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Chapter 4.2a Covalent Bonding

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welcome to the first video for

chapter 4 section 2 covalent bonding uh

in this video i'll be talking about some

covalent molecules including a very cool one

one

buckminster fullerene or buckyballs

which is just 60 carbon atoms covalently

bonded together

into a sphere that you may recognize

from soccer balls

and this this molecule buckminster

fullerene or buckyballs

is uh named after buckminster fuller who

is an architect who

designed the uh geodesic spheres that

are used um for

a whole bunch of things and uh gave gave

this molecule the name

all right so the learning objective for

this video

is to describe the formation of covalent bonds

bonds

so first off we'll define a covalent bond

bond

and a covalent bond is a bond that forms

due to the attraction of atoms that

results in the sharing of a pair of

electrons between

two atoms this is different than

an ionic bond that we talked about previously

previously

where that's an electrostatic attraction

between two charged particles

in a covalent bond there are no charged

particles um the

the electrons are being shared they are

not uh they have not been transferred from

from

one atom to a different one

before we talk about how covalent bonds

form i want to talk about some

characteristics of covalent compounds

the first thing to note is that they

have much lower melting points in

boiling points than ionic compounds

they are often liquids or gases at room

temperature where ionic compounds are

usually solid

a sort of typical covalent compound that

you can think of if you are curious or

or need to kind of think of something

concrete is water

water is a comp equivalent compound

um the fact that these have lower

melting points and boiling points than our

our

typical ionic compounds tends to

indicate that they have

weaker bonds than ionic compounds they

are weaker than

also many covalent compounds are

actually insoluble in water

that's not always the case water is

does not fit that but many uh covalent

compounds do fit this characteristic

where they're insoluble in water

they are also poor conductors of

electricity in any state

so what this means is that there's no

ions or charged particles flowing around

there's no ion movement um

and in any state that means in solid

liquid gas or

dissolved in water there's still no ion

movement and that's that's different

from what we saw with ionic compounds

what this tells us that is that the

smallest chunk of a covalent compound

is not an ion it's in fact a molecule

and it's still neutrally charged

um generally we're going to think about

covalent compounds our

covalent bonds are formed between two

non-metal ions

which means so our covalent compounds

generally consist of two non-metal ions

but what that really means is that

generally covalent bonds form between

atoms or elements with similar similar

electron affinities or ionization energies

energies

and that just tends to happen in the

non-metal part of the periodic table

what this means is that there is not

one atom that is significantly more

likely to give up or

gain an electron than the other one so

they are much more likely to share the electrons

electrons

a little bit more equally all right so

let's talk about how

covalent bonds tend to form this is a

diagram that can be

super helpful to explain a whole lot of

stuff in chemistry it's a really really

helpful diagram

uh it basically depicts the the

relationship between the distance

between two nuclei

and here we're looking at hydrogen atoms

versus the energy

on the x-axis is what we call the

internuclear distance so that's

essentially just right that's the

distance between

these two nuclei and it's given here in picometers

picometers

and then on the y-axis we've got the

energy and that's given in

in joules um we did have to choose

scientists had to choose a a zero point

a reference

point and so what we had to do was pick

basically the spot where um we're gonna

say that these these nuclei are so far

apart they're essentially infinitely far apart

apart

that is the point where the attractive

force between them is zero and the

energy of the system is uh

zero um and the reason we did that is

because we had to pick a reference state

that's the same for everything

and where there is no interaction is the

only point that it's the same for everything

everything

uh so that just means that all of our

energies are going to be

the ones that we're interested here are

going to be negative so it's a little

bit unfortunate but it's just how that works

works

all right so what this diagram shows is how

how

as these atoms get closer and closer

together the

energy of the system changes so as we see

see

when the atoms approach each other and

they begin to

interact the energy decreases so what's

going on here is that

remember that electron density or the the

the

volume of space around the atom around

the protons

around the nucleus that we tend to think

that electrons are occupying is actually

a distribu

a distribution of probabilities so what

we think is they're probably in this

volume most of the time but sometimes

they're not

as these volumes get closer and closer

together there's a greater and greater

chance that in fact

the electron from this nucleus or this

sorry this atom is going to be spending

time around this atom and it'll be

interacting with that nucleus

that's an additional attractive force

that that electron can experience

and that lowers the electron so the

um the energy very slowly goes down

and then we get to a point where

essentially the s orbitals overlap

so when the s orbitals begin to um

when the s orbitals begin to overlap

what we see is that

now there's actually a much greater

probability that

either one of these electrons could be

spending time around the other

nucleus at any given moment and that's

going to start to really drop

our energy down the more overlap we have

the more likely it is that

that these electrons are experiencing

these additional attractive forces so

they're they're experiencing

attractive forces between their own

nucleus and now the additional one

in other words they're beginning to be

shared and those additional attractive

forces decrease

the energy of the system and so

eventually we get down here

to where we see this minimum what

happens at this minimum

is that if we tried to force those uh

atoms any closer together or they just

randomly moved closer together the

positive charges in the nucleus

would start to repel each other and the

energy would begin to

increase rapidly this increase

in energy so remember that that the

vast majority of space in an atom is

empty right the the nucleus is

incredibly tiny compared to the volume

that we think of

of the atom but this repulsive force

between the nuclei is essentially why

you can't stick your finger through

tables um it's this repulsion between

the nuclei

in your finger and the nuclei in your in

your table

so that's what's going on this

minimum here if we come back to this

minimum down at the bottom of our what

we call an energy well

is uh is the is represents both the

energy that's released

by the formation of this bond and it

will uh

it represents the distance between the

nuclei that is the um the most stable nuclei

nuclei

uh the the most stable internuclear distance

distance

um for this bond or in other words the

bond length um between two hydrogen

so it's important to keep track of the

changes in energy that accompany

physical changes or chemical changes as

we see here

that's one of the key things that we

look for in chemistry one of the key

things that we do

to sort of help us understand the

changes in the world around us

so this can be a little bit confusing

but essentially as we see with the

formation of this bond and this energy well

well

when we form a bond or when atoms form a bond

bond

to the system or sorry to the

surroundings from the system to the surrounding

surrounding

this is what we call an exothermic process

process

um and exo is uh i think it's i think

it's a latin it might be greek but it's

a latin prefix that just means

exterior right outside so exothermic

just means releasing energy

uh thermic to the outside if you

break a bond that

um from the surroundings

and that is what we call an endothermic process

process endothermic

so you can think about this with um as

we see

when the when the atoms move close

together and then they form a bond

um the energy decreases and and that

essentially tells us that the energy is

being released to the surroundings

if we wanted to then break that bond and

return these atoms to their unbonded

state we would have to put in enough energy

energy

to uh to get the atoms out of this

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