Guided Inquiry Activity #2
Model 1. When at least two atoms are joined together by bonds, a molecule is formed. A compound is formed from the bonding of atoms from at least two different elements.
Figure 2.1. Covalent and ionic bonds.
In the compound sodium chloride, the sodium cation is always carrying a positive charge (Na+) and the chloride anion is always carrying a negative charge (Cl-), but sometimes the compound will be represented as NaCl – without the charges explicitly shown – see Activity 1 for a review of this concept.
1. Define the word molecule using the words atoms and bond.
2. How many electrons would you expect between the C and O atoms joined with a double covalent bond (also called a “double bond”) in Figure 2.1?
3. In the molecule glycine shown in figure 2.1, there are two different carbon atoms. The structure has been re-drawn below and the carbons labeled.
a. How many bonds are being made with each of the two carbon atoms? i.e. How many bonds are being made with carbon 1; with carbon 2?
b. How many electrons are represented by the bonds surrounding each individual carbon atom?
4. The NaCl does not have a covalent bond – i.e. there is no sharing of electrons. – rather it has an ionic bond. What seems to be holding the NaCl (i.e. table salt) molecule together?
Figure 2.2. Two representations of glycine
Since food molecules are most often going to contain carbon, oxygen, nitrogen and hydrogen. The top rows of the periodic table are the most important when thinking about food. To determine the number of electrons that an atoms brings along when engaging in bonding, simply count from left to right. Starting on the left, count to the right along a single row – the position of the element on the P.T. within its row is equivalent to the number of electrons it brings along to bonding. This “counting” strategy is synonymous with the Group Number for the column. For example, in the image of the Periodic table shown below, carbon is in Group IVB. The important number here being IV or “4”. Carbon is in group 4; carbon is also four spaces in from the left in row 2 (also called period 2).
Figure 2.3. The “biochemical” periodic table[footnoteRef:0]. Elements in red are present in bulk form in living things and are essential for life. Since food is made of material from living things, those elements are the most important to the science of food and cooking. Those elements in yellow are trace elements that are very likely essential for life. Those elements in blue are present in some organisms and may be essential for life. [0: This Periodic Table comes from Concepts in Biochemistry by Rodney Boyer (published by Wiley)]
Figure 2.4. Counting electrons within a molecule.
5. In Figure 2.4, why is there only one electron in the blue box drawn around the hydrogen? Please include a reference to the periodic table in your answer
6. How many electrons are inside the box drawn around the oxygen in Figure 2.4? How is this consistent with oxygen’s group number on the periodic table?
7. Based on the pattern of box drawing shown in Figure 2.4, how are lone pairs allocated when “assigning” electrons for the purposes of counting.
8. Complete the image below by drawing boxes around each atom in the molecule (using the drawing tool or inserting shapes) – include within the box electrons that “belong” to that atom. Then complete the table that follows.
|Atom||Number of electrons in the box||Group number for that element on the periodic table (Figure 2.3)||Number of covalent bonds formed with the atom||Number (if any) of lone pairs on the atoms (electrons not in a bond)|
What is the relationship between number of bonds and lone pairs an atom forms/has to group number?
Model 3. All living things (animals, plants microbes, and smaller life forms) are made of atoms and molecules. How those molecules are organized, interact and react are the building blocks for life. Molecules are often divided into two categories, organic[footnoteRef:1] (those molecules containing carbon atoms) and inorganic molecules (everything else). Food molecules can also be held together with covalent bonds (typical of, but not exclusive to organic compounds), ionic bonds (typical of, but not exclusive to inorganic compounds) and mixtures of both types of bonds. [1: The word “organic” in this context has nothing do to with methods of farming or food production; it is a broad chemical concept that describes all molecules that are found in living things. Since the molecules of living things are largely based on carbon, a chemist would consider carbon compounds to be synonymous with organic compounds. ]
Figure 2.5. Examples of organic and inorganic food molecules
As we saw in Activity 1, an element with too few electrons carries a positive charge and is called a cation, while an element with too many electrons carries a negative charge and is called an anion. In Figure 2.5, we can see an example of this in citric acid vs. calcium citrate. In citric acid, all the oxygens are neutral, while in calcium citrate, some of the oxygen atoms are carrying a negative charge. Let’s examine this more closely in Figure 2.6 below.
Figure 2.6. The difference in electron number between a neutral and charged oxygen.
Using the concept of the “electrons each atom brings along to bonding” developed in Model 2, the citric acid oxygen identified by the arrow in Figure 2.6 is bringing 6 electrons, while the comparable oxygen in citrate is bringing 7 electrons. It is the seventh electron that is extra; it is giving that oxygen a negative charge. Remember that oxygen is in Group 6 of the periodic table.
9. Study Figure 2.5. List below an example of a molecule that is held together with covalent bonds. Then also list an example of a molecule that is held together by ionic bonds. Then list an example of a molecule that is held together by both ionic bonds and covalent bonds. Then provide an example of a molecule that is organic and a molecule that is inorganic.
10. Calcium citrate is made with two citrate molecules and three Calcium ions. (An ion is an element that has too many or too few electrons, and is therefore carrying a charge). Why is this? (Hint: the overall molecule must be neutral)
11. In Figure 2.6, the extra electron present on the oxygen gives it a negative charge.
a. Why are 6 electrons ok for an oxygen atom, but 7 is too many?
b. How many electrons would be too many for a nitrogen atom?
c. How many electrons would be too few for a nitrogen atom?
d. How does your answer to (c) explain the structure of ammonium chloride below? Why is the nitrogen carrying a positive charge?
12. Table salt is an ionic compound – sodium chloride. Why might calcium citrate also be called the calcium salt of citric acid?
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