Bonding. Guided Inquiry Activity #2

Transcription

1 Bonding 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. ovalent 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 (l - ), but sometimes the compound will be represented as Nal 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. opyright 2016 Wiley, Inc. Page 1

2 2. ow many electrons would you expect between the and 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. ow many bonds are being made with each of the two carbon atoms? i.e. ow many bonds are being made with carbon 1; with carbon 2? b. ow many electrons are represented by the bonds surrounding each individual carbon atom? 4. The Nal 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 Nal (i.e. table salt) molecule together? opyright 2016 Wiley, Inc. Page 2

3 Model 2. 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. arbon is in group 4; carbon is also four spaces in from the left in row 2 (also called period 2). opyright 2016 Wiley, Inc. Page 3

4 Figure 2.3. The biochemical periodic table 1. 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. Figure 2.4. ounting 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 1 This Periodic Table comes from oncepts in Biochemistry by Rodney Boyer (published by Wiley) opyright 2016 Wiley, Inc. Page 4

5 6. ow many electrons are inside the box drawn around the oxygen in Figure 2.4? ow 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. omplete the image below by drawing boxes around each atom in the molecule include within the box electrons that belong to that atom. Then complete the table that follows. Atom 1 () 2 () 3 (N) 4 () 5 () 6 () 7 () 8 () 9 () 10 () 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? 9. In the structure of acetic acid below, the covalent bonds are drawn in, but any lone pairs have been deliberately omitted. This is not uncommon practice for chemists often opyright 2016 Wiley, Inc. Page 5

6 molecules are drawn without the lone pairs explicitly shown; however, the lone pairs are still there. Draw in any lone pairs on the structure of acetic acid below. Acetic acid is the pungent odor of vinegar. 10. The structure of cysteine is drawn below. ysteine is an important component of protein. ysteine contains a sulfur atom which has not been in any of our previous examples. owever, sulfur is located right below oxygen on the periodic table how can that information help you determine the number of lone pairs around sulfur? Using the line drawing below, fill in any and all lone pairs on all the atoms of cysteine. opyright 2016 Wiley, Inc. Page 6

7 Model 3. All living things (animals, plants microbes, and smaller life forms) are made of atoms and molecules. ow those molecules are organized, interact and react are the building blocks for life. Molecules are often divided into two categories, organic 2 (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. this really looks like but has been abbreviated for simplicity Na l sodium chloride (i.e. table salt) 2 2 itric Acid (found in all citrus fruits) K K l I potassium chloride (in bananas) potassium iodide (in iodized salt) citrate calcium ions citrate a 2+ a 2+ a 2+ alcium citrate (food preservative and f lavor additive) Figure 2.5. Examples of organic and inorganic food molecules Glucose (i.e. dextrose) sodium bicarbonate (baking soda) Na 2 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. opyright 2016 Wiley, Inc. Page 7

8 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. 11. Study Figure 2.5 and categorize the molecules according to whether or not they are held together with covalent bonds, ionic bonds or both. For the molecules that have both covalent and ionic bonds, circle and label the different types. Then, label each molecule as organic or inorganic. 12. alcium citrate is made with two citrate molecules and three alcium ions. (An ion is an element that has too many or too few electrons, and is therefore carrying a charge). Why is this? (int: the overall molecule must be neutral) 13. 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. ow many electrons would be too many for a nitrogen atom? c. ow many electrons would be too few for a nitrogen atom? opyright 2016 Wiley, Inc. Page 8

9 d. ow does your answer to (c) explain the structure of ammonium chloride below? Why is the nitrogen carrying a positive charge? 14. Table salt is an ionic compound sodium chloride. Why might calcium citrate also be called the calcium salt of citric acid? opyright 2016 Wiley, Inc. Page 9