MED109 Chemistry Lecture 02a – Chemical Bonding Introduction & Covalent Bonds Dr Olivier Missa Staff Room, Dept. of Medicine OMissa@dwu.ac.pg phone ext: 847 1 Lesson 2a – Learning Objectives • Draw the Lewis dot symbol for an element • Explain the formation of atomic bonds through the principle of the octet rule • Describe how different types of covalent bonds are formed • Illustrate the formation of molecular orbitals • Draw the Lewis dot structures of a molecule 2 Chemical Bonding The tendency of atoms to react with other atoms leads to the formation of chemical bonds A chemical bond between atoms involves their valence electrons & can be explained through different interactions between the atoms: Ø covalent bond (sharing electrons) Ø ionic bond (transfer of electrons): electrostatic 3 Lewis dot symbols The only electrons involved in bonding are the valence electrons & can be represented by the Lewis dot symbols These are represented by the symbol of an element & one dot for each valence electron 4 Lewis dot symbols • The electron configuration shows all electrons; Lewis dot symbols only show valence electrons: example: N 1s2 2s22p3 core electrons 5 valence electrons •• •N• • 5 Lewis dot symbols Problem: Which of the following is an incorrect Lewis dot symbol? • Ca • • •C• • 2A •• • O• •• •• 1A •• Ne •• •• • H •• • O• • 8A 3A 4A 5A 6A 7A 6 Lewis dot structure for Oxygen https://www.youtube.com/watch?v=o4J9mLbvWJA 7 Chemical Bond A chemical bond is formed because the bound atoms reach a more stable state (lower energy compared to unbound atoms) Noble gases (He, Ne, Ar, …) have a very low tendency to form chemical bonds (maximum number of electrons in their outermost shell = filled valence shell) outermost electronic configuration He: 1s2 Ne: 2s22p6 Ar: 3s23p6 8 Octet rule Noble gases have 8 electrons (only 2 for He) in the outermost shell; this configuration gives them great stability (no reactivity, no chemical bond) When forming chemical bonds, elements try to reach (mimick) the configuration of noble gases 9 Octet rule Octet rule: when forming a chemical bond, an element loses, gains, or shares its electron(s), in order to have 8 electrons in its outermost shell (or only 2 electrons for elements near He) 10 Octet rule Examples H (1s1): shares 1 e- (He) Na (3s1): loses 1 e- (Ne) Ca (4s2): loses 2 e- (Ar) N (2s22p3): shares 3 e- (Ne) S (3s23p4): gains 2 e- (Ar) C (2s22p2): shares 4 e- (Ne) 11 Covalent bond • Occurs between two nonmetal atoms sharing their electrons • Covalent compounds are also known as molecules • In the valence bonding theory the driving force for covalent bond formation derives from the octet rule (both atoms reaching the stable configuration of noble gases) 12 Covalent bond • If the bound atoms are identical the same electronegativity *) the called non-polar # (or have covalent bond is • In case of different atoms (having different electronegativity) the covalent bond is called polar # Ø * Lecture 2b Ø # Lecture 2c 13 How is a covalent bond formed ? Example: binding between two H atoms (1s1: each have one unpaired electron) Ø at a proper distance between the atoms (inter-nuclear bonding distance), each bonding electron is attracted by the nucleus of the other atom ➡ H• ➡ •H 14 How is a covalent bond formed ? Ø When the two H atoms get close to each other, their atomic orbitals with unpaired electrons interact, leading to the formation of a covalent bond H• •H H➖H 15 Covalent bond strength & length Attraction of bonding electrons by both nuclei determines the bond strength Strong repulsive forces keep the positive charge of nuclei apart after they have reached a distance corresponding to the minimum energy This distance between atoms at the point of minimum energy corresponds to the bond length 16 Single, double or triple bonds A covalent bond between two atoms may be: Ø single (1 shared pair of electrons) Ø double (2 shared pairs of electrons) Ø triple (3 shared pairs of electrons) In all cases the two elements involved (single, double or triple bond) reach a stable configuration, according to the octet rule 17 Single, double or triple bonds Instead of representing a covalent bond as two dots, it is represented by a line connecting the two atoms The single line corresponds to a single bond with a shared pair of electrons ex: H➖H 18 Single, double or triple bonds Examples: H• + •H → H• •H H➖H (single bond) H• + •F → H• •F H➖F (single bond) O: + :O → O: :O O=O (double bond) :C: + 2 :O → O::C::O N⋮ + ⋮N → N⋮ ⋮N O=C=O (2 double bonds) N≡N (triple bond) 19 Double bond formation using the Lewis dot structure? Formation of CO2: Ø carbon: 4 valence electrons Ø oxygen: 6 valence electrons • •• • O• • C • •• • • • C• • •• • O• •• •• • O• •• 20 Double bond formation using the Lewis dot structure? 1. Formation of a single bond: only 7 electrons (one shared with C) •• • •• • O • • C• • O • •• • •• only 7 electrons (one shared with C) only 6 electrons (one shared with one O, one shared with the other O) Ø octet rule not respected 21 Double bond formation using the Lewis dot structure? 2. Create new bonds: double bonds C •• O• • •• •• •• •• • •O •• 8 electrons around each atom Lewis dot structure for CO2 •• • • • •O = C = O • • 22 Lewis dot structures for molecules In the Lewis structure of a molecule, all valence electrons are shown, including the ones involved in the bond (2) & the others also referred as lone pairs (non-bonding pairs) Cl2 •• •• Cl➖Cl •• •• example 1: •• • •• 23 Lewis dot structures for molecules example 3: N2 O=O •• •• N≡N •• O2 •• example 2: •• •• 24 How to draw Lewis structures https://www.youtube.com/watch?v=1ZlnzyHahvo 25 Formation of a Molecular Orbital When two atoms get close, a new space accommodating one electron from each atom is formed through a combination of their atomic orbitals A new orbital named molecular orbital is formed that surrounds the molecule & corresponds to the area where the shared electrons are more likely to be found 26 Formation of a Molecular Orbital In a molecular orbital, no more than two electrons are accommodated (each with opposite spins like in atomic orbitals); the bonding electrons are mainly located at a higher density between the two nuclei The bonding interaction is achieved because the energy of the molecular orbital is lower (more favorable) than the energy of the atomic orbital 27 Formation of a Molecular Orbital Molecular orbitals that are symmetrical about the axis of the bond (head to head overlap of atomic orbitals) are called sigma (σ) molecular orbitals atomic orbital 1s H• + atomic orbital 1s σ molecular •H H• •H orbital 28 Formation of a Molecular Orbital Molecular orbital theory actually predicts that the combination of two atomic orbitals leads to the formation of two molecular orbitals named bonding molecular orbital & anti-bonding molecular orbital Ø bonding molecular orbitals are at lower energy than the atomic orbitals they were formed from Ø antibonding molecular orbitals are at higher energy than the atomic orbitals from which they were formed 29 Formation of a Molecular Orbital As a results the electrons participating in the bond will only occupy the bonding molecular orbital (here σ) & the antibonding molecular orbital (here σ*) will remain empty 30 Other Types of σ Molecular Orbitals σ molecular orbitals can be formed also between s & p atomic orbital as well as between p atomic orbitals atomic orbital 1s atomic orbital 2p atomic orbital 2p atomic orbital 2p σ molecular orbital σ molecular orbital 31 Detailed Formation of a Double or Triple Bond Example: double bond between 2 oxygen atoms •• •• •• •• O=O 32 Detailed Formation of a Double or Triple Bond The first molecular orbital σ forms through the linear combination of 2p atomic orbitals along the inter-nuclear axis (head to head overlap of atomic orbitals) atomic orbital 2p atomic orbital 2p molecular orbital σ 33 Detailed Formation of a Double or Triple Bond The second molecular orbital forms through the lateral combination (sideways overlap) of the remaining 2p orbitals This molecular orbital is named π (pi) atomic orbital 2p atomic orbital 2p molecular orbital π 34 Detailed Formation of a Double or Triple Bond The maximum electron density in the π orbital is above & below the inter-nuclear axis Attention: there is only one orbital, although the bonding electrons are accommodated in two apparently unconnected regions The π orbital occupies the empty space, not used by the σ orbital 35 Detailed Formation of a Double or Triple Bond Note stability of the π orbital (energy required to break π) < stability of the σ orbital < (energy required to break σ) As a result, in case of bond breakage, the first bond broken is the one corresponding to the π orbital, because of its lower stability 36 Detailed Formation of a Double or Triple Bond Example: triple bond between 2 nitrogen atoms The first molecular orbital formed is the σ orbital (head to head overlap of atomic orbitals) •• •• N≡N molecular orbital σ 37 Detailed Formation of a Double or Triple Bond Then two π orbitals form through the lateral combination of the remaining two 2p atomic orbitals (sideways overlap) v v one π above & below the inter-nuclear axis one π in front & behind the internuclear axis 38 Detailed Formation of a Double or Triple Bond •• •• N≡N 39 Hybrid Orbitals explained (video) https://www.youtube.com/watch?v=vHXViZTxLXo (12 min) 40 Bibliographic & Online Resources Nuclear Chemistry (2015) in Bettelheim. F. A., et al. Introduction to general, organic and biochemistry (Chapter 9 - 11th ed.). US: Cengage Learning. Glencoe Virtual Lab: How can you tell which elements form chemical bonds? http://www.glencoe.com/sites/common_assets/science/virtual_labs/E20/E 20.html General Chemistry Online – Glossary: Atoms, elements and ions htmlhttp://antoine.frostburg.edu/chem/senese/101/atoms/glossary.shtml • How to draw Lewis Structures : five easy steps: https://www.youtube.com/watch?v=1ZlnzyHahvo • UC Davis Chemwiki: Bonding and anti-bonding orbitals http://chemwiki.ucdavis.edu/Core/Theoretical_Chemistry/Chemical_Bonding /Pictorial_Molecular_Orbital_Theory/Bonding_and_antibonding_orbitals • PBS Learning Media (Covalent bonding) http://www.pbslearningmedia.org/asset/lsps07_int_covalentbond/ 41