Polar, Non-Polar, and Ionic Compounds: Explanation, Examples, and Practice

Summary

本视频深入探讨了分子极性的判断方法。首先分析了原子间的化学键，特别是两个原子间的电子亲和力差异，用以判断键的极性。通过具体例子（如HCl和N2）展示了如何通过电子亲和力差异来判断分子是否极性。视频中还介绍了如何通过路易斯结构、键的极性差异以及分子的对称性来综合判断分子的整体极性。以CH3Cl、NH3和H2O为例，详细解释了孤对电子对分子极性的影响，强调了极性在科学领域的重要性。

Takeaways

• 🔬 极性的理解首先从单个化学键开始，然后扩展到整个分子。
• 🌐 原子间的极性取决于它们电负性值的差异，差异大于0.5但小于2.0的键被认为是极性的。
• 📊 电负性值在周期表上可以找到，常用值包括氢（2.20）和氯（3.16）。
• 📉 电负性差异大于2.0的键是离子键，0.5以下的是非极性共价键。
• 🌟 电负性是原子吸引共享电子的能力，这些电子位于原子之间。
• 📚 电负性的趋势是，当你向氟（氟是第18族的元素）移动时，原子变得更电负。
• 🔍 极性分子的例子包括HF、HBr、NHI等，而非极性分子如N2、O2、F2等。
• 🧩 判断分子是否极性时，需要考虑分子的对称性和形状，即使单个键可能是极性的。
• 🌐 在更大的分子中判断极性时，需要遵循绘制路易斯结构、计算键的电负性差异、考虑分子形状和对称性的步骤。
• 💧 水分子（H2O）和氨分子（NH3）由于它们的非对称结构和孤对电子的影响，是极性分子。
• 🔍 极性是科学中一个极其重要的主题，它影响分子如何相互作用，从药物到建筑材料都与分子的极性有关。

Q & A

什么是极性分子？

极性分子是指分子内部电荷分布不均匀，导致分子的一端带有部分正电荷，另一端带有部分负电荷的分子。

如何判断一个化学键是极性还是非极性的？

通过比较两个原子的电负性差异来判断。如果电负性差异大于0.5但小于2.0，则化学键被认为是极性的；如果差异小于0.5，则认为是非极性的；如果差异大于2.0，则可能形成离子键。

为什么HCl被认为是极性分子？

HCl分子中氢的电负性为2.20，氯的电负性为3.16，两者的电负性差异为0.96，根据电负性差异的标准，HCl的化学键是极性的，因此HCl是极性分子。

为什么N2被认为是非极性分子？

N2分子中两个氮原子的电负性相同，都是3.04，因此电负性差异为0，这导致N2分子中的化学键是非极性的。

电负性是什么？

电负性是原子吸引共享电子对的能力，通常在路易斯结构图中表现出来。电负性高的原子会吸引电子对，使其更靠近自己。

为什么CCl4被认为是非极性分子？

尽管CCl4中的每个碳-氯键都是极性的，但由于CCl4分子具有四面体结构，这种结构是对称的，使得分子的极性相互抵消，因此整个分子是非极性的。

孤对电子对分子的极性有什么影响？

孤对电子对会影响分子的形状和对称性，从而影响分子的极性。例如，在NH3分子中，孤对电子的存在使得分子形状变为三角锥形，打破了对称性，使得NH3成为极性分子。

为什么H2O被认为是极性分子？

H2O分子中的氢氧键是极性的，并且由于氧原子上有两对孤对电子，使得分子形状成为V形，这种结构是非对称的，因此H2O是极性分子。

分子的极性对于科学领域有什么重要性？

分子的极性对于科学领域非常重要，它影响分子间的相互作用，这在药物设计、建筑材料以及许多其他领域都是关键因素。

如何确定一个分子是否是极性或非极性的？

确定分子是否极性需要考虑分子的路易斯结构、单个化学键的电负性差异以及分子的整体形状和对称性，包括孤对电子的影响。

Outlines

00:00

🔬 化学键和分子的极性

本段视频脚本介绍了如何通过分析化学键和整个分子来理解极性。首先，通过查看两个原子之间的极性，每个原子都有特定的电负性值。通过比较这些值的差异来判断键是极性还是非极性。视频中使用了一个简化的周期表来展示常用的电负性值，并以HCl（氢氯酸）为例，计算了氢和氯之间的电负性差异，得出0.96的值，表明HCl是一个极性分子。接着，介绍了电负性差异的分类标准：大于2.0为离子键，0.5到2.0之间为极性键，小于0.5为非极性键。此外，还讨论了如何通过路易斯结构图来分析分子的极性，包括考虑分子的形状和对称性。最后，通过CCl4（四氯化碳）的例子，说明了即使每个碳-氯键都是极性的，但由于分子的对称性，整个分子是非极性的。

05:02

🧠 分子的极性分析

第二段视频脚本继续探讨了如何在更大的分子中分析极性。首先，通过路易斯结构图来确定分子中各个键的极性，然后考虑分子的整体形状和对称性。以CH3Cl（氯甲烷）为例，虽然碳-氯键是极性的，但由于分子的四面体结构，整体分子是非极性的。接着，讨论了未成键的电子对（孤对电子）对分子形状、极性和对称性的影响，以NH3（氨）为例，展示了孤对电子如何改变分子的对称性，导致分子成为极性的。最后，通过H2O（水）的例子，说明了即使氢-氧键是极性的，但由于孤对电子的存在，水分子呈现出非对称性，因此是极性的。视频强调了极性不仅由原子间的电子共享不均导致，还受到分子形状或对称性的影响，这对于科学中的许多领域都非常重要。

Keywords

💡极性

极性是指分子中电荷分布的不均匀性，通常由于电子对的不对称分布造成。在视频中，极性是核心概念，用来解释分子如何通过电荷分布的不均匀性来影响其物理和化学性质。例如，HCl分子由于氢和氯之间电子吸引能力的差异而表现出极性。

💡电负性

电负性是原子吸引电子对的能力，通常用数值表示。视频中提到，电负性差异是判断化学键极性的关键因素。例如，氢和氯的电负性差异为0.96，这表明HCl是一个极性分子。

💡离子键

离子键是由正负离子之间的静电吸引力形成的化学键。在视频中，当两个原子的电负性差异超过2.0时，形成的是离子键。这是理解化学键类型及其性质的重要概念。

💡非极性共价键

非极性共价键是指两个原子共享电子对时，电子对均匀分布的情况。视频中提到，当电负性差异低于0.5时，形成的是非极性共价键，如N2分子中的氮氮键。

💡路易斯结构

路易斯结构是表示分子中原子如何通过共价键连接的图形表示方法。视频中通过绘制HF等分子的路易斯结构，来分析分子的极性。这是理解分子结构和性质的基础工具。

💡对称性

对称性是指分子结构的均匀和平衡状态。视频中强调，即使分子中的单个键是极性的，整个分子也可能是非极性的，这取决于分子的对称性。例如，CCl4分子虽然每个碳-氯键都是极性的，但由于其对称的四面体结构，整个分子是非极性的。

💡孤电子对

孤电子对是指未与其他原子共享的电子对。视频中提到，孤电子对会影响分子的形状和对称性，从而影响分子的极性。例如，NH3分子中的孤电子对导致分子呈现非对称的三角锥形结构，使其成为极性分子。

💡四面体结构

四面体结构是一种分子几何形状，其中有一个中心原子与四个等距的外围原子相连。视频中提到，CCl4分子具有四面体结构，但由于其对称性，整个分子是非极性的。

💡三角锥形结构

三角锥形结构是一种分子几何形状，其中一个原子位于顶点，三个外围原子位于底面的三角形顶点。视频中提到，NH3分子具有三角锥形结构，由于孤电子对的存在，使其成为极性分子。

💡电子对的共享

电子对的共享是指两个原子之间共享电子对形成化学键的过程。视频中讨论了电子对共享的不均匀性是导致分子极性的原因，如HF分子中氢和氟之间的电子对共享不均匀，导致分子极性。

Highlights

理解极性首先要看单个化学键，然后是整个分子。

通过原子的电负性值来确定键的极性，电负性差值大于0.5为极性键，小于0.5为非极性键。

电负性差值大于2.0则形成离子键，0.5到2.0之间为极性键，小于0.5为非极性键。

HCl分子的极性分析，氢和氯的电负性差值为0.96，属于极性分子。

电负性的趋势是向氟元素移动时增加，稀有气体的电负性通常不被考虑。

N2分子是非极性的，因为两个氮原子的电负性相同。

双原子分子如O2、N2、F2总是非极性的，因为电负性差值为零。

CCl4分子中每个碳-氯键都是极性的，但整个分子是否极性需要考虑分子的对称性。

CCl4分子具有四面体结构，是对称的，因此是非极性的。

CH3Cl分子的极性分析，碳-氯键是极性的，但整个分子的极性取决于其空间结构。

NH3分子的极性分析，氮-氢键是极性的，但分子的极性还受到孤对电子的影响。

H2O分子的极性分析，氧-氢键是极性的，分子的极性还受到孤对电子和分子对称性的影响。

极性是由电子的不均等共享导致的，包括键合电子和孤对电子。

分子的极性对科学领域中的许多事物都非常重要，如药物和建筑材料。

通过绘制路易斯结构、分析键的电负性差值以及考虑分子的形状和对称性，可以确定分子是否极性。

Transcripts

to understand polarity we'll first look

at individual chemical bonds and then

the entire molecule let's look at

polarity between two atoms first

each atom has a specific value for its

electronegativity to figure out if a

bond is polar or nonpolar we look at the

difference between these values

let's look at a condensed periodic table

with the values we'll use most

frequently let's try hcl hydrochloric

acid hydrogen has a value of 2.20 and

chlorine has a value of 3.16

the difference between these

2.20 minus 3.16 gives us 0.96

that's the difference in

electronegativity for h and cl

but what does that number mean chemical

bonds can be classified along a

continuum if the difference in

electronegativity is above 2.0 it's an

ionic bond we consider molecules between

2.0 and 0.5 to be polar and below 0.5 is

nonpolar

these are just guides you may be given

slightly different values back to hcl we

found the difference in

electronegativity to be 0.96

meaning that hcl is considered a polar

molecule

other examples of polar molecules

hf

hbr nhi

we've been talking about

electronegativity

often written as en is the ability of

atoms to attract shared electrons those

are the electrons that are between atoms

when we draw lewis structures

as we've seen in the periodic table

atoms have different values for

electronegativity

the trend is that atoms are more

electronegative as you move towards

fluorine

for group 18 the noble gases they rarely

form chemical bonds and we don't really

consider their electronegativity to be

important

so we know hcl is a polar molecule with

its difference in electronegativity

greater than 0.5 but less than 2.0

for something like n2 nitrogen gas we

can look up the value for n which is

3.04

so 3.04 minus 3.04 is 0.

back to our continuum we see that the

difference below 0.5 is nonpolar

covalent

at this point you probably realize you

need to memorize the numbers in our

continuum

when we have diatomic molecules like o2

n2 f2 these will always be nonpolar

because the difference when we subtract

the electronegativity values we'll be

zero

pause and take a moment to figure out if

each one of these molecules is polar or

nonpolar

for hf we have a difference of 1.78

meaning this is going to be a very polar

molecule with those shared electrons

spending most of their time around the

fluorine atom

for brcl the difference is

0.20

we can have different atoms and still

have a nonpolar bond

for i2 they're the same we'll have an

electronegativity value of zero that

means i2 is nonpolar

we can now find the bond polarity

between two atoms and even do simple

atoms like hcl or n2

next up we want to look at polarity in

larger molecules it's useful to follow

these steps

first have the lewis structure

second

we'll look at the individual bonds just

like we've been doing in this video and

finally we'll look at the shape and the

symmetry to figure out if the molecule

is polar or nonpolar overall

we'll start with ccl4 carbon

tetrachloride carbon has a value of 2.55

and cl has a value of 3.16

the difference between these two numbers

is 0.61

so we know that each bond is going to be

polar

we can write the structure like this

the arrows point towards the more

electronegative atom the delta symbol

that shows the charge

here cl has a negative charge because

it's more electronegative

at this point we've looked at the lewis

structure and we've calculated the

electronegativity difference between the

bonds each carbon-chlorine bond is polar

but be careful this alone won't tell us

if the whole molecule is polar or

nonpolar we need to consider the

symmetry of the molecule to answer that

question

ccl4 is a symmetrical molecule so watch

what happens we have a carbon here in

the center and we're going to add

chlorines so we add one chlorine and

then we add the second one and they

spread out they push away from each

other

the reason they do that is this atom

here the surface are all the electrons

and electrons are negative so when i try

to put two negatives together they'll

spread out

if i add another one

they spread out again you can see that

they're equidistant

and finally i'll add the fourth cl so we

have ccl4 and they're spread out in this

tetrahedral structure

this is symmetrical any angle you look

at it it's pretty much the same

that means that the surface of the

molecule will be the same everywhere

there'll be no poles and it won't be

polar

tetrahedral shaped molecules would be

nonpolar if they consist of carbon and

four of the same type of atoms attached

to that carbon

using the steps we've just covered pause

and try to figure out if ch3cl

is polar or nonpolar

for ch3cl we have the lewis structure

here

and we can calculate the en difference

for each of the bonds

you can see that ccl that's a polar bond

well the c h bond is nonpolar

so with our lewis structure we can take

a look at the actual shape of the entire

molecule we see we have the carbon with

four atoms attached and we know those

are going to spread out and form a

tetrahedral structure

we can see that we have two sides to

this molecule we have a side with the

chlorine atom which is more

electronegative and that means those

shared electrons between the chlorine

and the carbon will spend more time

around the chlorine atom making it more

negative

that means we have a negative pole and a

positive pole and a polar molecule

up until now we've only talked about

electrons that are between atoms they're

bonded electron pairs

we also have pairs of electrons that are

called unbonded electron pairs or lone

pairs they are not in between atoms but

they do have their own orbitals and that

means they influence the shape the

polarity and the symmetry of a molecule

nh3 is an excellent example

first we'll draw the lewis structure for

nh3

next we can calculate the differences in

electronegativity between bonds and we

see that the nh bond is indeed polar

but let's go back and look at the shape

of the molecule to see if it's

symmetrical

we have our nitrogen atom in the middle

and let's add three hydrogen atoms

as we add them they spread out to be as

far away from each other as possible

and we have this structure

when you look at it it looks like it

should be nonpolar each of the hydrogens

is pulling in an opposite direction and

they should cancel out

but we need to go back to our lewis

structure because we have a lone pair of

electrons we have to consider

when we add the lone pair

it influences the shape it actually

pushes down the hydrogens and now we

have a pyramidal also called pyramidal

structure

so the structure is no longer

symmetrical and that means we're going

to have a positive and a negative side

and we're going to have a polar molecule

it's important to stress that polarity

results from an unequal sharing of

electrons the ones that are bonded

shared between atoms but it also results

from the shape or the symmetry of the

molecule and this can be influenced by

unbonded electrons like we saw with nh3

let's take a look at one more

pause and determine if h2o is a polar or

nonpolar molecule

we'll first look at the lewis structure

for h2o

and then we'll calculate the difference

between bonds we can see that that h o

bond that's a polar bond

next let's look at the shape of the h2o

molecule to see if we have symmetry

so we'll start with our oxygen atom and

we'll put two hydrogen atoms on that

they spread out to be as far away as

possible from each other and it looks

like it would be symmetrical like these

two hydrogens would cancel out

however we have our lone pairs two of

them we'll put one

two

and now we can see that the molecule is

no longer symmetrical we have a distinct

top and bottom

because we have this distinct top and

bottom to the molecule water is a polar

molecule

polarity is a hugely important topic in

science everything from medicines to

building materials how the molecules

interact is largely a function of their

polarity to figure that out we drew the

lewis structures

then we looked at the individual bonds

the electronegativity difference between

those bonds finally we looked at the

shape and the symmetry including those

lone pair electrons to figure out if the

molecule was polar or nonpolar

this is dr b with polar and nonpolar

molecules and thanks for watching

you