If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Carbon and hydrocarbons

AP.BIO:
ENE‑1 (EU)
,
ENE‑1.A (LO)
,
ENE‑1.A.2 (EK)
The element carbon and why it's essential to life as we know it. Properties and bonding patterns of carbon atoms.

Introduction

Carbon isn’t a difficult element to spot in your daily life. For instance, if you’ve used a pencil, you’ve seen carbon in its graphite form. Similarly, the charcoal briquettes on your barbeque are made out of carbon, and even the diamonds in a ring or necklace are a form of carbon (in this case, one that has been exposed to high temperature and pressure). What you may not realize, though, is that about 18% of your body (by weight) is also made of carbon. In fact, carbon atoms make up the backbone of many important molecules in your body, including proteins, DNA, RNA, sugars, and fats.
These complex biological molecules are often called macromolecules; they’re also classified as organic molecules, which simply means that they contain carbon atoms. (Notably, there are a few exceptions to this rule. For example, carbon dioxide and carbon monoxide contain carbon, but generally aren't considered to be organic.)

The bonding properties of carbon

Why is carbon so popular for making molecular backbones? Why don’t we instead use, say, oxygen for the same purpose? For one thing, carbon-carbon bonds are unusually strong, so carbon can form a stable, sturdy backbone for a large molecule. Perhaps more important, however, is carbon’s capacity for covalent bonding. Because a C atom can form covalent bonds to as many as four other atoms, it’s well suited to form the basic skeleton, or “backbone,” of a macromolecule.
As an analogy, imagine that you’re playing with a Tinker Toy® set and have connector wheels with either two or four holes. If you choose the connector wheel with four holes, you’ll be able to make more connections and build a complex structure more easily than if you choose the wheel with two holes. A carbon atom can bond with four other atoms and is like the four-hole wheel, while an oxygen atom, which can bond only to two, is like the two-hole wheel.
Carbon’s ability to form bonds with four other atoms goes back to its number and configuration of electrons. Carbon has an atomic number of six (meaning six protons, and six electrons as well in a neutral atom), so the first two electrons fill the inner shell and the remaining four are left in the second shell, which is the valence (outermost) shell. To achieve stability, carbon must find four more electrons to fill its outer shell, giving a total of eight and satisfying the octet rule. Carbon atoms may thus form bonds to as many as four other atoms. For example, in methane (CHstart subscript, 4, end subscript), carbon forms covalent bonds with four hydrogen atoms. Each bond corresponds to a pair of shared electrons (one from carbon and one from hydrogen), giving carbon the eight electrons it needs for a full outer shell.

Hydrocarbons

Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen. We often use hydrocarbons in our daily lives: for instance, the propane in a gas grill and the butane in a lighter are both hydrocarbons. They make good fuels because their covalent bonds store a large amount of energy, which is released when the molecules are burned (i.e., when they react with oxygen to form carbon dioxide and water).
Image of a methane molecule, showing its tetrahedral shape and the bond angle of 109.5 degrees for each H-C-H unit.
Image credit: OpenStax Biology.
Methane (CHstart subscript, 4, end subscript), the simplest hydrocarbon molecule, consists of a central carbon atom bonded to four hydrogen atoms. The carbon and the four hydrogen atoms form the vertices of a three-dimensional shape known as a tetrahedron, which has four triangular faces; because of this, methane is said to have a tetrahedral geometry. More generally, when a carbon atom is bonded to four other atoms, the molecule (or part of a molecule) will take on a tetrahedral shape similar to that of methane. This happens because the electron pairs that make up the bonds repel each other, and the shape that maximizes their distance from each other is a tetrahedron.
Most macromolecules are not classified as hydrocarbons, because they contain other atoms in addition to carbon and hydrogen, such as nitrogen, oxygen, and phosphorus. However, carbon chains with attached hydrogens are a key structural component of most macromolecules (even if they are interspersed with other atoms), so understanding the properties of hydrocarbons is important to understanding the behavior of macromolecules.

Want to join the conversation?

  • piceratops seed style avatar for user pimentel.nestor
    Why is oxygen electronegative?
    (10 votes)
    Default Khan Academy avatar avatar for user
    • leafers ultimate style avatar for user Anthonie
      Oxygen is electronegative because it only needs 2 electrons to complete it's valence shell. Instead of losing an electron (like sodium, in sodium chlorine), it simply attracts to those 2 electrons. The more a atom wants an electron, the more electronegative it is and visa versa.

      To imagine whats more electronegative, simply look at the periodic table. At the far bottom left to the top right is the scale for how electronegative an atom is. The closer to the bottom left, the less an atom is electronegative. Closer to the top right, the more electronegative the atom is.

      Hope this helps ;)~
      (2 votes)
  • piceratops seedling style avatar for user Atharva.nigoskar
    Is it possible to artificially create diamond using huge hydraulic presses with super hot base plates such that all conditions to create diamond are fulfilled ?
    (6 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user 16sylvia.wright
    If the bonds in methane are repelled to each other, then wouldn't the bonds be too far apart that it would break the bonds?
    (6 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user Darmon
      The force that repels the pairs of electrons in the bonds in not as strong as the force that attract the electron to the protons of the carbon and hydrogen atoms. Thus, the best the electron bonds can do to stay away from each other is to form a tetrahedron. :)
      (2 votes)
  • duskpin ultimate style avatar for user Nausicaa
    Why are Carbon-Carbon bonds "unusually strong"? Is it just because of its capacity to form 4 covalent bonds?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • male robot hal style avatar for user usbatlle
    is there a program in which a person can mix and match different elements together and see what compounds they come up with, what compounds they form. then when the compound is formed the program gives a backstory o how that compound is used throughout life
    (2 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user Kachinsky
    How do people actually look and measure the angles of bonds if we haven't actually seen an atom?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • purple pi teal style avatar for user Michaela O'Connor
      I think it's just maths, based on the knowledge we already have. If a molecule has 4 hydrogens and 1 carbon (methane, as in the example above), and we know that electrons repel each other, then there's only one set of angles that allow those electrons to all be as far apart from one another as possible. The lower the number of electrons, the greater the angle, presumably.
      (2 votes)
  • piceratops ultimate style avatar for user Noah Price
    in the bottom half paragraph one, it says: What you may not realize, though, is that about 18% of your body (by weight) is also made of carbon. In fact, carbon atoms make up the backbone of many important molecules in your body, including proteins, DNA, RNA, sugars, and fats. which makes me wonder: could the overall weight of a person effect that? could say, a very big person change the percentage a carbon?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user /
    Why are carbon-carbon bonds stronger than other types of bond?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • starky tree style avatar for user Angelo Ben Charles
      Carbon-Carbon bonds are stronger cuz Carbon has 6 electrons (2,4) and the outermost shell has 4 valence electrons. Things get quite interesting when you release another carbon next to it. Both of these carbons are now thirsty for 4 electrons so as to attain a Noble Gas configuration. Let me explain this to you via an analogy... Suppose you are a fidget spinner collecting maniac and so is your friend. You want to get 8 fidget spinners to create a fidget spinner tower. Your friend also has the same idea. What will you do?

      The most practical approach to this is to share the fidget spinners such that both of you are satisfied with 8 spinners. Sharing the spinners will help both of you to satisfy. Your friend takes 8 spinners and make a video about the tower and you take 8 spinners and post a pic on Instagram. Now, your colleagues know that you don't have 8 spinners and wants to tease you and your friend for not having 8 spinners and post that on social media (Pretty mean, huh?). Now, the only option for both of you is to stick together... forever.

      This is the same thing that happens to carbon atoms. The only thing is that, for its case it is a life or death situation.
      (2 votes)
  • duskpin ultimate style avatar for user Zoe LeVell
    Wait, so burning fuel creates water? Wouldn't that kill the flames, even in it's gaseous state?
    (2 votes)
    Default Khan Academy avatar avatar for user
    • orange juice squid orange style avatar for user Stephen Allen
      Burning hydrogen does form water in the gaseous form, but gaseous water won't kill the flames. Liquid water is good at putting out fires as it can absorb a lot of heat and it can cut off the oxygen supply. Since that is not the case here, the flames will continue.
      (2 votes)
  • aqualine seedling style avatar for user Ameera E
    why does carbon bond with hydrogen?
    (2 votes)
    Default Khan Academy avatar avatar for user