Lewis Dot Diagrams & Structures

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Lewis Dot Diagrams & Structures

Lewis Dot Diagrams & Structures

How are electrons shared to create covalently bonded molecules?

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Why? Covalent bonds result from a sharing of electrons by two or more atoms (usually nonmetals). Lewis theory (Gilbert Newton Lewis, 1875-1946) focuses on the valence electrons, since the outermost electrons are the ones that are highest in energy and farthest from the nucleus, and are therefore the ones that are most exposed to other atoms when bonds form. Lewis dot diagrams for elements are a handy way of picturing valence electrons, and especially, what electrons are available to be shared in covalent bonds.

Model I: Valence Electrons / Lewis Dot Diagrams

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

Exploration I

  1. a)Write the long hand electron configuration for H, Li, Na and circle the valence electrons.

b)How many valence electrons do the atoms H, Li and Na each have?

c) Does this agree with Model 1 showing the Lewis Dot Diagrams?

  1. How many valence electrons do the atoms B and Al each have (refer to Model 1)?
  1. a)Write the long hand electron configuration for F and Cl and circle the valence electrons.

b)How many valence electrons do the atoms Fl and Cl each have?

c) Does this agree with Model 1 showing the Lewis Dot Diagrams?

  1. Create a rule about how many valence electrons an atom will have.
  1. Did the electron dot diagram for He violate the rule you made in #4, explain?

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Notice that the placement of the electron dots is very purposeful. The first two (if applicable) are always placed together on the right side of the element. Then, any remaining valence electrons are placed individually on each of the remaining three sides of the chemical symbol. After each side has one electron, then any remaining valence electrons are paired on the three sides.

NOTE: On the Internet, OGT and some textbooks, the Lewis Dot Diagrams are not drawn properly. For example, carbon is drawn: C

  1. A. Write the long hand electron configuration for Ge.

B. What do you notice about the energy level of the d orbitals vs the s and p orbitals for Ge?

C. Does this hold true for other atoms that have s, p, and d orbitals?

  1. What electrons are considered to be valence electrons: s, p, d, f? Explain how you came to this conclusion.
  1. Why are only these electrons considered valence electrons?
  1. Hypothesize: Why are the valence electrons placed around an atom in the manner described? (Think about the orbital configuration)

Application:Use the atom cards and Cheerios to build the atom before drawing it.

  1. Draw a Lewis dot diagram for the barium atom.
  1. Draw the Lewis dot diagram for the silicon atom.
  1. Draw the Lewis dot diagram for the iodine atom.
  1. Draw the Lewis dot diagram for the xenon atom.
  1. Hypothesize: Why are noble gases considered to be non-reactive?

Your group will check your answers with the instructor before moving on.

Model II: Lewis Dot Structures

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Covalent bonds generally form when a nonmetal combines with another nonmetal. Both elements in the bond are attracted to the unpaired valence electrons so strongly that neither can take the electron away from the other (unlike the case with ionic bonds), so the unpaired valence electrons are shared by the two atoms, forming a covalent bond. The shared electrons act like they belong to both atoms in the bond, and they bind the two atoms together into a molecule. The shared electrons are usually represented as a two dots ( ) or as a line (—) between the bonded atoms. These represent a single bond. (In Lewis structures, a line represents two electrons.)Atoms tend to form covalent bonds in such a way as to satisfy the octet rule, with every atom surrounded by eight electrons.(Hydrogen is an exception, since it is in period 1 of the periodic table, and only has the 1s orbital available in the ground state, which can only hold two electrons.)

WAIT!!! WHAT??? DID THIS JUST USE THE TERM OCTET? Yes, in this situation and every other time we use the term octet in this POGIL it is the proper use of the term. We need to evaluate octets to get the correct Lewis Dot Structures. It is ok to use the term when you are talking about bonding.

NOTE: A line (-) is used only for bonding electrons, never use it to represent a lone pair. In other words, you may not write a Lewis dot structure like this: He . Instead you must write it showing the individual electrons as shown in Model 1. However, you may see Lewis Dot structures drawn incorrectly on the web.

Example 1

Example 2

  1. Identify the number of valence electrons in each atom of example 1.
  1. How many total valence electrons are available to build the Lewis dot structure for HF?
  1. Identify the number of valence electrons in each atom of example 2.
  1. How many total valence electrons are available to build the Lewis dot structure for F2?
  1. How many valence electrons does each H atom have?
  1. How many total electrons are available to build the Lewis dot structure for H2?
  1. How many valence electrons does each bromine atom have?
  1. How many total electrons are available to build the Lewis dot structure for Br2?

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When building a Lewis dot structure for a molecule, it is important to first place one pair of electrons between the atoms to be bonded. Once the atoms have been bonded, then valence electrons are place in pairs around one atom until it has an octet. Then any remaining valence electrons are placed around another atom until it also has an octet. NEVER: place individual electrons! All unbonded electrons are called lone pair.

Extension Questions

Directions: Use the atom cards and Cheerios to build the molecule before drawing it.

  1. Draw a Lewis dot structure for the H2 molecule.
  1. Draw the Lewis dot structure for the Br2 molecule.
  1. Draw the Lewis dot structure for the HI molecule.

Your group will check your answers with the instructor before moving on.

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When building molecules with more than two atoms, the structure will begin with one central atom, placed in the middle, with up to four surrounding atoms, placed on the top, bottom, right and left sides of the central atom. Deciding which atom is the central atom? NEVER use hydrogen or a halogen! They typically only form one bond as they achieve an octet. The central atom should be the atom that is farthest from the fluorine atom on the periodic table, but not Hydrogen or a halogen.

Rules for building Lewis dot structures:

Directions: Read each rule and place a check after each statement, if you understand what it is saying. If you don’t get it, discuss the rule with your group.

  1. Count the total number of valence electrons in each atom and add them together to get the total number of electrons in the molecule. (For example, H2O has 21 + 6 = 8 valence electrons, CCl4 has 4 + 47 = 32 valence electrons.)
  1. Place the atoms relative to each other. Place the atom that is farthest from fluorine on the periodic table, as the central atom. Place the remaining atoms on the top, bottom, left and ride sides. Remember: Hydrogen and Halogens are never central atoms.
  1. Place a pair of electrons between each atom to form a bond. Each bond uses two valence electrons.
  1. Complete an octet on one surrounding atom before proceeding to the next surrounding atom. Once all surrounding atoms have octets, any remaining electrons should be placed on the central atom toachieve an octet. The number of electrons in the final structure must equal the number of valence electrons from Step 1.

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Lewis dot structure for CH4

Step 1: C has 4 valence electrons; each H has 1 valence electron. Total electrons = 8

Step 2: Place C as the central atomH

H C H

H

Step 3: Place electrons to form bondsH

H - C - H

H

Step 4: Check: There were 8 valence electrons in step 1 and 8 electrons in the Lewis dot structure.

Application: Use the atom cards and Cheerios to build the molecule before drawing it.

  1. Determine the number of valence electrons in CCl4. _____

Complete the Lewis dot Structure for CCl4.

  1. Determine the number of valence electrons inNH3 _____

Complete the Lewis dot structure for NH3

  1. Determine the number of valence electrons inH2O _____

Complete the Lewis dot structure for H2O

Model III: Complex Lewis Dot Structures / Polyatomic Ions

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Polyatomic ions are a group of atoms that are covalently bonded and act as a single unit with a charge. Building Lewis dot structures for polyatomic ions follows the same process as building a molecule. So what is the difference? The charge! A 2+ charge means that the Lewis dot structure will have 2 FEWER electrons in the structure than what is calculated in step 1. A 3- charge means that the Lewis dot structure will have 3 MORE electrons than what is calculate in step 1. Once the structure is completed, a bracket [ ]charge will be placed around the entire structure.

Directions: Insure you understand the above read this text box, if yes check here______. If not discuss with your team so that you understand it.

Directions: Use the atom cards and Cheerios to build the polyatomic ion before drawing it. Remember, you will need brackets and the charge as part of your drawings.

  1. Complete the Lewis dot structure for H3O+ valence electrons____
  1. Complete the Lewis dot structure for OH-valence electrons____
  1. Complete the Lewis dot structure for NH4+valence electrons____
  1. Complete the Lewis dot structure for SO42-valence electrons____

Your group will check your answers with the instructor before moving on.

Model IV: Double and Triple Bonds

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When following the four rules for building a Lewis dot structure, it may appear as though there are not enough valence electrons to complete the structure. If this happens, you will move a lone pair from the one of the surrounding atoms, and place it in between that atom and the central atom. This will create a double bond. If this doesn’t result in all atoms achieving an octet, move another lone pair from a surrounding atom and place it between that atom and the central atom. This sharing of three pair of electrons is called a triple bond. Atoms never share more than three pair of electrons! WARNING: Do not create double or triple bonds unless it is needed. Hydrogen and Halogens never have double or triple bonds.

Building O2: Total valence electrons = 12

Following steps 1-4: O - O

Move one lone pair: O - O to produce O = O Total electrons = 12

Directions: Use the atom cards and Cheerios to build the molecule. Before drawing it, check your cheerio model to ensure that H and He only have 2 electrons (-single bond) attached, and all other atoms are only surrounded by 8 electrons either as lone pairs or in bonds.

  1. Complete the Lewis dot structure for N2valence electrons____
  1. Complete the Lewis dot structure for CO2valence electrons____
  1. Complete the Lewis dot structure for COvalence electrons____
  1. Complete the Lewis dot structure for HCNvalence electrons____

Your group will check your answers with the instructor before moving on.

Model V: Resonance Structures— When One Lewis Structure Isn’t Enough

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Step 1: Build a Lewis dot structure for ozone, O3 Total valence electrons (36) = 18

Steps 2 and 3: Place one O in the center, and connect the other two O’s to it. Drawing a single bond from the central atom to each of the surrounding atoms.

O - O - O

Step 4: Place electrons around surrounding atoms first to complete octets. Place the remaining two electrons on the central atom.

We can satisfy the octet rule on the central O by making a double bond either between the left O and the central one or the right O and the center one:

O = O - O orO - O = O

The question is, which one is the “correct” Lewis structure? In reality, neither one is correct, due to the fact that the electrons are in constant motion. The bond lengths of each bond are equal to each other; approximately a 1 ½ bond. This is where the typical model fails, so we draw all possible structures with a double sided arrow between them. These are called resonance structures.

Application:Use the atom cards and Cheerios to build the molecule before drawing it.Remember, when drawing a polyatomic ion, you must include the bracket and ion charge. Also, review the warning listed in Model IV before proceeding.

  1. Complete the Lewis dot structure(s) for NO3-valence electrons____
  1. Complete the Lewis dot structure(s) for CO32-valence electrons____
  1. Complete the Lewis dot structure(s) for COCl2 valence electrons____
  1. Complete the Lewis dot structure(s) for NO2-valence electrons____
  1. Complete the Lewis dot structure(s) for ClO3-valence electrons____

Your group will check your answers with the instructor before moving on.

Model VI: Multi-Centered Molecules

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Molecules with more than one central atom are drawn similarly to the ones above. The octet rule can be used as a guideline in many cases to decide in which order to connect atoms.

C2H6 Total valence electrons = 14

Each carbon atom will represent a central atom and they will be bonded to each other. The remaining hydrogen atoms will be placed symmetrically around each central atom. Be sure your final structure has the correct number of electrons.

H H

H - C - C - H Total electrons = 14

H H

Application: Use the atom cards and Cheerios to build the moleculebefore drawing it. Before drawing it, check your cheerio model to ensure that H and He only have 2 electrons (-single bond) attached, and all other atoms are only surrounded by 8 electrons either as lone pairs or in bonds.

  1. Complete the Lewis dot structure(s) for C2H4 valence electrons____
  1. Complete the Lewis dot structure(s) for C2H2valence electrons____
  1. Complete the Lewis dot structure(s) for CH3OH (Hint: The three H atoms are connected to the C atom and the last H atom is connected to the O atom) valence electrons____
  1. Complete the Lewis dot structure(s) for NH2OHvalence electrons____
  1. Complete the Lewis dot structure(s) for N2F62+valence electrons____
  1. Complete the Lewis dot structure(s) for N2H4valence electrons____

Congratulations, you have completed this POGIL. Please check your for Model VI application before moving. By now you should be an expert at drawing Lewis Dot Structures. Check you expertise by completing Lewis Dot Worksheet 1 with only the use of a periodic table, atom cards, and cheerios. You may not refer to this POGIL or the Internet to complete worksheet 1.

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