< Return to Video

Covalent network solids | Intermolecular forces and properties | AP Chemistry | Khan Academy

  • 0:00 - 0:01
    - [Instructor] So we've already talked
  • 0:01 - 0:03
    about multiple types of solids.
  • 0:03 - 0:05
    We've talked about ionic solids.
  • 0:05 - 0:07
    That's formed when you have ions
  • 0:07 - 0:10
    that are attracted to each other,
  • 0:10 - 0:12
    and they form these lattice structures.
  • 0:12 - 0:16
    We have seen metallic solids,
  • 0:17 - 0:21
    and we've thought about
    them as these positive ions
  • 0:21 - 0:24
    in this sea of negatively
    charged electrons.
  • 0:24 - 0:28
    And we've also seen molecular solids,
  • 0:28 - 0:33
    which is formed from individual
    molecules being attracted
  • 0:33 - 0:37
    to each other through
    intermolecular forces.
  • 0:37 - 0:40
    Now, what's different about
    covalent network solids is
  • 0:40 - 0:45
    that they're entire networks
    formed by covalent bonds.
  • 0:45 - 0:47
    What we see here, for
    example, is a network
  • 0:47 - 0:50
    of silicons and carbons,
  • 0:50 - 0:53
    and this is silicon
    carbide right over here.
  • 0:53 - 0:55
    And now, some of you might thinking,
  • 0:55 - 0:57
    haven't we already seen
    covalent bonds involved
  • 0:57 - 1:00
    in a solid before, for
    example, in molecular solids?
  • 1:00 - 1:03
    And this right over here is an example
  • 1:03 - 1:06
    of a molecular solid that
    we studied in that video.
  • 1:06 - 1:07
    You have the molecules,
  • 1:07 - 1:10
    which are made up of atoms
    bonded with covalent bonds.
  • 1:10 - 1:13
    But the reason why they form a solid is
  • 1:13 - 1:15
    because the molecules are
    attracted to each other
  • 1:15 - 1:18
    through intermolecular forces.
  • 1:18 - 1:22
    And if you wanted to melt
    this molecular solid,
  • 1:22 - 1:23
    you have to essentially overcome
  • 1:23 - 1:25
    these intermolecular forces.
  • 1:25 - 1:28
    Well, in a covalent
    network solid, the solid,
  • 1:28 - 1:31
    to a large degree, is made
    up of these covalent bonds.
  • 1:31 - 1:34
    And if you wanted to melt this somehow,
  • 1:34 - 1:37
    you would have to overcome
    these covalent bonds,
  • 1:37 - 1:39
    which, generally speaking, are stronger
  • 1:39 - 1:41
    than these intermolecular forces.
  • 1:41 - 1:44
    And so you can imagine,
    covalent network solids
  • 1:44 - 1:47
    are going to have higher melting points.
  • 1:47 - 1:49
    You also don't see a
    sea of electrons here.
  • 1:49 - 1:52
    So unlike metallic solids,
  • 1:52 - 1:55
    they're not going to be good
    conductors of electricity.
  • 1:55 - 1:58
    But just to understand this
    point a little bit more clearly,
  • 1:58 - 2:01
    let's look at some more
    covalent network solids.
  • 2:02 - 2:04
    So what you see here on the left,
  • 2:04 - 2:06
    you might recognize as a diamond.
  • 2:06 - 2:08
    A diamond is just a bunch
  • 2:08 - 2:11
    of carbons covalently
    bonded to each other,
  • 2:11 - 2:14
    and this is the structure of
    how these carbons are bonded.
  • 2:14 - 2:15
    And as you might already know,
  • 2:15 - 2:18
    diamonds are the hardest
    solid that we know of.
  • 2:18 - 2:21
    These covalent bonds, the
    way that they are structured,
  • 2:21 - 2:25
    can take a lot of stress, a
    lot of pushing and pulling.
  • 2:25 - 2:26
    It's very hard to break it.
  • 2:26 - 2:30
    Now, what's interesting is
    that same carbon can form
  • 2:30 - 2:33
    different types of
    covalent network solids.
  • 2:33 - 2:35
    For example,
  • 2:35 - 2:37
    this right over here is graphite,
  • 2:37 - 2:39
    and graphite is probably something
  • 2:39 - 2:40
    you're quite familiar with.
  • 2:40 - 2:42
    When you write with a pencil,
  • 2:42 - 2:44
    you're essentially scraping graphite
  • 2:44 - 2:45
    onto that piece of paper.
  • 2:45 - 2:47
    And so this is what graphite looks like.
  • 2:47 - 2:49
    It's these covalent network sheets,
  • 2:49 - 2:53
    and each of these sheets
    actually are attracted each other
  • 2:53 - 2:55
    through intermolecular forces.
  • 2:55 - 2:57
    And that's why it's easy to scrape it,
  • 2:57 - 2:59
    because these sheets can
    slide past each other.
  • 2:59 - 3:02
    But if you really wanted to melt graphite,
  • 3:02 - 3:04
    you would have to break
    these covalent bonds.
  • 3:04 - 3:08
    And so you can imagine, to
    overcome the covalent bonds
  • 3:08 - 3:10
    and melt, say, diamond or graphite,
  • 3:10 - 3:13
    it takes a very, very high temperature.
  • 3:13 - 3:16
    Graphite, for example, sublimes
  • 3:16 - 3:21
    at 3,642 degrees Celsius.
  • 3:21 - 3:23
    The silicon carbide that we looked at
  • 3:23 - 3:24
    at the beginning of this video,
  • 3:24 - 3:28
    it decomposes at 2,830 degrees Celsius.
  • 3:29 - 3:32
    This right over here
    is a picture of quartz,
  • 3:32 - 3:36
    which is a very common
    form of silicon dioxide,
  • 3:37 - 3:39
    another covalent network solid,
  • 3:39 - 3:41
    and this has a melting point
  • 3:41 - 3:46
    of 1,722 degrees Celsius.
  • 3:47 - 3:49
    So the big takeaway over
    the last several videos is
  • 3:49 - 3:52
    there's many different
    ways of forming a solid.
  • 3:52 - 3:55
    It could be with ions,
    it could be with metals,
  • 3:55 - 3:57
    it could be with molecules
    that are attracted
  • 3:57 - 3:59
    to each other with intermolecular forces,
  • 3:59 - 4:01
    or you could have a network
  • 4:01 - 4:04
    of atoms formed with covalent bonds.
Title:
Covalent network solids | Intermolecular forces and properties | AP Chemistry | Khan Academy
Description:

more » « less
Video Language:
English
Team:
Khan Academy
Duration:
04:04

English subtitles

Revisions