has 12 electrons in its

valence shell which is known as an

expanded octet

this is often seen when non-metal

elements in period 3 onwards form

compounds

so next we'll have a summary of the

exceptions they are hydrogen helium

beryllium boron and period three

elements such as sulfur and phosphorus

both hydrogen and helium are stable with

two electrons in their valence shells

beryllium and boron are stable with four

and six electrons in their valence

shells respectively and finally we have

period three elements such as sulfur and

phosphorus these can have more than

eight electrons in their valence shells

which is known as an expanded octet