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Chapter 4.1a Ionic Bonding

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welcome to the first video for chapter

four section one on ionic bonding

uh the learning objective for this video

is to explain the formation of cations

anions and

ionic compounds before we jump into

the definitions of what exactly an ionic

bond is and

what it consists of essentially an ionic

compound is a compound that consists of

some ions that are held together through

an ionic bond

and the properties of ionic compounds

can actually tell us quite a lot about

the characteristics of that ionic bond so

properties of ionic compounds when we

think of ionic compounds a good example

of an ionic compound is sodium chloride

which is table salt

so if you're thinking about just a

generic ionic compounds you can think

about table salt to kind of get a sense

of what of what these things

are um so they tend to be very

crystalline they tend to exist in these

very well-defined uh

structures they're very rigid which

means they don't deform easily

uh they tend to be incredibly brittle so

if you try to deform them they will

shatter rather than deform

and they also have a very high melting

points and boiling points

and together these properties tell us

that an ionic bond is

very strong it uh

these ionic bonds hold the ions in place um

very strongly which gives us the high

melting point and high boiling point

it's very difficult takes a lot of energy

energy

to separate the particles also uh it

means that our ionic compounds are very rigid

rigid

they don't deform because of these

strong bonds and if you try to deform

them they

actually shatter due to these um strong

bonds and essentially it's easier to

shatter than to

deform them and they are also quite

crystalline um

because of the uh the way that they

combine into these

very well-defined crystal structures

with these very strong forces holding

those ions in place

they also tend to be very poor

conductors of electricity when they're

in their solid form

and again this is because there's no ion

movement uh

in the uh no ion movement there we go

when they're in their solid form if you dissolve

dissolve

or melt an ionic compound then they

become good conductors of electricity

uh and again that's because when you've

dissolved an ionic compound like sodium

chloride in water

you've allowed those ions to move around

freely in the water

and they can therefore conduct

electricity the movement of charged

particles is is what electricity really

is and same thing if you melt them

you've essentially disrupted this ionic bond

bond

but again that takes a lot of energy uh

if you have ever

seen molten sodium chloride you were in

very specific conditions

most people have never seen molten

sodium chloride it takes

i think it's around a thousand degrees

celsius to melt sodium chloride

and once you do that then they become

good conductors because those ions can

move around freely

all right the next thing we're going to

do is jump into some definitions

so um the definition of an ionic bond

is uh the electrostatic forces of

attraction between

oppositely charged ions so if you have a

positively charged ion

and you have a negatively charged ion

the force that holds them together

is is what we call the ionic bond and

it's an electrostatic force between this

positive charge and this negative charge

all right so um the next thing is the

definition of what these

charges are what we call them so we're

always going to call a positively charged

charged

ion a cation so this is a positively

charged ion

and an anion is a negatively charged ion

and the way that i remember this is that

um cation the t in cation looks like a

plus sign

so it's like there's a little plus held

inside the cation word so

uh also it's really important for us to

remember that ionic compounds

and um the ions that that make them up

have very different properties than

their neutral atoms

so this is an example so this is solid

sodium this is chlorine gas

which is a diatomic and this is table salt

sodium is a very soft metal it's uh

it it lives in family one in the people

table um

this is a chunk of metal that someone

has sort of squished down into a vial

you can cut it with a butter knife it's

it's really incredibly soft

the thing is that it it violently reacts

with water it will actually catch on

fire if you expose it to water

um so it's it's it's very

it's very reactive um

this liquid in this vial is actually

mineral oil so sodium is usually stored

under mineral oil to prevent

any water from getting in contact with

it uh because

it it just immediately catches on fire i

actually one of my professors

in um undergraduate my undergraduate

degree tells a story about uh

when he was working in a lab and someone

cut some sodium

on a piece of paper towel and then they

went and did their experiment but they

threw the paper towel away in a trash

can and that trash can caught on fire

and uh had caused the entire building to

be evacuated and that was just from the

traces of sodium left on a paper towel

so this is incredibly violent chlorine

gas is poisonous

it's really nasty stuff um you can see

this sort of yellowish it's it's really

nasty stuff

and then sodium chloride this is table

salt right this is yummy

right well it's i mean not straight but

you put on your food to make it taste

good right so compare that with this

poisonous chlorine gas

and the stuff that burns if it comes in

contact with

the water vapor in the atmosphere um

it's uh

yeah it's it's they the ions and this

and the ionic compounds have very

different properties

than the neutral elements

all right so now we're going to talk

about why ionic compounds form or

exactly how they form

and we're going to focus on binary

compounds which are just compounds that

consist of two elements

they tend to consist of a metal and a

non-metal um the

cation the positively charged guy is

generally the metal

and that is because it tends metals tend

to have

low ionization energies i'm going to

abbreviate that as low ie

ionization energies and that leads

to um the metals losing electrons

and becoming positively charged right

uh and they tend to react with nonmetals

and the nonmetals tend to become anions

because they have a high

electron affinity and i'm going to

abbreviate that as ea

and what that means is that they tend to

gain electrons

which means they become negatively

charged and hey

cation anion and these guys tend to react

react

to form an ionic compound

so um so that's what these guys tend to

be the next thing that we need to think

about is the ratio

in which they react and the key here is

that all ionic compounds are going to be

electrically neutral

think about sticking your finger in a

bowl of table salt you don't get shocked

it's electrically neutral um so that

actually gives us some

really helpful information either if we

have the ratio or we have the charges we

can figure out the other

so here i've given you the charges um

this is going to form aluminum oxide

and aluminum tends to form a three plus

cation oxygen

is uh forms a two plus or sorry two

minus anion oxide

and when these guys react we have to

react them in a ratio that is

electrically neutral

for example if we tried a one-to-one

ratio um

let's see if this works we have a plus

three and a minus two

this guy would have a plus one charge on

it but we know that doesn't work out

so we're going to try a different ratio

and you can fiddle around with this and

just keep trialing and erroring

but sometimes when you have a an even odd

odd

mismatch it's actually a lot easier to

just multiply them by each other

so the low the least common multiple of

three and two is going to be six

and hey that turns out that's going to

be the the charge so

uh we're going to wind up needing two aluminums

aluminums

and three oxygens to make aluminum oxide

and let me just show you the math for

that so we have two

aluminums that are three plus

so that's a plus six and then we have three

three

oxygens that are a two minus

so that's going to be minus six so when

we add these up together

that gets us to neutrally charged life

is good

so this is the ratio and you can kind of

see how if we

only had these ratios we could work

backwards to figure out the uh the

charges of our ions

all right so the last thing i'm going to

talk about in this video before i move

on to

part two is that the formula is

not a physical arrangement it's really

tempting to if we look at aluminum oxide

or if we take a simpler case and we look

at sodium chloride

it's really tempting to say oh well okay

so sodium chloride

it's a one-to-one so there must be an

atom of sodium and an atom of chlorine

or rather an ion and an ion right a

sodium ion and then a chloride ion

hanging out and then there's another

pair of them over here

this is not the case um in fact what's happening

happening

is that as we said these guys are

crystalline they arrange themselves in

these very strong

tightly held crystal structures and

exact how exactly how the ions pack

there's a few different ways that they

tend to pack

and it depends on some factors but

essentially what happens is each ion

is exerting what we call an isotropic

attractive force so isotropic just means

it goes out in all directions

um three-dimensional right so it's in

all directions in three-dimensional space

space

and it's attracting uh this is a

negatively charged guy this is our anion

the chloride

is attracting the cation here the sodium

to it

uh in all directions and this turns out

to be the most stable form

um this uh this sort of cubic lattice is our

our

is our the most stable form for this

particular substance

this goes on for a really long ways

right this this is not just

nine atoms by nine atoms if you're

looking or nine ions by

nine ions if you're looking at a grain

of table salt

um you're probably looking at hundreds

of thousands or millions of atoms

on one side of your grain of salt um

remember how

infinitesimally tiny atoms are and ions

are um

uh anions tend to be slightly larger and

cations tend to be slightly smaller

um but they're you know comparable sizes

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Chapter 4.1a Ionic Bonding