Lewis Structure of Molecules Lecture

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 carbonhydrogen 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 8. is diselenium dibromide 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 Mono Number of atoms 1 atom of that type of element in the molecule Example 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 atoms present in the molecule carbon dioxide (CO2)