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Lewis Dot Structure practice test, Exercises of Chemistry

Lehigh University (LU)Chemistry
Prof. John Freeman

Practice on Lewis dot structure

Typology: Exercises

2022/2023

Uploaded on 12/08/2023

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Lewis Dot Diagrams & Structures
How are electrons shared to create covalently bonded molecules?
Read This First
Why Lewis structures? 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.
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?
2. How many valence electrons do the atoms B and Al each have (refer to Model 1)?
3. 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?
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Lewis Dot Diagrams & Structures

How are electrons shared to create covalently bonded molecules?

Read This First

Why Lewis structures? 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.

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?
  2. How many valence electrons do the atoms B and Al each have (refer to Model 1)?
  3. 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.
  2. Did the electron dot diagram for He violate the rule you made in #4, explain? Read This There is an ongoing dispute about the placement of electrons around elemental symbols. We will be using the unpaired electron approach which mimics Lewis’s original intent to show the potential valance of atoms. The existence of atomic and hybrid orbitals was twenty years into the future when Lewis first established his proposal and many modern texts use the atomic orbitals in the place order of placing the electrons designating a pair for the s orbital and then filling the p orbitals as one would in a box diagram.
  3. A. Write the long hand box 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?
  4. What electrons are considered valence electrons: s, p, d, f? Explain how you came to this conclusion.
  5. Why are only these electrons considered valence electrons?
  6. Hypothesize: Why are the valence electrons placed around an atom in the manner described? (Think about the orbital configuration) Application:
  7. Draw a Lewis dot diagram for the calcium atom.
  8. Draw the Lewis dot diagram for the silicon atom.
  9. Draw the Lewis dot diagram for the iodine atom.
  10. Draw the Lewis dot diagram for the xenon atom.
  11. Hypothesize: Why are noble gases considered to be non-reactive?
  1. How many total electrons are available to build the Lewis dot structure for H 2?
  2. How many valence electrons does each bromine atom have?
  3. How many total electrons are available to build the Lewis dot structure for Br 2? Important: 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. Do not place individual electrons! All unbonded electrons are called lone pairs. There are some exceptions to the placing of pairs that will be covered later. Extension Questions

Directions

  1. Draw a Lewis dot structure for the H 2 molecule.
  2. Draw the Lewis dot structure for the Br 2 molecule.
  3. Draw the Lewis dot structure for the HI molecule. Read This! 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 and avoid halogens! They typically only form one bond as they achieve an octet(duet). The central atom should be the atom that is the least electronegative, I.E. farthest from the fluorine atom on the periodic table, but not Hydrogen 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, H 2 O has 21 + 6 = 8 valence electrons, CCl 4 has 4 + 47 = 32 valence electrons.)
  2. Place the atoms relative to each other. Place the atom that the least electronegative as the central atom. Place the remaining atoms on the top, bottom, left and right sides. Remember: Hydrogen is never a central atom. Halogens tend to be terminal atoms as well.
  3. Place a pair of electrons between each atom to form a bond. Each bond uses two electrons.
  1. Determine the number of remaining electrons by subtracting the number of electrons in the bonds from the total valance electrons.
  2. 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 to achieve an octet. The number of electrons in the final structure must equal the number of valence electrons from Step 1. Read This Lewis dot structure for CH 4 Step 1: C has 4 valence electrons; each H has 1 valence electron. Total electrons = 8 Step 2: Place C as the central atom H H C H H Step 3: Place electrons to form bonds H H — C — H H Step 4: Check: There were 8 valence electrons in step 1 and 8 electrons in the Lewis dot structure. Application:
  3. Determine the number of valence electrons in CCl4. _____ Complete the Lewis dot Structure for CCl4.
  4. Determine the number of valence electrons in NH 3 _____ Complete the Lewis dot structure for NH 3
  5. Determine the number of valence electrons in H 2 O _____ Complete the Lewis dot structure for H 2 O

Model III: Complex Lewis Dot Structures / Polyatomic Ions 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 H 3 O+^ valence electrons____
  2. Complete the Lewis dot structure for OH-^ valence electrons____
  3. Complete the Lewis dot structure for NH 4 +^ valence electrons____
  4. Complete the Lewis dot structure for SO 4 2-^ valence electrons____ Your group will check your answers with the instructor before moving on. Model IV: Double and Triple Bonds Read This! 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 O 2 : 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 N 2 valence electrons____
  2. Complete the Lewis dot structure for CO 2 valence electrons____
  3. Complete the Lewis dot structure for CO valence electrons____
  4. Complete the Lewis dot structure for HCN valence electrons____ Your group will check your answers with the instructor before moving on.

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

Read This!

Step 1: Build a Lewis dot structure for ozone, O 3 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.

  1. Complete the Lewis dot structure(s) for ClO 3 -^ valence electrons____ Your group will check your answers with the instructor before moving on. Model VI: Multi-Centered Molecules Read This!

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.

C 2 H 6 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 molecule before 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 C 2 H 4 valence electrons____
  2. Complete the Lewis dot structure(s) for C 2 H 2 valence electrons____
  1. Complete the Lewis dot structure(s) for CH 3 OH (Hint: The three H atoms are connected to the C atom and the last H atom is connected to the O atom) valence electrons____
  2. Complete the Lewis dot structure(s) for NH 2 OH valence electrons____
  3. Complete the Lewis dot structure(s) for N 2 F 6 2+^ valence electrons____
  4. Complete the Lewis dot structure(s) for N 2 H 4 valence 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.