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mitchell's we use them in countless ways every day
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they're important to all societies and have been for a very long time
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why are people so useful one reason is that they have great strength yet they
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can be pressed or molded into almost any shape
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most medals also conduct electricity
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how can we explain these properties
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and how do we explain the many differences between them laid for
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instance is a soft people
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a sleeping tenfold aluminium is much lighter
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but also more rigid unlike most mitchell's
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mercury is not a solid at normal temperatures but a liquid
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to begin to explain any of this we first need to ask
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water beetles made of when we look closely at a metal surface we can see
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groups and bumps
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yeah it looks like one continuous piece of material
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what is this material a highly magnified view
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shows that the piece of metal is actually made minute but clearly
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separate sections called crystal
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but then we might ask what are the crystals mind of
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this is an even more highly magnified view of tiny specks of gold
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taken with an electron microscope image reveals the crystal structure of the
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mitchell
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but it reveals more we can see that a crystal is actually night out the
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separate particles
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arranged in a regular pattern these particles are called atoms
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the basic building blocks all major the unimaginably small
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alive seven million gold atoms would stretch no more than a millimeter
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although no one has yet found a way of getting a close-up view of the National
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different experiments have made it possible to build up a picture or model
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what items consist of the colors in these models
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of course used simply to distinguish between different particles
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in the center the National is a nucleus
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this is made up even smaller or sub-atomic particles called protons
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and neutrons the protons each carry a positive electrical charge
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the neutrons I was hideous protons but carry no charge
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this means that overall the nucleus is positively charged
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orbiting around it and negatively charged particles called electrons
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the signs that the negative charge carried by electron
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equal to the positive charge carried by approach on
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now the electrons are moving at incredible speed
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in fact three quarters at the speed of light so what stops in shooting of in
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any direction
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if we could pull a net on the power it into its components
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we'd see that a neutral atom has the same number of electrons protons
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and it's the force of attraction between the positive and negative charges
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that prevents the electrons escaping each type that in
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is identified by the number of protons and electrons at Hans
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a hydrogen atom has just one product on
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and one electron an oxygen atom
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has and protons and eight electrons and on atom
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has 26 protons and 26 electrons
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but this is not enough to explain the behavior and properties of most
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substances
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if we could slow the electrons down
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which see that they move around the nucleus in predictable and different
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patterns
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and to understand how particles bond with each other
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we need to look closely at these patent remember there are conflicting
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electrical forces with the Nini anthem
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H negatively charged electrons is attracted to the protons in the nucleus
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that repealed from other electrons the result of this tussle
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is that electrons tend to occupy a seat at more or less stable regions around
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the nucleus
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and these regions are basically arranged in layers or shells
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they exist around the nucleus every action spices to be potentially field
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but like the rooms in a large hotel there really are all filled
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as we've seen the hydrogen atom has just one electron
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she'll say actually divided into smaller regions called sub chills
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and age can hold only a certain number of electrons
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the in about its chill number one is unusual
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in that it has only one sub she'll this is called the one
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is some chill and can hold up to two electrons
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the second chill has to sub chills called S&P
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a gang its sub she'll can hold only two electrons but the peace up shell can
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hold up to seeks
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the food shell is even bigger again
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its its sub she'll can hold two electrons the police up she'll seeks
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and there is also a d-sub shiel which can hold up to 10 electrons
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a vital concept at this model is that all the electrons within a sub she'll
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have the same energy
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but the electrons in different subfields have different energy levels
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as we move up through the sub chills the energy levels increase
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notice that the four a sub she'll is at a lower energy level than the three-day
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sub shiel
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so the forests up she'll feels boost
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the shells continued and summer the subscales have different shapes
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but we've seen enough to get the general idea the more electrons in an atom hence
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the more shells will be occupied
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the most important point for us here infect the key to understanding how old
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bonding in metals works
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is that a shell filled with electrons is more stable than a shell its only partly
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filled
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but of course in most atoms the outer electron shell
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with the highest energy electrons are usually located
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is not filled this makes the atom unstable
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if the outer shell of an atom is almost field yet and will attract electrons to
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feel it
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on the other hand Evans with only one to or three electrons in the outer shell
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will really shaped them to achieve the field outer shell
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an aluminium adam has 13 protons in its nuclear its
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around it a food chain electrons arranged like this
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the foods chill is filled by two electrons
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the second chill which can take up to eight electrons is also filled
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this leaves three electrons in the third chill
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these outer shell electrons a hill feeling loosely
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when aluminium atoms come in contact with each other
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the officials are the left and this allows the outer electrons to escape
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from the individual action
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they become delocalized moving at random to form a sea of electrons between the
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engines
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this have dramatic effects the actions become positive on Iran's
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sometimes called catch irons because they now have more protons the electrons
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that strongly attracted to the negatively charged
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see it outer shell electrons in Villepin them this attraction is the glue that
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holds the particles together
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because all the islands at the same
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they pack up into a regular leftists arrangement and because they have
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effectively lost the outer electrons and have filled out and chills
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then now much more stable than they were a separate actions
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this model basically applies to all the angels
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and explains why they're such good conductors of electricity
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normally the delocalized electrons move
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random through the letters this 98 movement one while the other
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but if the positive charge is applied to one point if the mitchell
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and a negative charge to another the delocalized electrons will tend to move
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towards the positive charge
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this is what an electric current Ian's a flow of electrons
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in our model is a solid metal the catch on sat down together in a regular three
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dimensional letters
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with day localized electrons migrating throughout the Levites
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although the islands have fixed positions within the lattice
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that continuously vibrating as the metal is heated
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all the particles gain energy and move more rapidly
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this is one metals are also good conduct is a feat
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eventually this movement overcomes the forces of attraction holding the letters
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together
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the middle changes to a liquid state
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the individual particles are moving with some freedom
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the fixed structures the left it's no longer exists
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but this still attraction between the Catalans and the delocalized electrons
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different metals have different melting points
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Saudi melts at 98 degrees Celsius
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aluminium notes at 660 celsius
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on doesn't bill till it reaches more than 1500 celsius
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how do we explain these differences
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the melting point to the middle depends on the strength the bond in the letters
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as you'd expect the stronger the charge
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the stronger the attraction between the cantons at the sea electrons
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lithium iron magnesium ions are the signs signs
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but the lithium-ion carries one charge magnesium
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to this is the main reason why the melting point of magnesium
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is so much higher than for lithium the other main factors that affect melting
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point is the size of the on
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sodium and potassium both carry the same charger one
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its idea mines a smaller this means that they paid tighter
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and potassium ions so the attraction between the ions and electrons see
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stronger this is why the melting point
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sodium is ninety eight celsius while for potassium
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it's only 63 so the melting point to the metal depends both on the signs that the
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island
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and the size of the charge it carries win
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the submittal metals has a new substance painful
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non all that's changed is the streets a bonding between the particles
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we can say that aluminium has changed States
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but it still aluminium one scientist is that when it cools
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it will return to the solid state just as it was before
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was melted in Malton form the particles carry a high level of Canadian energy
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as they lose energy to the surroundings the former heat loss
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they move more slowly and the attractive forces between the particles begin to
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control the position
12:12
the ions begin to form up into a regular pattern
12:16
while the outer shell electrons remind a localized
12:19
moving freely throughout the letters a metallic crystal has begun to form
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this process will begin at many points as a liquid metal cools
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so what happens when the growing crystals meet basically they become
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bonded together by the same electrical forces holding the particles within the
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crystal
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because the material shrinks in size as the islands for not going to let this is
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gets will be left between the crystals at some points
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over on some of the uneven boundaries actually
12:51
help to in to lock the crystals the bonding at the uneven surfaces a
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crystals
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will always be the weak link within each crystal
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the onto for regular lettuce inside the bonding is strong
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but because the crystals grow at random irregular shapes
13:08
when they meet each other the ions on the boundaries will never line up
13:12
as well as the islands within a crystal win a piece of metal cracks or breaks
13:19
it's usually the bonding between the crystals that gives way
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in some cases
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extreme stress will actually break the letter structure the crystal
13:30
you can see the fracture lines through the letters in this highly magnified
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view
13:39
for a long time it's being none that the right to who the molten metal
13:43
fix the white crystals form and hence the why the solid maple the hands
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let's see what happens when this red-hot Nadal is cool
13:51
rapidly the needle is brutal Daisley snaps
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in this case
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the crystal shapes form so fast many gets a lift between the crystals
14:05
one suspects is closed off there's no way for any remaining Malta non-stick 18
14:10
the gap States is empty space creating lines have weaknesses in the metal
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structure
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now what is the natal is cool slowly
14:19
it will be deemed it is very difficult to break
14:23
the reasons that a needle cool slowly has larger crystals
14:28
if you would joins and there's much more opportunity for Bolton particles to flow
14:32
and Phil gets
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the result is more closely fitting boundaries with stronger bonding
14:38
sewing needle slowly it's very difficult to play
14:41
his principal is used to make metal products with different properties
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decorative cast I N
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is made by pouring molten steel to a mall this allows intricate happens to be
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created
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the middle has limited strength forged tools
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where red-hot but solid me for season two shy
15:03
and allow to cool slime have much right straight
15:07
when Sam arise made this swords
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they used to process a repeatedly heating Beijing out and cool in the
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steel
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each cycle formed larger more strongly interconnected crystals
15:22
produced applied that was almost unbreakable how do we explain
15:30
other common features a mitchell's for instance that the malleable
15:34
in other words like BB into a priest stepped into sheet
15:38
most metals are also ductile meaning they can be drawn into a while
15:43
in both cases the middle change its shape
15:47
but remains a solid essentially
15:50
the crystals rearrange their shapes much as a bee in bed
15:53
reshapes itself when someone sits in it and steel is rolled out into sheets
15:58
you can see how the crystals change while the crystals a distorted to form
16:03
new shapes
16:04
the left us within the crystals the pattern in which the ONS group around
16:08
each other
16:08
remains the same just what form this patent takes depends on the particular
16:14
Mitchell sizeof the diane's at the strength they charge
16:18
in some metals like iron
16:21
the ONS will form up so that if we look at anyone on
16:25
we can say that it has eighteen-year snipers for bob and for the love this is
16:30
called
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tight coordination it's also called body centered cubic
16:39
because a giant sits in the center of a cube shape formed by its your snipers
16:44
in other metals with smaller islands or strong
16:51
charges the on some all strongly attracted and this allows them to pet
16:55
closer
16:56
so this time each one is directly down to twelve other islands
16:59
this is called 12 coordination examples include aluminium and copper
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the closer the pecking the stronger the bonding
17:08
as we've seen these patents have bonding determine the melting point
17:13
other properties at the Mitchell
17:23
what happens when different metals a heated together when chromium is added
17:28
to Malton I'm
17:29
the result is called stainless steel it has different properties to either of
17:34
the two original medals on the run
17:36
for instance stainless steel doesn't trust I'm like I am
17:40
how can we explain this change he's a model
17:44
the regular lesson since all I am once melted the letters breaks down
17:49
when a small amount of multiple crimean is added
17:52
the two types of particles makes at random as the mixture cools and the
17:57
particles lose energy
17:58
the left this is the major component in this case on
18:02
reforms but crimea mines and now distributed at random within the letters
18:07
this changes the properties of the material
18:11
any substance like stainless steel
18:15
might have a mixture two or more metals is called family
18:18
over the centuries many different alloys have been created to make a whole
18:28
variety of useful products
18:30
in fact most middle product to families rather than pure
18:33
rentals
18:40
when middle to combine changing the properties is often for Mattie
18:43
small amounts of titanium added to aluminium
18:47
make elmo is used in the construction a crowd of it still
18:51
life that much stronger than pure aluminium some of these alloys up to ten
18:56
times stronger than steel
18:58
months jewelry is made
19:02
ellwood's the mixtures at metals like gold
19:05
silver copper are often closely guarded secrets
19:09
soldier is a mixture of Tina blade
19:13
it has a low melting point which makes it extremely useful as a kind of blue
19:18
joins to give the wires and other components electrical circuits
19:22
and being a Mitchell conduct electricity will there are countless billions
19:27
evasions
19:28
in even the finest Nadal to understand the properties in the middle
19:34
we need to look at it structure on different levels first the size and
19:38
shape of the crystals in the material
19:40
and the way they connected at a more basic level
19:44
we also need to understand the way the particles are organized into our regular
19:48
let us within the crystal
19:50
both the crystals and the individual our minds
19:54
a hill together by the electrical attraction between the positive ions
19:57
and the envelop thing see delocalized electrons
20:01
Mabel's adopt this arrangement because the atoms have a week hold on the outer
20:05
shell electrons
20:07
so when we get right down to it the bonding patterns within the material
20:12
depend on the number of electrons in the atoms outer shell
20:15
this is true not just metals it applies to the basic particles in all substances