Chemistry
Students Come First
Chapter 4

1. Covalent compounds revisited

We will leave the realm of ionic compounds behind and we will focus on covalent compounds. In the next sections we will look at how to “balance” covalent compounds and how to give them a name.

Covalent compounds are those compounds that are entirely made out of non-metal atoms (read the section on metals and non-metals). Examples of covalent compounds are H2O (water), CH4 (methane) and C6H12O6 (glucose).

Exercises

  1. Determine for each of the following compounds whether they are covalent compounds or not.
    1. H2O
    2. NaCl
    3. H2SO4
    4. C2H6
    5. CO2
    6. K3PO4
    7. HCl
    8. C2H5OH

Answers

    1. covalent compound
    2. not a covalent compound; this is an ionic compound
    3. not a covalent compound; this is an acid
    4. covalent compound
    5. covalent compound
    6. not a covalent compound; this is an ionic compound
    7. not a covalent compound; this is an acid
    8. covalent compound


2. Bohr diagrams

To understand the chemical structure of covalent compounds we need to study atoms more closely. In the section on inside atoms you read that atoms consist of protons, electrons and neutrons. In the center of the atom, the nucleus, you find the protons and neutrons. The electrons circle the nucleus. The atomic number of an atom is the number of protons in the nucleus and determines its position in the periodic table. For example, lithium has atomic number 3 because it has 3 protons and therefore it is the third element in the periodic table. The number of electrons in an atom is always the same as the number of protons in an atom, thus a lithium atom also has 3 electrons.

The electrons of an atom circling the nucleus are organized as well. As you can see in the picture of the lithium atom,

Bohr diagram of a lithium atom Bohr diagram of a hydrogen atom Bohr diagram of a helium atom Bohr diagram of a beryllium atom
there are two circles around the nucleus called shells. The first shell has two electrons in it and the second shell has one electron in it. For any atom each shell can contain a fixed maximum number of electrons. For example, the first shell can have at most two electrons in it, as is the case for a lithium atom. The second shell can have at most eight electrons in it; lithium has only one electron in the second shell. Look at the following table to see how the shells of each atom are filled with electrons.
shell maximum number of electrons
1 2
2 8
3 8

Hydrogen has only 1 electron and puts this electron in the first shell. Helium has two electrons and puts both of them in the first shell. The first shell is full with two electrons in it, therefore, lithium who has 3 electrons can only put 2 electrons in the first shell and has to put 1 electron in the second shell. Beryllium has 4 electrons to put into shells, 2 electrons can go into the first shell and the remaining two go into the second shell. Boron has 5 electrons to put into shells, 2 electrons can go into the first shell and therefore the remaining 3 have to go into the second shell. The second shell can have at most 8 electrons in it, and therefore sodium who has 11 electrons to put into shells, will fill the first shell with 2 electrons, the second shell with 8 electrons and the remaining electron will have to go into the third shell.

The electron configuration is a list of numbers showing how many electrons are in each shell. For example the electron configuration for boron is 2, 3 and the electron configuration for sodium is 2, 8, 1. A Bohr diagram is therefore nothing more than a graphical representation of an electron configuration.

name number of protons total number of electrons number of electrons in 1st shell number of electrons in 2nd shell number of electrons in 3rd shell
hydrogen 1 1 1 0 0
helium 2 2 2 0 0
lithium 3 3 2 1 0
beryllium 4 4 2 2 0
boron 5 5 2 3 0
carbon 6 6 2 4 0
nitrogen 7 7 2 5 0
oxygen 8 8 2 6 0
fluorine 9 9 2 7 0
neon 10 10 2 8 0
sodium 11 11 2 8 1
magnesium 12 12 2 8 2

The electron configuration can easily be read from a periodic table, where each row in the table represents a shell and the position of the element in the row indicates how many electrons are in the outermost shell. For example, sodium is in the third row of the periodic table and thus has 3 shells, the first two shells are filled with electrons and the third shell has 1 electron in it because sodium is the first element in this row. Magnesium also has 3 shells because it is mentioned in the third row of the periodic table but it has 2 electrons in the third shell because it is the second element in the third row.

Exercises

  1. Write down for each of the following atoms their electron configuration.
    Example:
    Li: 2,1 (meaning 2 electrons in the first shell and 1 electron in the second shell)
    S: 2, 8, 6 (meaning 2 electrons in the first shell, 8 in the second shell and 6 in the third shell)
    1. Co
    2. Ne
    3. Br
    4. Xe
    5. Mo
    6. H
    7. O
    8. Ca
    9. Pt
    10. As
    11. P

  2. Write down for each of the following atoms how many electron shells they have.
    Example:
    He has 1 electron shell
    N has 2 electron shells
    1. Li
    2. C
    3. Cl
    4. Al
    5. Rb
    6. Hg
    7. V
    8. Ne
    9. At
    10. H
    11. Te
    12. Ra

  3. Write down the names of the atoms with the following electron configuration.
    1. 2, 1
    2. 2, 7
    3. 2, 8, 3
    4. 1
    5. 2, 8, 8, 18, 13
    6. 2, 8, 8, 1

Answers

    1. 2, 8, 8, 9
    2. 2, 8
    3. 2, 8, 8, 17
    4. 2, 8, 8, 18, 18
    5. 2, 8, 8, 18, 6
    6. 1
    7. 2, 6
    8. 2, 8, 8, 2
    9. 2, 8, 8, 18, 18, 24
    10. 2, 8, 8, 15
    11. 2, 8, 5

    1. 2
    2. 2
    3. 3
    4. 3
    5. 5
    6. 6
    7. 4
    8. 2
    9. 6
    10. 1
    11. 5
    12. 7

    1. lithium
    2. fluorine
    3. aluminum
    4. hydrogen
    5. indium
    6. potassium


3. Lewis diagrams

In the previous section we looked at Bohr diagrams where we had to put all an atom’s electrons into their respective shells. Most of chemistry can be understood by only looking at the electrons in the outer shell. The outer shell is called the valence shell and the electrons in this shell are called the valence electrons.

Lithium has 1 valence electron.
Lithium has 1 valence electron.
Carbon has 4 valence electrons.
Carbon has 4 valence electrons.
For example, lithium has only one electron in the outer shell and therefore has one valence electron; carbon has four electrons in its outer shell and thus it has four valence electrons. The number of valence electrons an atom has is the same as the position it has in its row of the periodic table. For example, lithium is the first element in its row and therefore has only one valence electron, while carbon is the fourth element in its row and thus has four valence electrons. You can therefore use the periodic table to find out how many valence electrons an atom has.

Exercises

  1. How many valence electrons do the following atoms have?
    1. beryllium
    2. fluorine
    3. silicon
    4. hydrogen
    5. oxygen
    6. potassium
    7. argon
    8. magnesium
    9. phosphorus
    10. lithium
    11. nitrogen

Lewis diagram of a carbon atom
A Lewis diagram is a diagram that only shows the valence electrons of an atom and does not show the different shells an atom has like a Bohr diagram does. Carbon has four valence electrons and the Lewis diagram of carbon can be seen on the right. The way the valence electrons are drawn around the nucleus is of importance: start at the top with the first valence electron, then go clockwise. Look at the following examples where each atom has one more valence electron than the previous one:

Lewis diagrams

Lithium has one lone valence electron drawn on top, beryllium has a second lone electron drawn on the side in a clockwise rotation, boron has a third lone electron drawn at the bottom, and carbon has a fourth lone electron drawn on the side in a clockwise rotation. Nitrogen has a fifth electron that will pair up with the first drawn electron, oxygen has a sixth electron that will pair up with the second drawn electron, fluorine has a seventh electron that will pair up with the third drawn electron and neon has an eight electron that will pair up with the fourth drawn electron. Electrons that are not paired up are called lone electrons, thus for example boron has three lone electrons and fluorine has one lone electron.

Exercises

  1. Draw the Lewis diagrams for each of the following atoms.
    1. carbon
    2. sulfur
    3. oxygen
    4. argon
    5. chlorine
    6. nitrogen
    7. silicon
    8. neon
    9. hydrogen

  2. How many extra electrons do the following atoms need for a full valence shell?
    Example:
    nitrogen has 5 electrons in its valence shell and the maximum number of electrons that fit in this valance shell is 8 and therefore nitrogen need 3 more electrons to fill its valence shell.
    1. hydrogen
    2. silicon
    3. carbon
    4. oxygen
    5. sulfur
    6. argon
    7. chlorine
    8. helium

Answers

    1. 2
    2. 7
    3. 4
    4. 1
    5. 6
    6. 1
    7. 8
    8. 2
    9. 5
    10. 1
    11. 5

    1. Lewis diagram of a carbon atom

    2. Lewis diagram of a sulfur atom

    3. Lewis diagram of an oxygen atom

    4. Lewis diagram of an argon atom

    5. Lewis diagram of a chlorine atom

    6. Lewis diagram of a nitrogen atom

    7. Lewis diagram of a silicon atom

    8. Lewis diagram of an oxygen atom

    9. Lewis diagram of a hydrogen atom


    1. 1
    2. 4
    3. 4
    4. 2
    5. 2
    6. 0
    7. 1
    8. 0


4. Forming covalent compounds

Non-metal atoms combine into covalent compounds in order to pair up all lone electrons. Only electron pairs exist in a covalent compound. Let us look at H2O (water) to give you an idea how this works. H2O consists of two hydrogen atoms and one oxygen atom, and a hydrogen atom has one lone electron and an oxygen atom has two lone electrons. Each hydrogen atom with its lone electron will pair up with one of the lone electrons of the oxygen atom until there are no lone electrons left.

Lewis diagram of H2O
Lewis diagram of H2O indicating bonding pairs
The lone electron from one of the hydrogen atoms that has paired up with a lone electron from the oxygen atom forms what is called a bonding pair. H2O therefore has two bonding pairs. Each bonding pair represents a covalent bond or in other words the hydrogen atoms are bonded to the oxygen atom. A covalent bond is a bond that consists of two electrons, one from each atom involved.

Lewis diagram of CH4
Let us have a look at CH4, where carbon has four lone electrons and each hydrogen has a lone electron. Both carbon and hydrogen would like their lone electrons to pair up. Note that four hydrogen atoms are needed to pair up all of carbon’s lone electrons. After carbon’s lone electrons are paired up with four hydrogen atoms, CH4 has four bonding pairs. Also, carbon is then surrounded by eight electrons meaning that it has eight electrons in its valence shell making this a full valence shell for carbon. Hydrogen is surrounded by two electrons meaning that it has two electrons in its valence shell making this a full valence shell for hydrogen. The general rules for covalent bonds are:
  • electrons are always paired up
  • the valence shell of each atom must be full
Covalent compounds are also called molecules.

Exercises

  1. Draw the Lewis diagrams for each of the following molecules
    1. HCl
    2. F2
    3. H2O
    4. CCl4
    5. SF2
    6. NCl3
    7. NF3
    8. CH4

  2. Determine the number of bonding pairs and lone pairs (i.e. electron pairs that are not bonding pairs) for each of the molecules in exercise 1.

Answers

    1. Lewis diagram of HCl

    2. Lewis diagram of F2

    3. Lewis diagram of H2O

    4. Lewis diagram of CCl4

    5. Lewis diagram of SF2

    6. Lewis diagram of NCl3

    7. Lewis diagram of NF3

    8. Lewis diagram of CH4


    1. Lewis diagram of HCl

    2. Lewis diagram of F2

    3. 2 bonding pairs and 2 lone pairs
    4. 4 bonding pairs and 12 lone pairs
    5. 2 bonding pairs and 8 lone pairs
    6. 3 bonding pairs and 10 lone pairs
    7. 3 bonding pairs and 10 lone pairs
    8. 4 bonding pairs and 0 lone pairs


5. Lewis Structures

Drawing a Lewis diagram is considerable work and for that reason a more convenient way of drawing molecules is available. Structural formulas take away much of the laborious work that drawing Lewis diagrams entails. Let us look at a familiar example, H2O, also known as water.

Lewis diagram of H2O
Skeletal structure of H2O
The Lewis diagram can be seen on the left, while the Lewis structure can be seen on the right. The individual atoms are visible and the lines represent bonding pairs of electrons. The Lewis structure above does not show the lone electron pairs (non-bonding pairs), but you are free to add them to the Lewis structure.

Let us look at another example, CH4, also known as methane.

Lewis diagram of CH4
Skeletal structure of CH4
You can see the Lewis diagram on the left and its corresponding Lewis structure on the right. All of CH4 electron pairs are bonding pairs.

Lewis structures are like Lego blocks, where the atoms are the blocks and the lines coming out of them (the bonds) connect each block with another block. Some of the building blocks are

Lewis structures
where you can see that oxygen atoms always make two bonds with other atoms, and carbon always makes four bond with other atoms and hydrogen makes only one bond with another atom. With these building blocks you can build anything your imagination can conceive, such as the molecules below, you just have to make sure that each atom makes the right number of bonds with other atoms.

random covalent compound 1
random covalent compound 2
random covalent compound 3

Exercises

  1. Draw the Lewis structures for each of the following molecules.
    1. CH4
    2. C2H6
    3. CH3OH
    4. C2H5OH

  2. Draw the Lewis structures for each of the following molecules.
    1. H2O2
    2. NH3
    3. PCl3
    4. NF3

Answers

    1. Lewis diagram of CH4

    2. Lewis diagram of C2H6

    3. Lewis diagram of methanol

    4. Lewis diagram of ethanol



6. Double and triple bonds

We have seen that non-metal atoms bond to other non-metal atoms to form covalent compounds. It is possible that a non-metal atom binds more than once to another non-metal atom, as is the case in diatomic (molecules that consist of only two atoms) oxygen gas O2

double bonded oxygen atoms
These kinds of bonds are called double bonds. If a non-metal atom bonds three times with another non-metal atom the bond is called a triple bond, such as in C2H2
triple bonded carbon atoms
Important diatomic molecules are H2 (hydrogen gas), N2 (nitrogen gas), O2 (oxygen gas), F2 (fluorine gas), Cl2 (chlorine gas), Br2, I2 and S8.

Exercises

  1. Draw the Lewis structures for each of the following molecules.
    1. C2H4
    2. C2H2
    3. N2
    4. C3H6
    5. HNO
    6. CS2
    7. HPO
    8. CO2

  2. Which of the following covalent compounds do not exist?
    1. Lewis diagram of a molecule

    2. Lewis diagram of a molecule

    3. Lewis diagram of a molecule

    4. Lewis diagram of a molecule

    5. Lewis diagram of a molecule

    6. Lewis diagram of a molecule


Answers

    1. Lewis diagram of a molecule

    2. Lewis diagram of a molecule

    3. Lewis diagram of a molecule

    4. Lewis diagram of a molecule

    5. Lewis diagram of a molecule

    6. Lewis diagram of a molecule

    7. Lewis diagram of a molecule

    8. Lewis diagram of a molecule


    1. this molecule does not exist because one of the carbon atoms has five bonds, while carbon can only make four bonds.
    2. this molecule exists. Each carbon atom makes four bonds, each oxygen atom makes two bonds and each hydrogen atom makes one bond.
    3. this molecule does not exist because nitrogen can only make three bonds instead of four bonds.
    4. this molecule exists. Each carbon atom makes four bonds, each oxygen atom makes two bonds and each hydrogen atom makes one bond.
    5. this molecule exists. Each carbon atom makes four bonds, each oxygen atom makes two bonds, each hydrogen atom makes one bond and each chlorine atom makes one bond.
    6. this molecule exists. Each carbon atom makes four bonds and each hydrogen atom makes one bond.


7. Skeletal formulas

Because carbon and hydrogen atoms are so common in covalent compounds, they are often not even mentioned, for example the following covalent compound

Lewis diagram
can also be represented by
Skeletal structure
Do you see how you can convert one structure into the other? The second structure that omits the explicit mentioning of carbon and hydrogen atoms provides more clarity, especially when covalent compounds get very large. This type of structure is called a skeletal formula. The lines in a skeletal formula represent the bonds between carbon atoms and the hydrogen atoms are added to the carbon atoms as much as needed to ensure that each carbon atom makes four bonds.

Exercises

Alexander Fleming
The structural formula of the molecule in question 1e is the structural formula of penicillin, an antibiotic produced by the Penicillium fungi and discovered by the Scottish scientist Sir Alexander Fleming in 1928.
  1. Convert the following skeletal formulas into Lewis structures
    1. Skeletal formula

    2. Skeletal formula

    3. Skeletal formula

    4. Skeletal formula

    5. Skeletal formula

    6. Skeletal formula


Answers

    1. Lewis diagram of a molecule

    2. Skeletal formula

    3. Skeletal formula

    4. Skeletal formula

    5. Skeletal formula

    6. Skeletal formula



8. Naming covalent compounds

You previously learned to name ionic compounds and how to name acids. Covalent compounds have their own naming scheme. For this reason it is important that before you start naming a compound that you first determine what kind of compound it is, i.e. a covalent compound, acid or an ionic compound.

Let us look at the familiar example of H2O, also known as water, and give it a scientific name. First we need to recognize that H2O consists of two hydrogen atoms and one oxygen atom. This information enables us to name H2O:

step 1: two hydrogen one oxygen
step 2: di hydrogen mono oxygen
step 3: di hydrogen mono oxide

In the first step you write down the number and the type of atoms that make up the compound, in case of H2O that is two hydrogen atoms and one oxygen atom. In step 2 you replace the numbers with their Greek / Latin translation:

1mono6hexa
2di7hepta
3tri8octa
4tetra9nona
5penta10deca

and in step 3 you have to make the name end with -ide. The scientific name for H2O is therefore dihydrogen monooxide (often written as dihydrogen monoxide).

Let us look at one more example: CH4, also known as methane. CH4 is made of one carbon atom and four hydrogen atoms and therefore the three steps mentioned above would result in:

step 1: one carbon four hydrogen
step 2: mono carbon tetra hydrogen
step 3: mono carbon tetra hydride

and therefore the name for CH4 would be monocarbon tetrahydride. Unfortunately this is not quite right because there is one more rule that you need to remember: a scientific name never starts with mono. For that reason the name of CH4 will not be monocarbon tetrahydride, but will be carbon tetrahydride. You can just leave out the mono in front of the carbon.

Exercises

  1. Name the following covalent compounds.
    1. CH4
    2. C2H6
    3. H2O
    4. N2O4
    5. Cl2O
    6. NH3
    7. N2O
    8. P2O5
    9. CO2
    10. CS2
    11. CO
    12. C4H10

  2. Write down the chemical formula for the following covalent compounds.
    Example:
    nitrogen trifluoride = NF3
    1. nitrogen monoxide
    2. boron monoxide
    3. dinitrogen tetraoxide
    4. carbon disulfide
    5. sulfur dioxide
    6. tricarbon disulfide
    7. phosphorus trinitride
    8. hydrogen monochloride
    9. sulfur difluoride
    10. carbon dioxide

Answers

    1. carbon tetrahydride
    2. dicarbon hexahydride
    3. dihydrogen monoxide
    4. dinitrogen tetraoxide
    5. dichloride monoxide
    6. nitrogen trihydride
    7. dinitrogen monoxide
    8. diphosphorus pentaoxide
    9. carbon dioxide
    10. carbon disulfide
    11. carbon monoxide
    12. tetracarbon decahydride

    1. NO
    2. BO
    3. N2O4
    4. CS2
    5. SO2
    6. C3S2
    7. PN3
    8. HCl
    9. SF2
    10. CO2


9. Organic compounds

An important class of compounds are the organic compounds. Those compounds that are primarily made out of carbon and hydrogen atoms and might also have some oxygen, nitrogen and some other non-metals are called organic compounds. Organic compounds are the molecules that carry out most of the chemical reactions that keep an organism alive. You can say that organic chemistry, the study of organic compounds, is the chemistry of living organisms and the knowledge of organic chemistry is useful in biology, medicine, ecology, etc. A compound that is not an organic compound is called an inorganic compound, and is studied in inorganic chemistry.

Lewis diagram of ethanol
ethanol

An example of an organic compound is ethanol, C2H5OH. The compound has hydrogen and carbon atoms and therefore qualifies as an organic compound. Ethanol has many different usages such as fuel for cars, alcoholic beverages, hand sanitizers and as solvent for paints.

Not all organic compounds are covalent compounds, organic acids which are ionic compounds, do also exist, such as carbonic acid, H2CO3.

Exercises

  1. Determine for each of the following compounds whether they are organic or inorganic compounds.
    1. C3H8
    2. H2O
    3. Na3PO4
    4. C6H12O6
    5. H2SO4
    6. CH4
    7. CO2
    8. C2H5OH

Answers

    1. organic
    2. not organic; does not contain carbon atoms
    3. not organic; not a covalent compound
    4. organic
    5. not organic; does not contain carbon atoms
    6. organic
    7. not organic; does not contain hydrogen atoms
    8. organic