Molecular Polarity

Summary

This video explains molecular polarity, focusing on how electronegativity differences between atoms lead to polar or non-polar molecules. It reviews key concepts like electronegativity, bond polarity, and dipole moments. Using examples like ammonia (NH3) and carbon tetrachloride (CCl4), the video shows how molecular geometry and bond differences determine overall polarity. A step-by-step approach is provided, guiding viewers to draw Lewis structures, predict 3D shapes using VSEPR theory, and evaluate molecular polarity based on electronegativity and molecular symmetry.

Takeaways

• 🔍 Understanding molecular polarity is essential to explain phenomena like boiling water or DNA complexity.
• ⚡ Electronegativity is a key concept for determining how atoms in a bond pull electrons toward themselves.
• 🧲 A higher electronegativity creates a dipole moment, which results in a molecule having a positive and negative pole.
• ➡️ Dipole moments are represented using arrows, where the arrow points towards the more electronegative atom.
• 📐 The overall molecular polarity depends on both the bond polarities and the molecular geometry.
• 🔺 In a trigonal pyramidal molecule like ammonia (NH3), the overall dipole moment points towards the nitrogen, making it a polar molecule.
• ⚛️ Symmetrical molecules, such as carbon tetrachloride (CCl4), have polar bonds but are non-polar overall due to the symmetry that cancels the dipoles.
• 📏 To determine polarity: 1) Draw the molecule, 2) Determine bond electronegativities, and 3) Sum the dipoles considering molecular symmetry.
• 🔄 Symmetry is a critical factor—symmetrical molecules tend to be non-polar, even if the individual bonds are polar.
• ✅ Non-symmetrical molecules or those with varying peripheral atoms can exhibit overall polarity if the dipoles do not cancel out.

Q & A

What is the importance of understanding polarity in molecules?

Understanding polarity is crucial because it helps explain how molecules interact, from how water boils to how DNA gives rise to complex life. Polarity affects molecular behavior, including solubility, boiling points, and chemical reactions.

What is electronegativity, and how does it relate to bond polarity?

Electronegativity is a measure of an atom's ability to attract electrons in a bond. In a bond between two atoms, the atom with higher electronegativity pulls electrons more strongly, creating a polar bond with a negative and a positive end.

How is a dipole moment represented in a bond?

A dipole moment is represented by an arrow pointing toward the more electronegative atom. The arrow has a plus sign on the electropositive end, indicating the movement of electron density towards the more electronegative atom.

How does the shape of a molecule affect its overall polarity?

The shape of a molecule determines how the bond dipoles add up. In symmetrical molecules, the dipoles may cancel out, making the molecule non-polar. In asymmetrical molecules, the dipoles can add up to create a net dipole moment, making the molecule polar.

Why is ammonia (NH3) a polar molecule?

Ammonia (NH3) is polar because it has a trigonal pyramidal shape, with the dipole moments from the hydrogen atoms pointing towards the more electronegative nitrogen atom. This creates an overall dipole moment towards the nitrogen.

Why is carbon tetrachloride (CCl4) a non-polar molecule despite having polar bonds?

Carbon tetrachloride (CCl4) is non-polar because it has a symmetrical tetrahedral shape. The dipoles from the C-Cl bonds cancel each other out, resulting in no net dipole moment, making the molecule non-polar.

What is the significance of a 'variant shape' in determining molecular polarity?

A 'variant shape' refers to an asymmetrical molecule. In such shapes, the bond dipoles do not cancel out, leading to an overall molecular dipole, which results in a polar molecule.

How do lone pairs of electrons affect molecular shape and polarity?

Lone pairs of electrons influence the shape of a molecule by repelling bonded atoms, altering the molecular geometry. While lone pairs themselves don't directly determine polarity, their effect on molecular shape can influence the overall dipole moment.

What steps should be followed to determine if a molecule is polar or non-polar?

First, draw the Lewis structure to understand the molecular geometry. Second, determine the bond polarities by calculating electronegativity differences. Finally, assess if the dipoles cancel out due to symmetry. If not, the molecule is polar.

How do symmetrical molecules with polar bonds become non-polar overall?

In symmetrical molecules, even if the individual bonds are polar, the bond dipoles are arranged such that they cancel each other out in three-dimensional space. This results in no overall dipole moment, making the molecule non-polar.

Outlines

00:00

🔍 Understanding Molecular Polarity Through Electronegativity

This paragraph introduces the concept of molecular polarity, explaining that it is essential for understanding phenomena like boiling water and DNA's complexity. The key concept of electronegativity is reviewed, describing how atoms in a bond interact, with more electronegative atoms pulling electrons more strongly. The idea of polar bonds, similar to the poles of Earth, is introduced, using dipoles and visual cues like arrows to indicate electron density distribution. Electronegativity differences between atoms in a bond determine polarity, which can be represented by dipole arrows and delta notations.

05:00

🧪 Explaining Molecular Polarity Using Ammonia and Carbon Tetrachloride

This section delves deeper into how molecular polarity is determined using the examples of ammonia (NH3) and carbon tetrachloride (CCl4). By analyzing the molecular structure and electron configuration of ammonia, the paragraph explains how dipole moments form when electrons are drawn toward more electronegative atoms (like nitrogen). Ammonia is a polar molecule because of its trigonal pyramidal shape. Conversely, carbon tetrachloride is non-polar due to its symmetrical tetrahedral structure, where individual polar bonds cancel each other out, resulting in no overall polarity.

Keywords

💡Polarity

Polarity refers to the distribution of electrical charge over the atoms in a molecule. In the video, polarity is essential for understanding why molecules possess an overall positive or negative charge. For example, the video explains how the polarity of water molecules contributes to boiling and how it influences the molecular structure of ammonia (NH3) and carbon tetrachloride (CCl4).

💡Electronegativity

Electronegativity is the tendency of an atom to attract electrons towards itself in a chemical bond. The video emphasizes that electronegativity differences between bonded atoms create polar bonds, which is central to determining the molecule's polarity. For example, nitrogen is more electronegative than hydrogen, making NH3 polar.

💡Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule. The video describes how molecules like ammonia have an overall dipole moment pointing toward the more electronegative atom, indicating its polarity. In contrast, symmetric molecules like CCl4 have no net dipole moment despite having polar bonds.

💡Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. The video's theme includes explaining how a molecule's geometry influences its polarity. For instance, NH3 has a trigonal pyramidal shape, leading to an overall dipole moment, while CCl4 has a tetrahedral shape that results in non-polarity due to symmetry.

💡Lewis Structure

A Lewis structure is a diagram that shows the bonding between atoms in a molecule and the lone pairs of electrons that may exist. The video mentions drawing Lewis structures as the first step in understanding molecular polarity. By showing where electrons are located, Lewis structures help visualize how polarity arises.

💡Symmetry

Symmetry in molecular geometry plays a crucial role in determining whether a molecule is polar or non-polar. The video explains that even if a molecule has polar bonds, its overall polarity may cancel out if the molecule is symmetrical, as seen with the example of carbon tetrachloride (CCl4).

💡Vector Sum Addition

Vector sum addition involves combining individual bond dipoles to determine the overall polarity of a molecule. The video explains how adding up the arrows representing individual bond dipoles helps in finding out the molecule's net dipole moment, which is crucial for identifying polarity.

💡AXE Notation

AXE notation is a system used to describe the general formula of molecules where 'A' represents the central atom, 'X' denotes the number of atoms bonded to 'A,' and 'E' represents lone pairs of electrons. The video refers to ammonia as an AX3E molecule, indicating its shape as trigonal pyramidal, impacting its polarity.

💡Bond Polarity

Bond polarity arises when two atoms with different electronegativities form a bond, leading to an unequal sharing of electrons. The video discusses how the electronegativity difference between atoms like nitrogen and hydrogen in NH3 leads to polar bonds, contributing to the molecule's overall polarity.

💡Non-Polar Molecule

A non-polar molecule has an even distribution of electron charge, resulting in no net dipole moment. The video uses carbon tetrachloride (CCl4) as an example, where despite having polar bonds, the symmetrical tetrahedral shape causes the bond dipoles to cancel out, making it non-polar.

Highlights

Understanding molecular polarity is essential for explaining complex phenomena, from why water boils to how DNA functions.

Polarity in molecules is determined by electronegativity, which measures an atom's ability to attract electrons in a bond.

A polar bond forms when there is a difference in electronegativity between two bonded atoms, creating a positive and negative end.

Electronegativity differences are symbolized with arrows pointing towards the more electronegative atom, indicating electron density.

The concept of dipole moment describes the direction of electron density within a bond, contributing to overall molecular polarity.

Molecular polarity is calculated by summing the vector addition of dipole moments within the molecule.

Ammonia (NH3) is used as an example of a polar molecule, where the dipole moments add up towards the nitrogen atom.

The shape of the molecule, determined by VSEPR theory, influences the overall molecular polarity.

Carbon tetrachloride (CCl4) is an example of a non-polar molecule despite having polar bonds due to its symmetrical tetrahedral shape.

Symmetry in molecules can cause the dipole moments to cancel out, resulting in a non-polar molecule even if the bonds are polar.

If a molecule has different peripheral atoms or varying electronegativities, it can still be polar even if the shape is symmetrical.

The arrow in the dipole moment points towards the more electronegative end, indicating the direction of electron density.

Determining molecular polarity involves considering both the three-dimensional shape and the electronegativity of each bond.

A non-symmetrical molecule with polar bonds will result in an overall molecular dipole, making the molecule polar.

The procedure to determine polarity includes drawing the molecule, determining its shape, calculating bond polarities, and summing the dipole moments.

Transcripts

now since you're watching this video I'm

going to make the assumption that you're

one of those individuals that wants to

explain everything from why and how

water boils all the way up to how DNA

gives rise to the complexity that is

life and you can't do this without

understanding polarity that's right you

need to be able to explain how it is

that molecules possess an overall

polarity now how do we do that well we

need to go back and quickly review

electr negativity now if you remember

electro negativity is a value that's

assigned to atoms based on how they

interact with other atoms in a bond and

the one that exhibits a greater pole for

the electrons within a bond we say has a

higher electro negativity and the one

that has a lesser pole of the two has a

higher

electropositivity so what that gives us

is a polar bond that is much like the

poles on the very Earth that you're

sitting sitting on right now or standing

on or hovering slightly above is that it

has a north and a South Pole we can say

in fact that it has a dipole there are

two poles and much like that with a bond

we have two ends of a pole as well we

have a positive end and we have a

negative end and that's determined or

established by the electro negativity in

fact within a given Bond we can even

indicate that and symbolize that by an

arrow and this arrow is a little

different than a normal Arrow it's got a

plus sign at the end the electropositive

end showing the dipole moment that is

the density or increasing density of the

electrons around the more

electronegative atom and we can also use

a lowercase Delta to help us figure that

out as well or help us signify that as

well with the Delta negative being the

electro negative end and the Delta

positive being the electropositive end

but how does that help us explain

molecular polarity well we have to

remember that electro negativity is

calculated between two bonded atoms only

two bonded atoms so even if we have

multiple atoms in a molecule and

multiple Bonds in a molecule we don't

ever add these up numerically anyway but

we can add these up in sort of a vector

sum addition if we consider the overall

movement or moment of those electrons

towards the electr negative elements and

we're going to use again much like we

did in the last video ammonia to help us

out so the procedure that we are going

to use is first we have to go through

and draw leis structure

then we have to represent this in

three-dimensional space and as we can

see here we have an ax3 class molecule

sorry an ax3 eClass molecule it is a

trigonal pyramidal molecule and we have

the hydrogens at the peripheral lens and

we have a central nitrogen with a lone

pair of electrons around it now it

should be noted that even though that

lone pair of electrons they are negative

electrons uh do possess a negative

charge they aren't considered when we

figure out the overall polarity not

directly at least but they do impact the

overall shape and therefore how all of

these arrows are going to add up to give

us the overall polarity of the molecule

if any now what we do is we calculate

the individual electro negativity

difference between each one of these and

we can see that if we do that the

nitrogen has a slight Electro negative

charge while the hydrogens the terminal

ends are slightly electropositive and if

we put the arrows in to indicate that we

can see that the overall moment of this

particular mole that is if we take a

look at the sum of all of the arrows

they all seem to be pointing towards the

nitrogen so we would say that within

this particular molecule it has a dipole

or possesses a dipole moment that is

moving towards the nitrogen and we can

draw an overall molecular dipole like

this and we can say in fact therefore

that this ammonia molecule this NH3 is a

polar molecule but if we take a look on

the other hand at say this molecule

which is carbon tetrachloride we can see

that this is an a X4 class tetrahedral

molecule both of the molecule examples

that we've taken a look at have the same

number of electron domains it's just

that our electron domain geometry is a

little bit different with this one

because all of the electron domains

possess bonded pairs of electrons well

there's one lone pair with ammonia now

as we move over to this one we can

calculate all of the electr negativities

but if we look at all of these Electro

negativities even though there is a very

great electro negativity difference

towards the chlorines at the end of all

of those bonds and away from that

Central carbon there is no clear end of

this molecule that is it exists

symmetrically in three-dimensional space

so any way you move it symmetrically it

is the same so there's no top or bottom

to this and as a result even though all

of these bonds are polar the molecule

itself is not so even though again we

have highly polar bonds here the

molecule due to its symmetry is not

itself a polar molecule so what we have

to remember is that all of these parent

shapes because they're symmetrical if

any of these bonds are the same and that

is they cancel each other out in terms

of their dipoles even if all of the

bonds are polar the molecule itself due

to its symmetry is non-polar it should

be noted though that if we have

different peripheral atoms within a

variant shape that it is certainly

possible to have a polar molecule there

and it is possible to have a non-polar

molecule variant if the electro

negativity difference is quite small so

as we go through drawing this a good

step-by-step procedure would be to First

figure out the three-dimensional shape

second figure out the electro negativity

difference of each of the bonds in that

particular molecule third try and figure

out which way the overall dipole moment

is moving if it's a symmetrical molecule

and all of the bonds are the same

they're all going to cancel cancel out

and you're not going to have a polar

molecule however if you have a variant

shape as I like to call them and we have

all of these bonds being the same or

even some of them having different

Electro negativities what you're likely

to find is that there's an overall

movement or moment towards one end of

the molecule hence making it a polar

molecule remember that the arrow points

towards the more electr negative end of

the bond and the more electr negative

end of the molecule because the electron

density is greatest around there so we

say that the dipole moment moves towards

the more Electro negative elements in a

particular molecule and then the other

end is the electropositive end so in

summary then if we don't have a

symmetrical molecule with all of the

bonds and their bond polarities being

the same we can take all of these

individual Bond polarities create a

vector sum addition of all of these

which will in turn create a molecular

dipole and in fact tell us whether or

not the molecule we have is polar or

non-polar so now hopefully after

watching this and several other videos

in the series you're able to take a

molecular formula draw two dimension

structure use Vesper Theory to figure

out its threedimensional

structure use the electr negativities to

establish the bond polarities of all of

the different bonds within that

particular molecule and then if any

figure out the overall polarity of that

molecule to establish whether or not the

molecule you've drawn is polar or

non-polar thanks for watching