Week 7 & 8: Covalent Compounds Part 1: Lewis Structures Part 2: Skeleton Structures
Section 1: Lewis Structures
Section 1: Lewis Structures / Objectives After this lesson I can define covalent bonding. use the octet rule to determine how many covalent bonds an element will usually form. draw Lewis structures for simple molecules and compounds. draw Lewis structures for molecules with double and triple bonds. draw Lewis structures for, and define, isomers. discuss some of the unique properties of carbon and why it is basis for all living things.
Covalent Bonding Recall from a previous week that ionic bonding is when electrons are transferred between metals and non-metals. The opposite of ionic bonding is covalent bonding. In covalent bonding, electrons are shared between non-metal elements. The octet rule applies to covalent bonding just like it applies to ionic bonding. In ionic bonding the octet rule told us what ion an element will form. In covalent bonding the octet rule tells us how many covalent bonds an element will make.
Octet Rule and Covalent Bonding Recall that the octet says elements want to get to 8 or 0 valence electrons depending on on what column they are in. Since covalent bonding only happens between non- metals, we only need to look at the elements in columns 4,5,6, and 7 because those are the columns non-metal elements are in (and hydrogen). Remember the goal for elements in those columns (except hydrogen) is still to get to 8 valance electrons. The number of covalent bonds formed is equal to the number of electrons an element wants to gain. Column 5 = 5 valence electrons = wants to gain 3 = 3 covalent bonds. Column 6 = 6 valence electrons = wants to gain 2 = 2 covalent bonds. Column 7 = 7 valence electrons = wants to gain 1 = 1 covalent bond. Carbon forms 4 covalent bonds. Hydrogen forms 1 covalent bond. The covalent bonding creates covalent compounds or molecules. Covalent compounds are modeled in a couple of different ways. One way we will model them is by using Lewis structures.
Lewis Dot Diagram for the first 20 Elements Image Credit: i.pinimg.com
Lewis Structures Lewis structures (also known as Lewis dot diagrams, electron dot diagrams, and electron dot structures) are models that show the valence electrons on single atoms and the bonding that happens between atoms. The simplest way to draw Lewis structures is to just connect the single dots. When your structure is done, it will look like this: Carbon/Silicon: 4 bonds; no lone pairs (may form single, double or triple bonds) Nitrogen/Phosphorus: 3 bonds; 1 lone pair (may form single, double or triple bonds) Oxygen/Sulfur/Selenium: 2 Bonds; 2 lone pairs(may form single or double bonds) F, Cl, Br, I (Halogens): 1 Bond; 3 lone pairs (single bonds only) H: 1 bond; no lone pairs (single bonds only)
General Guidelines for Lewis Structures 1) Draw the atom that makes the largest number of bonds first because it will be your central atom. 2) Move on to the element that makes the 2 nd largest number of bonds next 3) Save Hydrogen and elements in column 7 for last. 4) Remember that the single dots make the bonds and that lone pairs do not make bonds. 5) Think of Lewis structures as a puzzle and do not get frustrated! The atoms can only fit together in a certain way. Lewis structures is a skill that takes time to master.
Lewis Structure Examples for H2, Cl2, H2O, CH4, NH3
Lewis Structures for molecules with double and triple bonds Many elements are capable of forming more than 1 bond to the same atom. Including an atom of the same element Elements in column 6 like oxygen and sulfur, can form double covalent bonds. O 2 which accounts for 21% of the air we breathe has a double bond. Elements in column 5 like nitrogen and Phosphorus can form double & triple covalent bonds. N 2, which accounts for 78% of air has a triple bond Carbon can also form double and triple bonds.
Lewis Structure Examples for H2, Cl2, H2O, CH4, NH3
Isomers Sometimes there may be one, two, or several thousand different possible Lewis structures for a single chemical formula. Each one of these Lewis structures models a unique chemical compound. The different structures for the same chemical formula are said be isomers of each other. Usually isomers have similar properties such as melting and boiling points, color, and reactivity. However, the properties are never completely identical. Occasionally, isomers can have very different properties.
Isomers for C4H10
Carbon s Unique Properties Carbon is very important to biology and a big part of why becomes apparent when drawing Lewis Structures. Carbon makes four covalent bonds, and more importantly it can form long chains with itself. (even chains thousands of carbons long) Carbon can also form cyclic compounds. Even though it seems like elements in columns 5 & 6 should be able to forms limited chains and cyclic compounds as well, in reality chains of nitrogen, phosphorus, sulfur and oxygen don t exist. Making four bonds, being able to form chains, & being able to form cyclic compounds allows carbon to create many diverse and unique compounds. There are approximately 150,000 known ionic compounds creating a diverse array of rocks an minerals. However, there are more than 18 million cataloged carbon compounds that are either synthetic or naturally occurring, with more being added every day.
VIDEOS! Show the Video(s) CCB #1 Video: Why Carbon is a Tramp CCC #30 Video: Hydrocarbon Power Note: For this video will watch the first half only
Part 1 Additional Resources Crash Course Biology #1 Video: Why Carbon is a Tramp Crash Course Chemistry #30 Video: Hydrocarbon Power
Section 2: Skeleton Structures
Section 2: Skeleton Structures / Objectives After this lesson I can Explain why skeleton structures are used. Identify the number & kinds of atoms present in skeleton structures or write the chemical formula for compounds when presented with a skeleton structure Convert a Lewis structure to a skeleton structure & vice versa
Skeleton Structures Because drawing Lewis structures for carbon compounds that contain more than just a few carbons can become tedious, a short hand version was created called skeleton structures In Skeleton structures, carbons are represented with small dots or even just where two bonds connect (when no other element is given.). Hydrogens that are connected to carbon are not drawn but are still there. Hydrogens connected to other elements are shown like in Lewis Structures. The other elements are drawn in the same way as Lewis structures. The carbons must be staggered when they are placed in a chain because in reality that s how they are positioned.
Skeleton, Lewis, and Ball and Stick Model for C4H10
Skeleton Structure for Ibuprofin C 13 H 18 O 2
Skeleton Structure for Tylenol C 8 H 9 NO 2
Skeleton for Caffiene C 8 H 10 N 4 O 2
Skeleton for Prozac C 17 H 18 NOF 3