Group Activity: Chemical Formulas
and Compounds

Molecular Compounds

H2O

NH3

CH3CH2OH

Now you will turn to learning about combinations of atoms. Atoms can combine in a couple of different ways. One way that atoms combine is as molecules. Molecules are covalently bonded collections of atoms that behave as a unit. Some common molecules are water (H₂O: 2 H atoms and 1 O atom), carbon dioxide (CO₂: 1 C atom and 2 O atoms), oxygen (O₂: 2 O atoms), ozone (O₃: 3 O atoms), nitrogen (N₂: 2 N atoms), hydrogen peroxide, (H₂O₂: 2 H atoms and 2 O atoms), alcohol (CH₃CH₂OH: 2 C atoms, 6 H atoms and 1 O atom), and ammonia (NH₃: 1 N atom and 3 H atoms).

Notice that some of these molecules contain only one kind of atom. These molecules are not compounds but elements. There are seven elements whose most common form is a diatomic molecule: H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂. The elemental form of oxygen can be either O₃ or O₂. We breathe O₂ but O₃ is poisonous (but essential for absorbing some of the sun’s UV radiation). When elements can occur in more than one form then those forms are called allotropes. Another common example of an allotrope is diamond: pure carbon can have the form of charcoal or graphite but it can also be a diamond. It is simply a matter of how the atoms are arranged.

Most molecules are compounds: substances in which atoms of different elements are chemically bonded to one another. Compounds are formed through the chemical reactions of elements and other compounds with each other. Molecular compounds are characterized by consisting of collections of atoms which are all covalently bonded to one another. Covalent bonds form when atoms share their electrons more-or-less equally. The mutual attraction of two atomic nuclei for the shared electrons creates the bond. They are not the same as ionic compounds. In ionic compounds the connection between atoms comes about because of the attraction of positive and negative charges for each other. In ionic bonds whole atoms have negative and positive charges due to the loss or gain of electrons. These opposite charges are attracted to each other, creating the bond.

Molecular compounds have certain defining characteristics. The atoms in a molecule are bound together and move and act as a unit. They have relatively low melting points (H₂O mp = 0°C), low boiling points (H₂O bp = 100°C) and make poor conductors of electricity even when melted into a liquid. In addition, molecular compounds are non-electrolytes.

An electrolyte is a substance that, when dissolved in water, allows the solution to conduct electricity. They do this because electrolytes are ionic compounds (made of positive and negative ions) and these ions break up when in water. Once the ions can move they can carry an electric current.You may be familiar with electrolytes from ads for sports drinks that claim to replace electrolytes lost during exercise. The electrolytes they are talking about are usually Na⁺, K⁺, Mg²⁺, and Ca²⁺. The sodium they get from table salt: NaCl. A non-electrolyte may dissolve in water but the resulting solution does not conduct electricity because molecular compounds cannot break up into ions.

Ionic Compounds

When ions form through the process of a chemical reaction they can combine to make ionic compounds. Ionic compounds have a different kind of bond than molecular compounds. When sodium metal reacts with

chlorine gas (2Na + Cl₂ —> 2NaCl) the neutrally charged sodium atoms each lose one electron and the neutrally charged chlorine atoms each gain one electron. It is an even exchange and both kinds of atom become charged. Sodium atoms have a +1 charge and chlorine atoms have a –1 charge.

You may remember that objects with opposite electric charges attract one another. Sodium ions and chloride ions attract each other very strongly and form ionic bonds. The ions Na⁺ and Cl^- have a one-to-one ratio in NaCl but you can never have just one unit of NaCl.

In the pictures above are three representations of NaCl. In the first you can see that for each of the small atoms (Na⁺) there are six of the large atoms (Cl^-). The same is true for each of the Cl^- ions. All of the ions are really in contact with each other, like in the middle picture. Salt is always a crystal with all of the positive charges surrounded by negative charges and all of the negative charges surrouned by positive charges.

Ionic compounds have relatively high melting points (NaCl mp = 801°C), high boiling points (NaCl bp = 1413°C) and make poor conductors of electricity. That is, when they are solids. When ionic compounds melt they make excellent conductors of electricity because the ions are free to move around. Moving ions can carry electric current in the same way that electrons in a wire carry electric current. The cations and anions can move freely and are attracted to electrical wires carrying a current. Their motion toward the wire they are attracted to serves to carry electrical charge from one wire to another, making it possible for electric current to flow.

Ionic compounds are strong electrolytes. Strong electrolytes work well to conduct electricity when they dissolve because they break up completely into cations and anions. The ions in liquid solution can move around, just as they could if the ionic compound is melted. Weak electrolytes, which form ions from molecular compounds, only partially break up into ions. The majority of a sample of a weak electrolyte stays whole and is neutral. Only a small fraction of the total sample breaks up into ions that can carry a current. An example of what happens when a strong electrolyte dissolves is shown at right. When sodium chloride (NaCl) dissolves it turns complete into Na⁺ and Cl^- ions: NaCl --> Na⁺ + Cl^-.

Formulas for ionic compounds are always written in lowest terms. In NaCl there is one sodium cation for every chloride anion. You could write Na₂Cl₂ or Na₄₂Cl₄₂ and the ratio of Na⁺ to Cl^- would be the same. But the point is to make things as simple as possible so you will just write NaCl every time you mean sodium chloride.

The reason there is always just one Na⁺ for every Cl^- is that each ion has one unit of charge (+ or -). In ionic compounds the charges always balance. That is, all ionic compounds are electrically neutral. For Na⁺ and Cl^- there is a +1 charge and a -1 charge. These add up to zero, making the combination electrically neutral. Take another example: calcium fluoride. Calcium has a +2 charge (Ca²⁺) and fluoride has a -1 charge (F^-)—you could predict this also from the electron configurations. To write the formula you would need to have two F^- for every Ca²⁺ because (2 × -1) + (+2) = 0. This makes the compound electrically neutral and you write the formula as CaF₂. An easy way to do this is to realize that the subscript on the cation will be equal to the charge on the anion and that the subsript on the anion will be equal to the charge on the cation. Use aluminum oxide as an example: Al³⁺ and O^2- combine to form Al₂O₃.

Naming Compounds

Take a good look at the ions and their names in the table you received with this packet. Notice that nearly all of the cations are metals and that most of the anions are nonmetals. Ionic compounds tend to form between metals and nonmetals. Some of the anions and one of the cations contain more than one atom. These ions are polyatomic ions. The ions that consist of only one atom are monatomic ions. It will be helpful to you to become familiar with the more commonly used names since naming compounds correctly is necessary for good communication. The names of ionic compounds are always named with the cation first, followed by the anion. It is left to you to figure out the formula because once you know the name of an ion you can figure out its charge. If you know the charge of both ions in a compound then you can figure out its formula.

Molecular compounds are named slightly differently. They are formed mostly from nonmetallic elements and come in discrete molecular units. When a compound contains more than one of a particular atom then Greek numerical prefixes are used in the name to make that clear.

• carbon monoxide (CO)
• carbon dioxide (CO₂)
• sulfur dioxide (SO₂)
• sulfur trioxide (SO₃)
• nitrogen dioxide (NO₂)
• dinitrogen tetroxide (N₂O ₄)

Carbon, sulfur and nitrogen all form several different compounds with oxygen. It is important to specify which one you mean. It is common to add the letters -ide to the end of the name of a molecular compound: HCl is hydrogen chloride, SiC is silicon carbide. Some compounds do not use the prefix system: CH₄ is methane, H₂O is water, NH₃ is ammonia. What these compounds have in common is that they contain hydrogen. They are very common compounds and just go by their non-sytemmatic names.

Some elements, especially metals, can have more than one charge. Iron commonly has a +2 or a +3 charge. These ions are called iron(II) (Fe²⁺, say “iron two”) and iron(III) (Fe³⁺, say “iron three”). Other metal ions that take different charges at different times can be named in the same way.

Electronegativity

The covalent bonds and ionic bonds that you have read about so far are only the two extreme ends of a spectrum of types of chemical bonds. Linus Pauling (1901 - 1994), one of the most influential chemists of the 20^th century, studied the chemical bond in great detail. In his studies of the chemical bond he found it useful to describe bonds according to where electrons in bonds spend most of their time. In some bonds, electrons are exchanged entirely: ionic bonds. In others, electrons are shared completely equally: covalent bonds. In completely equal covalent bonds the atoms share electrons just the way people share a cake: two equal slices. This type of bond is called non-polar covalent. There are also bonds that are somewhere on a spectrum in between. These bonds are called polar covalent bonds. Nearly all bonds between atoms are at least slightly polar. In polar covalent bonds the electrons spend more time closer to one atom than to the other. Polarity in a molecule is when part of the molecule has a slight negative charge and another part has a slight positive charge.

The degree of polarity of a chemical bond can be calculated using the relative scale of electronegativity developed by Pauling. Electronegativity is the ability of an atom to attract toward itself the electrons in a bond. Atoms that are more electronegative tend to draw the negatively charged electrons toward themselves. Water is a great example of a compound with very polar bonds. Oxygen is more electronegative than hydrogen and therefore draws electrons toward itself. This causes there to be a partial positive charge on hydrogen and a partial negative charge on oxygen (see the picture). These charges are fractional, not full electric charges. Water (H₂O) is an excellent example of a polar molecule.

To find out how polar a bond is use the electronegativity values from the chart on the next page. If the difference between two values is greater than 2.0 then the bond is ionic. If the difference is between 0.5 and 2.0 then the bond is polar covalent. If the difference is between 0 and 0.4 then the bond is non-polar covalent.

Electronegativity ties everything together that you have read so far. Bonds between atoms of very different electronegativities are ionic. Bonds between atoms of similar electronegativities are non-polar covalent. In between there are the polar covalent bonds.

Pauling Electronegativity Values

Questions and Problems

Comprehension Questions

Answer the following questions using one or more complete sentences. Everyone in the group must write down complete answers. Discuss among your group members what the best way to answer the question is and then write it down. All members must write down the answer or risk losing participation points.

1. Define the following terms.
   1. allotrope:
   2. molecule:
   3. covalent bond:
   4. ionic bond:
2. What are the defining characteristics of a molecular compound?
3. What are the defining characteristics of an ionic compound?
4. What makes a compound an electrolyte?
5. Devise an experiment to test a series of white powders to see whether they are electrolytes. In several complete sentences describe what steps you would take, what results you would look for, and how to interpret those results.
6. What is the difference between a weak electrolyte and a non-electrolyte? That is, what is different about what happens at the molecular level when a substance is dissolved?
7. What is electronegativity?
8. Describe the difference between a polar covalent and a non-polar covalent bond.

Periodic Trends

The periodic table tells you the atomic number, the average atomic mass and also how many valence electrons an element has. It can also reveal certain other trends in the properties of the elements. By making a graph you will explore the trend of electronegativity values in the periodic table.

9. Construct a graph using an entire sheet of paper. Set up the x-axis to contain all of the atomic numbers from 1 - 82 (from hydrogen to lead). Set up the y-axis to contain all of the electronegativity values from the chart on the previous page of this packet. Graph the electronegativity vs. the atomic number to see the trend in the periodic table. Lightly draw a line connecting the dots. Label the noble gases and the halogens in the graph using their atomic symbols.

10. What trend do you see for elements in groups 1 & 2 and 13 through 18?
11. What trend do you see for elements in groups 3 through 12?
12. What trend do you see for elements in in the lanthanide and actinide series (the rare earth metals)?

13. Using the mini-periodic table at right, draw two arrows showing the overall trend in the periodic table for electronegativity. Draw the arrows from low electronegativity pointing toward high electronegativity.
14. What element is most electronegative? Which one is least electronegative?
15. Without looking at the chart of electronegativity values circle the element that is more electronegative in each of the following pairs. Use your understanding of the overall trend to help you.

16. Imagine that a chemical bond has formed between each of the following pairs of elements. Calculate the absolute value of the difference in electronegativity between each pair and write it in the blank. Then circle Polar, Non-polar, or Ionic.

17. Figure out the polarity of the compound and then circle the word water if it will dissovle in water. If it will dissolve in oil, then circle oil.