11 3.1 The Octet Rule and Lewis Dot Diagrams

Figures

Figure 3.1.1: Lewis Electron Dot Diagrams

A diagram showing the Lewis electron-dot diagrams for neutral atoms, arranged by groups on the periodic table, with the first three rows of the table are shown. The dots represent electrons and may appear on the four sides of each elemental symbol with the number of dots representing the number of valence electrons. Note that it does not matter where the first dot is placed, and you must fill up each side with a single dot before pairing dots together when drawing dot diagrams.

Group 1 elements, hydrogen, lithium and sodium, each have a single dot, indicating one valence electron. Group 2 elements, beryllium and magnesium have two dots, for two valence electrons. Group 13 elements, boron and aluminum, have three dots and group 14 elements, carbon and silicon have four dots.

Now the group 15 elements, nitrogen and phosphorus, have five dots and because each side is now full, there is a pair of dots on one side, and single dots on the other three sides. Continuing with this pattern: Group 16 elements, oxygen and sulphur, have six dots, with two pairs and two single dots. Group 17 elements, fluorine and chlorine, with seven dots, has pairs on the left, right, and above, and a single dot below. Lastly, Group 18 elements, neon and argon, have eight dots, with pairs of dots on all four sides, representing a full valence shell.

Practice questions

Multiple-choice questions

1. Choose the correct Lewis structures of the molecule SO₃.
   1. The Lewis structure of sulphur trioxide has one sulphur atom and three oxygen atoms. Each oxygen is connected by a double bond to the sulphur atom, which is at the centre. The sulphur atom does not have a lone pair, while three oxygen atoms have two lone pairs.
   2. The Lewis structure of sulphur trioxide has one sulphur atom and three surrounding oxygen atoms. Each oxygen is connected by a double bond to the sulphur atom. There are two lone pairs around the top oxygen and one pair around the other two oxygens, respectively.
   3. The Lewis structure of sulphur trioxide has one sulphur atom and three surrounding oxygens. Each oxygen is connected by a double bond to the sulphur atom. Two lone pairs are around the bottom, and two oxygens and three electrons are around the top oxygen.
2. Choose the correct Lewis structures of the molecule COCl₂.
   1. The Lewis structure has carbon as the central atom with single bonds to chlorine atoms and double bonds to the oxygen atom. There are two lone pairs of electrons on the oxygen atom and each chlorine atom.
   2. The Lewis structure has carbon as the central atom with single bonds to two chlorine atoms and one OH. There are three lone pairs of electrons on each chlorine atom. No electrons surround the OH.
   3. The Lewis structure has carbon as the central atom with single bonds to chlorine atoms and double bonds to the oxygen atom. There are two lone pairs of electrons on the oxygen atom and three lone pairs on each chlorine atom.
3. Choose the correct Lewis structures of the molecule HF.
   1. Hydrogen (H) and fluoride (F) are connected by a single bond. There are three lone pairs surrounding the fluoride.
   2. Hydrogen (H) and fluoride (F) are connected by a double bond. There are two lone pairs surrounding the fluoride.
   3. Hydrogen (H) and fluoride (F) are connected by a single bond. There are two lone pairs surrounding the fluoride.
4. Choose the correct Lewis structures of the molecule SF₆.
   1. The Lewis structure for sulphur hexafluoride involves a central sulphur atom bonded to six fluoride atoms with single bonds. There is one lone pair surrounding each fluoride atom.
   2. The Lewis structure for sulphur hexafluoride involves a central sulphur atom bonded to six fluoride atoms with single bonds. There are three lone pairs surrounding each fluoride atom.
   3. The Lewis structure for sulphur hexafluoride involves a central sulphur atom bonded to six fluoride atoms with single bonds. There are two lone pairs surrounding each fluoride atom.
5. Choose the correct Lewis structures of the molecule CS₂.
   1. The Lewis structure for carbon disulphide (CS₂) involves a central carbon double bonded to two sulphur atoms on each side. Each sulphur atom contains two lone pairs.
   2. The Lewis structure for carbon disulphide (CS₂) involves a central carbon double bonded to two sulphur atoms on each side. Each sulphur atom contains three lone pairs.
   3. The Lewis structure for carbon disulphide (CS₂) involves a central carbon double bonded to two sulphur atoms on each side. Each sulphur atom contains one lone pair.
6. Choose the correct Lewis structures of the molecule SeF₄.
   1. The Lewis structure of selenium tetrafluoride involves a selenium (Se) atom single-bonded to four fluoride (F) atoms. There are no extra electrons surrounding the fluoride or selenium.
   2. The Lewis structure of SeF₄ (selenium tetrafluoride) involves a selenium (Se) atom single-bonded to four fluoride (F) atoms. Each fluoride contains three lone pairs, while selenium contains one lone pair.
   3. The Lewis structure of selenium tetrafluoride involves a selenium (Se) atom single-bonded to four fluoride (F) atoms. Each fluoride contains two lone pairs, while selenium contains no lone pairs.
7. Choose the correct Lewis structures of the molecule AlCl₃.
   1. The Lewis structure for aluminium trichloride involves a central aluminium atom single-bonded to three chlorine atoms, with two lone pairs around each chloride. There are no extra electrons surrounding aluminium atoms.
   2. The Lewis structure for aluminium trichloride involves a central aluminium atom single-bonded to three chlorine atoms, with one lone pair around each chloride. There are no extra electrons surrounding the aluminium atom.
   3. The Lewis structure for aluminium trichloride involves a central aluminium atom single-bonded to three chlorine atoms, with three lone pairs around each chloride. There are no extra electrons surrounding aluminium atoms.
8. Choose the correct Lewis structures of the molecule Cl₂O.
   1. The Lewis structure for dichlorine monoxide involves a central oxygen atom with one lone pair and is double bonded to left chloride with one lone pair. Another chloride with three lone pairs is single-bonded to central oxygen.
   2. The Lewis structure for dichlorine monoxide involves a central oxygen atom single-bonded to two chloride atoms, with the oxygen atom having two lone pairs and each chloride having two lone pairs.
   3. The Lewis structure for dichlorine monoxide involves a central oxygen atom single-bonded to two chloride atoms, with the oxygen atom having two lone pairs and each chloride having three lone pairs.
9. Choose the correct Lewis structures of the molecule N₂O.
   1. The Lewis structure for nitrous oxide involves two triple-bonded nitrogen atoms connected to one oxygen atom by a single bond. There are three lone pairs around oxygen. There is one lone pair around the left nitrogen.
   2. The Lewis structure for nitrous oxide involves two triple-bonded nitrogen atoms connected to one oxygen atom by a double bond. There is one lone pair surrounding oxygen and the left nitrogen, respectively. The Lewis structure for nitrous oxide involves two triple-bonded nitrogen atoms connected to one oxygen atom by a double bond. There are two lone pairs around the oxygen.
10. Choose the correct Lewis structures of the molecule SiBr₄.
    1. The Lewis structure of selenium tetrabromide involves a silicon (Si) atom double-bonded to four bromine (Br) atoms with three lone pairs around each bromine.
    2. The Lewis structure of selenium tetrabromide involves a silicon (Si) atom single-bonded to four bromine (Br) atoms with three lone pairs around each bromine.
    3. The Lewis structure of selenium tetrabromide involves a silicon (Si) atom single-bonded to four bromine (Br) atoms with two lone pairs around each bromine.

Solutions

Multiple-choice questions

1. (a)
2. (c)
3. (a)
4. (b)
5. (a)
6. (b)
7. (c)
8. (c)
9. (a)
10. (b)