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Lesson 4 Ionic and Molecular Compounds

Octet Rule

Why do atoms bond?



B4.2 Modified Bohr diagrams of the first three noble gases
To understand why atoms bond, we will first look at a group of elements that rarely form compounds: the noble gases. Group 18 elements were once known as inert gases, because it was thought that they could not form chemical bonds. Chemists explain the stability (resistance to chemical reaction) of these atoms as resulting from their atomic structures. Specifically, they contain a full valence energy level. With the exception of helium (which only has two valence electrons in the first energy level), having eight valence electrons is considered to be a full and stable (unreactive) valence energy level. Other elements react with each other in order to obtain full outer energy levels, thus making them more stable.

This is called the octet rule.

Elements can form an octet by gaining electrons, losing electrons, or sharing electrons. When an atom gains or loses electrons, it becomes an ion, or charged particle.

  Digging Deeper

© Wikimedia Commons
B4.6 Comparison of atom radius to ion radius

Did you know that cations have a smaller atomic radius than the atom they are formed from? And that anions have a larger atomic radius than the atom they are formed from? https://blausen.com/en/video/loss-and-gain-of-electrons/

Learn More

Non-metal atoms have almost-full valence energy levels and will gain electrons to attain an octet.

As an example:

  • Fluorine has seven valence electrons; so by gaining one electron, it will form an ion with a stable octet.
  • It will no longer be a neutral atom; instead it will be a charged particle called an ion, specifically, an anion.
  • Anions are given unique names:  The element name ending changes to ide (e.g., this example anion is called a fluoride ion).
  • As well, the symbol has the negative charge included (e.g., this example becomes F–).


B4.3 a modified Bohr diagram of fluorine gaining an electron to form a fluoride ion


The charge on an anion indicates the number of electrons gained. An anion with a charge of 2– (e.g., O2–) for example, means that that element must have had six valence electrons and gained two to become stable, or attain a full valence.

Metal atoms have almost-empty valence energy levels and will lose electrons to attain an octet.

As an example:

  • Sodium has one valence electron; so by losing one electron, it will form an ion with a stable octet.
  • It will no longer be a neutral atom; instead it will be a charged particle called an ion, specifically, a cation.
  • Cations retain the element name (e.g., this example cation is called a sodium ion).
  • The symbol has the positive charge included (e.g., this example becomes Na+).


             
B4.4 A modified Bohr diagram of sodium losing an electron to form a sodium ion


The charge on a cation indicates the number of electrons lost. A cation with a charge of 2+ (e.g., Ca2+) for example, means that that element must have had two valence electrons and lost them both to become stable, or attain a full valence. 

B4.5 Two fluorine atoms bonding together to form a molecule with a stable octet

Examples

Each example has a video to go with it. To play the video, click on the play icon next to the example.

  1. Using the octet rule and a modified Bohr diagram,
 


Recall:

The atomic number represents the number of protons in an element, and in a neutral atom, the number of protons equals the number of electrons
Recall:

To determine the number of neutrons, round the atomic molar mass to the closest whole number and subtract the number of protons.


Using the periodic table, determine the number of protons, neutrons, and electrons in the atom.

protons = 12
electrons = 12
neutrons = 12
B4.7 Magnesium on the periodic table
Draw the modified Bohr diagram.
B4.8 Modified Bohr diagram of magnesium
Determine if the atom will gain electrons (does it have five or more valence electrons?) or lose valence electrons (does it have three or less valence electrons?).

Mg will lose two valence electrons.
B4.9 Modified Bohr diagram showing Mg losing two electrons
Draw the modified Bohr diagram of the ion, showing that it lost two valence electrons.
B4.10 Modified Bohr diagram showing Mg as an ion
Name the ion. If it is a cation, it will retain the element name.

This ion is called a magnesium ion.
Write the ion symbol. If the ion lost electrons, it will have a positive charge.

protons (+) = 12
electrons (–) = 10
net charge = 2+

The symbol for this ion is Mg2+.
Example 2
  1. Using the octet rule and a modified Bohr diagram,
 


Using the periodic table, determine the number of protons, neutrons, and electrons in the atom.

protons = 7
electrons = 7
neutrons = 7
B4.11 Nitrogen from the periodic table
Draw the modified Bohr diagram.
B4.12 Modified Bohr diagram for nitrogen
Determine if the atom will gain electrons (does it have five or more valence electrons?) or lose valence electrons (does it have three or less valence electrons?)

N will gain three valence electrons.
B4.13 Modified Bohr diagram showing nitrogen gaining three electrons

Draw the modified Bohr diagram of the ion, showing that it gained three valence electrons.
B4.14 Modified Bohr diagram of a nitride ion
Name the ion. If it is an anion, it will have the ending change to “ide.”

This ion is called a nitride ion.
Write the ion symbol. If the ion gained electrons, it will have a negative charge.

protons (+) = 7
electrons (–) = 10
net charge = 3–

This symbol for this ion is N3–.


Example 3
  1. Using the octet rule and a modified Bohr diagram,


 

Using the periodic table, determine the number of protons, neutrons, and electrons in the atom.

protons = 17
electrons = 17
neutrons = 18
B4.15 Chlorine from the periodic table
Draw the modified Bohr diagram.
B4.16 Modified Bohr diagram of chlorine

Determine if the atom will gain electrons (does it have five or more valence electrons?) or lose valence electrons (does it have three or less valence electrons?)

Cl will gain one valence electron.
B4.17 Modified Bohr diagram of chlorine gaining one electron

Draw the modified Bohr diagram of the ion, showing that it gained one valence electron.
B4.18 Modified Bohr diagram of a chloride ion

Name the ion. If it is an anion, it will have the ending change to “ide.”

This ion is called a chloride ion.
Write the ion symbol. If the ion gained electrons, it will have a negative charge.

protons (+) = 17
electrons (–) = 18
net charge = 1–

The symbol for this ion is Cl–.

    Example 4
    1. Using the octet rule and a modified Bohr diagram,
     



    Using the periodic table, determine the number of protons, neutrons, and electrons in the atom.

    protons = 13
    electrons = 13
    neutrons = 14
    B4.19 Aluminium from the periodic table

    Draw the modified Bohr diagram.
    B4.20 Modified Bohr diagram of aluminium
    Determine if the atom will gain electrons (does it have five or more valence electrons?) or lose valence electrons (does it have three or less valence electrons?)

    Al will lose 3 valence electrons
    B4.21 Modified Bohr diagram of aluminium losing 3 electrons

    Draw the modified Bohr diagram of the ion, showing that it lost three valence electrons.
    B4. 22 Modified Bohr diagram of the aluminium ion

    Name the ion. If it is a cation, it will retain the element name.

    This ion is called an aluminium ion.
    Write the ion symbol. If the ion lost electrons, it will have a positive charge

    protons (+) = 13
    electrons (–) = 10
    net charge = 3+

    The symbol for this ion is Al3+.




      Watch This

    What Are Ions @ YouTube FuseSchool – Global Education 


    This video looks at why ions form.

    There are a few differences to keep in mind when watching the video. 1. In the modified Bohr diagrams they use, they place the electrons separate in the first energy level. 2. At times, they place two electrons in one orbital. 3. They also use an alternate group numbering system (e.g., group 6 is group 16).
     


        Read This

      Please read pages 33, 34, and 35 to 38 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the octet rule and how this relates to why ions form. Remember, if you have any questions or you do not understand something, ask your teacher!

        Practice Questions

      Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

      1. In your own words describe the octet rule.

        Noble gases are stable and unreactive because they have a full valence energy level, consisting of eight valence electrons. Other elements gain or lose electrons to attain a stable octet.
      2. Fill in the missing information in the following chart.

        Ion Name
        		 Number of
        Protons
        		 Modified Bohr
        Diagram
        		 Number of Electrons
        Lost or Gained
        		 Number of
        Electrons in Ion
        		 Ion Symbol

        		3
        		lose 1



        B4.23 Modified Bohr diagram


        calcium 20
        		lose 2

        sulfide







        		gain 3
        		 18





        		Be2+


        Ion Name
        		 Number of
        Protons
        		 Modified Bohr
        Diagram
        		 Number of Electrons
        Lost or Gained
        		 Number of
        Electrons in Ion
        		 Ion Symbol
        lithium 3
        B4.24 Modified Bohr diagram of lithium ion
        		 lose 1
        		 2 Li+
        oxide
        		 8
        B4.25 modified Bohr diagram of oxygen ion
        		 gain 2
        		 10 O2–
        calcium 20
        B4.26 Modified Bohr diagram of calcium ion
        		 lose 2
        		 18 Ca2+
        sulfide 16
        B4.27 Modified Bohr diagram of sulfur ion
        		 gain 2
        		 18 S2–
        phosphide 15
        B4. 28 Modified Bohr diagram of phosphorus ion
        		 gain 3
        		 18 p3–
        beryllium 4
        B4. 29 Modified Bohr diagram of beryllium ion
        		 lose 2
        		 2 Be2+
      3. What do you notice about the number of electrons for each of the ions in question 2? What elements have the same number of electrons in their atoms?

        Notice that the number of electrons in an ion is always 2, 10, or 18. They are the same as the number of electrons of the noble gases.

      Making Connections

        1. Recall that the periodic table got its name from periodicity, or patterns on it. Fill in the following chart using the answers from the practice questions to help you identify patterns on the periodic table.

          Element Group
          Number
          		 Number of
          Valence Electrons
          		 Ion Charge
          sodium
          potassium
          beryllium
          magnesium
          calcium
          aluminium
          nitrogen
          phosphorus
          oxygen
          sulfur
          flourine
          chlorine

          Element Group
          Number
          		 Number of
          Valence Electrons
          		 Ion Charge
          sodium 1 1 1+
          potassium 1 1 1+
          beryllium 2 2 2+
          magnesium 2 2 2+
          calcium 2 2 2+
          aluminium 13 3 3+
          nitrogen 15 5 3–
          phosphorus 15 5 3–
          oxygen 16 6 2–
          sulfur 16 6 2–
          flourine 17 7 1–
          chlorine 17 7 1–

          Notice that all ions in group 1 have a charge of 1+.
          Notice that all ions in group 2 have a charge of 2+.
          Notice that all metallic ions in group 13 have a charge of 3+.
          Notice that all non-metallic ions in group 15 have a charge of 3–.
          Notice that all non-metallic ions in group 16 have a charge of 2–.
          Notice that all ions in group 17 have a charge of 1–.

        Extending Your Knowledge

        The great thing about the periodic table is that the patterns continue! You can only model the first 20 elements using the modified Bohr diagram, but you can extend your knowledge and understanding of the relationship between group numbers, number of valence electrons, and the octet rule to accurately predict ion charges of elements past the first 20!

        1. Fill in the blanks for each of the following questions. You may need to recall the relationship between group number and the number of valence electrons. Organization of Periodic Table
          1. Arsenic is in group ___ and has ___ valence electrons. This means it needs to ____ ____ electrons in order to have a full octet and it will have an ion charge of ___.

            Arsenic is in group 15 and has five valence electrons. This means it needs to gain three electrons in order to have a full octet and it will have an ion charge of 3–.
          2. Barium is in group ___ and has ___ valence electrons. This means it needs to ____ ____ electrons in order to have a full octet and it will have an ion charge of ___.

            Barium is in group 2 and has two valence electrons. This means it needs to lose two electrons in order to have a full octet and it will have an ion charge of 2+.
          3. Iodine is in group ___ and has ___ valence electrons. This means it needs to ____ ____ electron in order to have a full octet and it will have an ion charge of ___.

            Iodine is in group 17 and has seven valence electrons. This means it needs to gain one electron in order to have a full octet and it will have an ion charge of 1–.