Lewis Structures & Covalent Bonding PDF

Topic: Lewis structures, Covalent bonding and polarity Chemistry Mr. Miller Covalent Bonding Covalent bonding is a chemical bond between two or more non-metals in a fixed ratio. The covalent bond consist of two or more non metal atoms sharing their val...

Topic: Lewis structures, Covalent bonding and polarity Chemistry Mr. Miller Covalent Bonding Covalent bonding is a chemical bond between two or more non-metals in a fixed ratio. The covalent bond consist of two or more non metal atoms sharing their valence electrons with each other. The substance formed after a covalent bond is done is called a molecule because it consists of only non metals. Therefore the special name for covalent compounds are called molecules. Types of covalent bond Types of covalent bond continued Single Bond- This is a covalent bond that consists of the sharing of 1 pair of valence electrons therefore has 2 electrons Double Bond- This is a covalent bond that consists of the sharing of 2 pairs of valence electrons ons therefore has 4 electrons Triple Bond- This is a covalent bond that consists of the sharing of 3 pairs of valence electrons and therefore has ons therefore has 6 electrons Bond strength increases in the order: Single bond->double bond->triple bond Sigma bonds and pi bonds Sigma and pi bonds are types of covalent bonds that differ in the overlapping of atomic orbitals. Covalent bonds are formed by the overlapping of atomic orbitals.. Parameters that govern sigma bond and pi bonds Various bond parameters such as bond length, bond angle, and bond enthalpy depend on the way the overlapping of atomic orbital takes place. This overlap occurs in two major ways, giving rise to two primary types of covalent bonds, i.e. sigma and pi bonds Sigma (σ) Bond Sigma bonds are a result of the head-to-head overlapping of atomic orbitals whereas pi bonds are formed by the lateral overlap of two atomic orbitals along the internuclear axis. Sigma bonds are the strongest covalent bonds, owing to the direct overlapping of the participating orbitals. The electrons participating in a σ bond are commonly referred to as σ electrons. Types of sigma bonds s-s Overlapping In this kind of overlapping, one ‘s’ orbital from each participating atom undergoes head-on overlapping along the internuclear axis. An s orbital must be half- filled before it overlaps with another. continued The overlapping of two s orbitals resulting in a sigma bond is illustrated above. This type of overlap occurs in H2 molecules, where each hydrogen atom has a half- filled s orbital. s-p Overlapping Here, one half filed s orbital overlaps with one half-filled p orbitals along the internuclear axis, forming a covalent bond. This condition is illustrated below. This type of overlapping can be observed in ammonia An NH3 molecule features 3 sigma bonds, formed by the overlap of the 2px, 2py, and 2pz orbitals belonging to the nitrogen atom and the 1s orbitals of the three hydrogen atoms. p-p overlapping In this condition, one half-filled p orbital from each participating atom undergoes head-on overlapping along the internuclear axis. This type of overlapping is illustrated below. P to P overlap A Cl2 molecule features a p-p overlap of the 3pz orbitals of two chlorine atoms. It is important to note that the head-to-head overlapping of two p orbitals gives a sigma bond whereas the lateral overlap of these orbitals leads to the formation of pi bonds. The Pi (π) Bond Pi bonds are formed by the lateral (sidewise / positive (same phase) overlap of atomic orbitals along a direction perpendicular to the internuclear axis. During the formation of π bonds, the axes of the atomic orbitals are parallel to each other whereas the overlapping is perpendicular to the internuclear axis. This type of covalent bonding is illustrated below. Weakness of pi bonds Pi Bonds are generally weaker than sigma bonds, owing to the significantly lower degree of overlapping. Generally, double bonds consist of one sigma and one pi bond, whereas a typical triple bond is made up of two π bonds and one σ bond. It is important to note that a combination of sigma and pi bonds is always stronger than a single sigma bond. Amount of sigma and Pi Bonds that exists exists in Double and Triple Bonds? A triple bond consists of two pi bonds and one sigma bond. A double bond contains one sigma and one pi bond. Single bonds are always sigma bonds. Free rotation Sigma bonds are defined as having their electron density along the bond axis, while pi bonds have their electron density above and below the bond axis. What this mean is that pi bonds cannot rotate the same way as sigma bonds since rotation would break the pi bond interaction. Question would there be rotation across a single, double and triple bond? Rotation, length, strength differences Differences between sigma bonds and pi bonds questions What are the Possible Combinations of Orbitals in Sigma Bonds? The three most common overlap conditions that result in sigma bonds are: 1.s-s overlap 2.s-p overlap 3.p-p overlap What is the Number of Sigma and Pi bonds in a Benzene Molecule?  The benzene ring consists of six carbon-carbon single bonds, all of which are sigma bonds. Additionally, there exist six carbon-hydrogen sigma bonds. Therefore, the total number of sigma bonds in a benzene molecule is 12. The aromatic features alternating double bonds between carbon atoms. Therefore, the total number of pi bonds in a benzene molecule is 3. Lone pairs You also need to be able to know what are lone pairs of electrons and how to identify how many lone pairs is present in a molecule Lone pairs are pair of valence shell electrons that did not take part in the covalent bond to form the molecule. However if you take that formed molecule and react it with another substance the lone pair becomes essential as it will participate in the chemical reaction. How much lone pairs and bonded pairs does water have? Lewis structure of atoms Bohr Model Lewis structures of atoms The lewis structures for atoms only show the valence shell with the valence electrons and the symbol for the element. Know how to draw them. Lewis structure of molecules Lewis structures also can be written with electrons only instead of lines. Electron pairs on the valence shell of the atom that did not take part in the covalent bond is called lone pairs Lewis structures for molecules: Converting dot cross structures to Lewis structures o r Lets try it for SF6-sulfur hexafluoride and Co2- Carbon dioxide Convert this Lewis structure for urea to a dot- cross diagram and Identify the types of covalent bonds, the number of pi and sigma bonds and the number of lone pairs and bonded pairs of electrons present Answer The benzene ring consists of six carbon-carbon single bonds, all of which are sigma bonds. Additionally, there exist six carbon- hydrogen sigma bonds. Therefore, the total number of sigma bonds in a benzene molecule is 12. The aromatic molecule features alternating double bonds between carbon atoms. Therefore, the total number of pi bonds in a benzene molecule is 3. Steps drawing lewis structures for covalent substances/ molecules: 1. Write the electronic configuration of your atoms 2. Determine the groups of your atoms and hence the total amount of valence electrons and in essence how much electrons are needed by each to get a full octet. 3.Identify the central atom: Please note that the largest atom( less electronegative atom ) should be placed in the center and all other atoms surround it. Be aware that the central element is usually the one with the lowest electronegativity. Exception: the water molecule actually has the more electronegative at center. Moreover If you are using Carbon or Nitrogen in a covalent bond, ensure that carbon or Nitrogen is the central atom even if they are the smallest If both Carbon and nitrogen is present Carbon will be the central atom. If you have more than one carbon they will be joined side by side in a chain and all other atoms hanging on them.. Steps continued 3. draw the structure of your atom( use dots to represent the electrons one the atoms. Ensure that you check the stability of all atoms. If you cant stabilize with a single bond then try with a double bond or triple bond. What is done for one atom is done for the next. NB> For cases where you were not given the formula to draw the lewis structure, but rather the name of the elements: if you cannot stabilize with only using two atoms by a single , double or triple bond then try adding another atom that is not the central atom. And try again with the single or double or triple bond. 4. Write the chemical formula and give the name of the formula for the substance formed.  Steps for drawing Lewis structures- shortcut method 1. Find the total number of valence electrons for all atoms in the molecule. So you can keep track of the amount of electrons you are using in lewis structure of molecule. 2. Write the electronic configuration of your atoms 3. Determine the groups of your atoms and hence the total amount of valence electrons. This is important if you are also determining if it is a metal or a non- metal or both non- metals, if they are both non metals you will do a covalent bond. 4. Identify the central atom: Please note that the largest atom( less electronegative atom ) should be placed in the center and all other atoms surround it. Be aware that the central element is usually the one with the lowest electronegativity. Exception: the water molecule actually has the more electronegative at center. Moreover If you are using Carbon or Nitrogen in a covalent bond, ensure that carbon or Nitrogen is the central atom even if they are the smallest If both Carbon and nitrogen is present Carbon will be the central atom. If you have more than one carbon they will be joined side by side in a chain and all other atoms hanging on them.. 5.Draw the Lewis structure of the molecule with single bonds first. 6. Determine how many electrons are needed to satisfy the octet rule of the outside atoms and stabilize them first by distributing the correct amount of valence electrons to them. 7.Check the stability of the central atom. If it was not stabilized by the single bonds then Stabilize the central atom with double bond(s) or triple bonds and lone pairs. To do make the double bond, use a lone pair of electrons from an electronegative non-central atom and make it into a bonding pair shared with the central element that lacks electrons. Ensure you use up all your electrons 8.Check to make sure you have the lowest formal charge for each atom( see next PowerPoint). Also ensure the octet rule is not violated. 9. Write the formula of the molecule and name it. How to write chemical formulas and formula name for covalent molecules Steps in writing formula for covalent molecules 1. Asses the structure and determine how many atoms of each element exists in the structure. 2. Write the Formula by writing the symbol of the central atom/ atoms first and the other atoms next. Use subscripts ( numbers to bottom right)to identify the number of each. Water is an exception ( Eg. H2O) Writing names for covalent molecules 3. Write the name of the formula by using the Prefixes that you have learnt. The prefix comes before the element name when writing the name of chemical formulas (See example 3). Please note there is no need to write mono if the first element is only one but you must use mono if the second element is only one( see example 4 and 5 ). 4. Change the last part of the name of the second element in the formula to “ide” 5. E.g CO2 is carbon dioxide 6. Eg. CO is Carbon monoxide 7. Eg. Se2Br2 is diselenium dibromide 8. More examples on slide 21 How to write the names for the molecular formulas of covalent molecules In naming a binary molecule/compound(binary compound is a substance composed of exactly two different elements, which are substances that cannot be simplified further by chemical means) molecular compound, the number of atoms of each element present in the molecule is noted by using prefixes. When naming, the Use of suffix prefix Prefix Number of atoms Example Mono 1 atom of that type of element in the molecule Carbon Monoxide (CO) Di 2 atom of that type of element in the molecule Carbon dioxide(CO2) Diphosphorous monochloride ( P2Cl) Tri 3 atom of that type of element in the molecule Carbon trioxide(CO3) Tetr or tetra 4 atom of that type of element in the molecule Carbon tetrachloride( CCl4) Pent 5 atom of that type of element in the molecule Diphosphorous pentoxide( P2O5) Hex 6 atom of that type of element in the molecule Sulfur hexafluoride(SF6) Use of suffix -ide Tells us that There are only 2 types of carbon dioxide (CO2) atoms present in the molecule Special names However many covalent substances have special names brought down by history E.g. H2O(WATER) and not di hydrogen monoxide and NH3 Ammonia All acids do not go by the naming system you just learnt but have special names eg. HCl- (hydrochloric acid), H2SO4 ( Sulphuric Acid) and many more MANY OTHER ORGANIC COMPOUNDS EG CH4- METHANE, C2H4 ETHENE, HAVE SPECIAL NAMES OR TOTALLY DIFFERENT NAMING SYSTEMS as well(IUPAC) Bond the following and show the name and formulas where necessary and convert to Lewis structure also. Cl2 O2 N2 H2O Carbon and oxygen ( example when only names of element given and not formula- look out for these) Sulphur and oxygen Carbon and hydrogen Carbon and fluorine C2F4 Sulphur and fluorine Carbon and chorine C2H4 CH4 C2H6  Questions Draw and name Lewis structures for the following NBr3 SiCl4 SF2 H2O2- CS2 PI5 violation of octet rule- incomplete and expanded octet BH3 NO- count valence electron total first as its odd numbered elctrons  Expanded octets In a number of compounds of there can exist elements that have more than eight valence electrons around the central atom. This is termed as the expanded octet. Obviously the octet rule does not apply in such cases. Some of the examples of such compounds are: PF5, SF6, H2SO4, and a number of coordination compounds. This can occur for some elements in period 3 -7 as they have vacant d orbitals that can contribute to bonding with the aid of hybridization. Period 3 vs period 2 and expanded octets Pcl5 ICl5 So42- Po43- Formal charge Formal Charge Definition: “The formal charge over an atom of a polyatomic molecule or ion is the difference between the valence electron of that atom in the elemental state and the number of electrons assigned to that atom in Lewis structure.” Formal charge equation F.C. = [Total no. of valence e– in the free state] – [total no. of e– assigned in Lewis structure] or F.C. = [Total no. of valence e– in the free state] – [total no. of non- bonding pair e– (lone pair)] – 1/2 [total no. of bonding e–] Example using sulphur trioxide Writing the formulas of covalent molecules what is a covalent compound? It is called a molecule and it only has two or more non metal atoms sharing electrons with each other. Important--carbon only forms covalent bonds. It can form 4 covalent bonds as it is tetravalent. What is the difference between an ionic compound and a covalent molecule- Note that hydrogen can be a cation if bonded to non- metal and a anion if bonded to metals.  The formula of a covalent compound gives the exact number of atoms of each element in one molecule of the compound i.e its molecular formula. Moreover it is not necessary to know valences as the name will give the number of atoms by using a prefix.  The prefix table on the board In naming a binary(binary compound is a substance composed of exactly two different elements, which are substances that cannot be simplified further by chemical means) molecular compound, the number of atoms of each element present in the molecule is noted by using prefixes. When NamingPrefixthem Number of atoms Example Mono 1 atom of that type of element in the Carbon Monoxide (CO) molecule Di 2 atom of that type of element in the Carbon dioxide(CO2) molecule Diphosphorous mono chloride ( P2Cl) Tri 3 atom of that type of element in the Carbon trioxide(CO3) molecule Tetr or tetra 4 atom of that type of element in the Carbon Use of suffix molecule tetrachloride( CCl4 ) -ide Tells us that There are carbon dioxide (CO2) only 2 types of atoms Pent 5 atom of that type of element in the Vanadium present in the molecule pentoxide( V2O5) molecule Hexa 6 atom of that type of element in the molecule 7 hepta- Octa 8 9 nona 10 Deca  The previous table will be used to explain the formation of the following compounds CO is the formula for carbon monoxide SO2 is the formula for sulphur dioxide SO3 is the formula for sulphur trioxide Steps in writing formula and names for covalent molecules 1. Asses the structure and determine how many atoms of each element exists in the structure. 2. Write the Formula by writing the symbol of the central atom/ atoms first and the other atoms next. Use subscripts to identify the number of each. Except water 3. Write the name of the formula by using the Prefixes that you have learnt. The prefix comes before the element name when writing the name of chemical formulas (See example 7). Please note there is no need to write mono if the first element is only one but you must use mono if the second element is only one( see example 5 and 6 ). 4. Change the last part of the name of the second element in the formula to “ide” 5. E.g CO2 is carbon dioxide 6. Eg. CO is Carbon monoxide 7. Eg. Se2Br2 is diselenium dibromide 8. More examples on slide 21 The prefix is placed BEFORE THE ELEMENT TO WHICH IT IS REFERRING afterwards you must end word in the prefix -ide. If you don’t see a subscript for the first element in the formula write the name of the first element as seen in periodic table. Special cases/ Special names--It is a good idea to learn the molecular formula of covalent compounds as you come across them as there are exceptions to the above rules as some names cannot be derived from their compounds/molecules eg. H2S hydrogen sulphide, Ammonia(NH3), methane for CH4, water H2O some of these were named long before it was discovered what atoms they consist of. Practice Name the following covalent molecules N2O3 – N2O4- SF6- XeF6- CCl4 – CF4- SN- Convert the following from word formulas to chemical formulas Write the formula from the names of the covalent substances below 1. Selenium trioxide – 2. Carbon disulphide- 3. Carbon tetrabromide- 4. silicon dioxide- 5.silicon trioxide- 6. nitrogen dioxide- Special Names and exceptions H O- water 2 All acids eg HCl (Hydroclhoric acid), H2SO4(Sulphuric acid). Ammonia NH3 Ammonium NH4+ And all hyrdocarbons eg CH4 is methane The diatomic molecules- All of these compounds can be found in nature as diatomic molecules. Meaning there is two atoms of the same kind combined together in a covalent bond like a set of twin holding hands. If any of these elements is being used or produced in a chemical reaction ensure the subscript two is beside them Eg. Structure of oxygen gas as found in nature Name ( in free state) Formula Hydrogen H2 Iodine I2 Bromine Br2 Chlorine Cl2 Oxygen O2 Nitrogen N2 Fluorine F2 Popular acids – Memorize them- You can consider their names as special cases of names given for covalent substances which they are Acid name Formula Hydrochloric acid HCl Hydrobromic Acid HBr Hydroiodic Acid HI hydrofluoric acid HF Hydrogen H2S Sulphide/hydrosulfuric acid CH3COOH Acetic acid/ Ethanoic acid H3PO4 phosphoric acid H2CO3 carbonic acid H2SO3 Sulfurous Acid H2SO4 Sulfuric acid HNO3 Nitric Acid HClO4 Perchloric Acid HClO3 Chloric Acid  Drawing Lewis structures for ions 1.Find the total number of valence electrons for all atoms in the molecule. 2.Draw the Lewis structure of the molecule with single bonds first. 3.Determine how many electrons are needed to satisfy the octet rule of the outside atoms and stabilize them first by distributing the correct amount of valence electrons to them. 4.Check the stability of the central atom. If it was not stabilized by the single bonds then Stabilize the central atom one double bonds or triple bonds. To do this, use a lone pair of electrons from an electronegative non-central atom and make it into a bonding pair shared with the central element that lacks electrons. Lets draw 5.Check to make sure you have the lowest formal charge for each the lewis structure for the nitrate anion and atom. Also ensure the octet rule is not violated. carbonate anion – 6.Put the molecule in a parenthesis with the charge to the top right hand NO - corner. 3 CO32- Coordinate( Dative bond) covalent bonding A coordinate or dative covalent bond is formed when both electrons in the shared pair come from the same atom. In ordinary covalent bonding each atom contributes one electron to the shared pair of electrons. This is described as simple/ ordinary covalent bonding. The sharing of electrons can occur in another way where all of the electrons in the bond is contributed by one of the combining atoms. The first atom uses a pair of non- bonded electrons( lone pair) to bond with the second atom. The second atom contributes nothing to the bond. The ammonium cation The hydronium ion  Polarity In chemistry substances can be considered polar or non-polar. Polar- Molecules and ionic substances( formula units) can be considered Polar as they have positive and negative charge). A polar molecule( Polar covalent substances) arises when one of the atoms exerts a stronger attractive force on the electrons in the bond. The electrons get drawn more towards that atom, so that the molecule exhibits a slight charge imbalance. It is worthy to mention that all ionic substances are polar because formula units have cations( +) and anions(-). Non- Polar- ( No opposite charge difference).A non polar molecule consists of atoms with the same electronegativity so that there is not much charge imbalance or very little difference in charge. Important: When dissolving substances it is worthy to mention the all time favorite idea that: Like dissolves Like This means that you need a polar substance to dissolve a polar substance and you need a non-polar substance to dissolve a non-polar substance. Electronegativity can be calculated from Pauling scale of electronegativity values Non-Polar covalent molecules Polar covalent molecules Other examples of polar covalent molecules include HF , HCl and NH3 (ammonia) Please know though that majority of covalent substances are non- polar