The pi bond dominates the chemistry of ethene. How do the electrons get from one half of the pi bond to the other if they are never found in between? It's an unanswerable question if you think of electrons as particles.Įven if your syllabus doesn't expect you to know how a pi bond is formed, it will expect you to know that it exists. Taking chemistry further: This is another example of the curious behaviour of electrons. It would be quite misleading to think of one living in the top and the other in the bottom. Those two electrons can live anywhere within that space. It is a region of space in which you can find the two electrons which make up the bond. In almost all cases where you will draw the structure of ethene, the sigma bonds will be shown as lines.īe clear about what a pi bond is. Note: The really interesting bond in ethene is the pi bond. An ordinary line represents a bond in the plane of the screen (or the paper if you've printed it), a broken line is a bond going back away from you, and a wedge shows a bond coming out towards you. The various sorts of line show the directions the bonds point in. A bond formed in this way is called a pi bond.įor clarity, the sigma bonds are shown using lines - each line representing one pair of shared electrons. In this one the electrons aren't held on the line between the two nuclei, but above and below the plane of the molecule. This sideways overlap also creates a molecular orbital, but of a different kind. Notice that the p orbitals are so close that they are overlapping sideways. In the diagram, the black dots represent the nuclei of the atoms. The p orbitals on each carbon aren't pointing towards each other, and so we'll leave those for a moment. These are sigma bonds - just like those formed by end-to-end overlap of atomic orbitals in, say, ethane. The various atomic orbitals which are pointing towards each other now merge to give molecular orbitals, each containing a bonding pair of electrons. The two carbon atoms and four hydrogen atoms would look like this before they joined together: The remaining p orbital is at right angles to them. The three sp 2 hybrid orbitals arrange themselves as far apart as possible - which is at 120° to each other in a plane. sp 2 orbitals look rather like sp 3 orbitals that you have already come across in the bonding in methane, except that they are shorter and fatter. The new orbitals formed are called sp 2 hybrids, because they are made by an s orbital and two p orbitals reorganising themselves. They use the 2s electron and two of the 2p electrons, but leave the other 2p electron unchanged. When the carbon atoms hybridise their outer orbitals before forming bonds, this time they only hybridise three of the orbitals rather than all four. In the case of ethene, there is a difference from, say, methane or ethane, because each carbon is only joining to three other atoms rather than four. It is important that you have first met the idea of hybridisation in the more simple methane case. Use the BACK button on your browser to come back here when you have finished. Note: If you haven't read about bonding in methane, follow this link before you go any further. The carbon atom is now said to be in an excited state. The extra energy released when these electrons are used for bonding more than compensates for the initial input. There is only a small energy gap between the 2s and 2p orbitals, and an electron is promoted from the 2s to the empty 2p to give 4 unpaired electrons. This is exactly the same as happens whenever carbon forms bonds - whatever else it ends up joined to. The carbon atom doesn't have enough unpaired electrons to form the required number of bonds, so it needs to promote one of the 2s 2 pair into the empty 2p z orbital. Each line in this diagram represents one pair of shared electrons.Įthene is actually much more interesting than this.Įthene is built from hydrogen atoms (1s 1) and carbon atoms (1s 22s 22p x 12p y 1). You may also find it useful to read the article on orbitals if you aren't sure about simple orbital theory.Īt a simple level, you will have drawn ethene showing two bonds between the carbon atoms. Important! You will find this much easier to understand if you first read the article about the bonding in methane.
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