hello matt here from chemistrystudent.com in this video we're going to look at sigma and pi bonds we're going to talk about what sigma and pi bonding actually is how sigma and pi bonds arise and compare them both by looking at the carbon bonding in ethane and ephene covalent bonding has been covered in a separate video check the links in the description below before we talk in detail about sigma and pi bonding there are a few essential ideas you need to be comfortable with a covalent bond is formed when two atoms share a pair of electrons it is an example of an atomic bond when a covalent bond forms half-filled atomic orbitals from two different atoms overlap creating a bonding orbital that a pair of electrons can exist in electrons are negatively charged and the positively charged nuclei of both atoms are attracted to the electron density in the new in orbital this pulls both atoms together and creates a covalent bond this is drawn as a single line between the two atoms there are different shaped orbitals electrons can be in depending on their distance from the nucleus of an atom the orbital shape just refers to the area that an electron pair is likely to be in at any one time they are essentially constantly moving around within this space at this level s and p shaped orbitals are the ones most commonly studied other half-filled atomic orbitals can sometimes also overlap or merge creating another bonding orbital between the two atoms leading to a double bond the atoms are harder to split apart when double bonded together although the second bond is weaker than the first single bond recap done let's go as mentioned to form a covalent bond between two atoms atomic orbitals must overlap or merge to create a bonding orbital the two atomic orbitals face each other and if the orbitals can overlap easily the bond formed is called a sigma bond shown with the greek symbol for sigma the bonding orbital is close to the nuclei of both atoms and the nuclei both have a high level of attraction to the electrons in the orbital making the bond very strong single bonds are always examples of sigma bonds as atoms will always try and arrange themselves to maximize the overlap of two orbitals for example in an oxygen molecule each oxygen atom has a half-filled p-shaped orbital the p-shaped orbitals have lobes that stick out of each side of the nucleus and the load from each p orbital from each atom can point towards each other and the two can overlap and merge the bond formed is a sigma bond as the both orbitals overlap directly the bond is also free to rotate as rotating each nuclei has no impact on the bonding orbital before the oxygen atoms formed a sigma bond they each had two half-filled p orbitals the sigma bond was formed from the direct overlap of one of these from each atom this means that each oxygen atom still has a half-filled p orbital and wants to form another bond now things can get a bit interesting remember that there are three p shaped orbitals around the nucleus p x p y and p z and their lobes are each pointing in different directions or 90 degrees to each other if two half-filled p orbitals overlap fully to form a sigma bond the half-filled orbitals left over cannot directly line up or overlap they are simply pointing in the wrong direction as the atoms are now quite close together due to the single sigma bond the other half-filled orbitals can actually bend inwards slightly and overlap sideways to create a new bonding orbital between the two atoms the electrons in this orbital are further from the nuclei of both atoms compared to the sigma bond and the bond is therefore weaker this type of bond is called a pi bond shown with the greek symbol pi due to each orbital having two lobes the p orbitals overlap sideways creating two areas of electron density above and below the sigma bond already formed this means the bond is now unable to rotate freely if one atom rotates the pi bond in orbitals will try to rotate as well meaning they'll become twisted and if twisted too far will break sigma bonds are covalent bonds that form from the direct overlap of two orbitals from two atoms electrons in the bond are close to the nuclei of both atoms and the bond is strong pi bonds are covalent bonds that form from the sideways overlap of two p-shaped orbitals from two atoms electrons in the bond are further from the nuclei of both atoms and as a result pi bonds are weaker than sigma bonds at this level the carbon bonding in ethane and ethene is often used to model sigma and pi bonding in ethane the carbon atoms are bonded by the direct overlap of two orbitals a sigma bond each carbon atom has three bonds to hydrogen atoms and this leaves one half-filled p-shaped orbital left over the carbon atoms arrange themselves in such a way that enables the p shaped orbitals to overlap in ifene the carbon atoms are again bonded by the direct overlap of two orbitals a sigma bond as well as a sideways overlap of two other orbitals a pi bond this is referred to as a carbon-carbon double bond each carbon atom has two bonds to hydrogen atoms and this leaves two half-filled p-shaped orbitals left over to start with the carbon atoms will arrange themselves to have that direct overlap of two orbitals and the sigma bond will form this will leave two p shaped orbitals left over that can overlap sideways between the two atoms forming a pi bond there are now essentially two bonds between the carbon atoms a sigma bond and a pi bond this is called a double bond and is shown as two lines between the carbon atoms one line represents the sigma bond and the other the pi bond as the carbon atoms are held by two bonds a carbon-carbon double bond is stronger and harder to break than a single carbon bond sigma bonds are stronger than pi bonds however meaning that a carbon-carbon double bond isn't twice as strong as a carbon-carbon single bond it is easier to break a pi bond than a sigma bond this actually gives ethene a higher reactivity of an ethane and explains why alkenes are more reactive than alkanes if a pi bond is broken the sigma bond between the atoms still remains so to summarize covalent bonds are formed from the overlap of two atomic orbitals from two atoms sigma bonds form from the direct overlap of atomic orbitals from two atoms electrons in the bond are close to the nuclei of both atoms given a high level of attraction that is hard to break making sigma bonds strong sigma bonds are free to rotate pi bonds formed from the sideways overlap or bending of p shaped atomic orbitals from two atoms electrons in the bond are further from the nuclei of both atoms compared to a sigma bond given a weaker attraction and a bond that is easier to break as p shaped orbitals have two lobes a pi bond has two areas of electron density one above and one below a sigma bond already formed between the atoms there is restricted rotation and the pi bond is unable to freely rotate and twist i hope you found this video useful please check out other relevant videos in the links given in the description below and visit chemistryshooting.com for free notes and revision materials