Investigation 3 Activity 3

3.2: Same Molecules but Different Representations Scientists develop and use molecular models for a variety of reasons. One reason is that molecular models help scientists visualize the shapes of molecules and how those molecules behave. Although a chemical formula tells which atoms a molecule is made up of, a chemical formula does not provide information about the shape of the molecule. It is important to be able to visualize the shape of a molecule because a molecule's shape determines some of its properties This reading discusses two types of molecular models, which you have already explored in class. One is called a space-filling model and the other is called a ball-and-stick model. The different ways these models represent molecules emphasize different things. Space-filling Model Atoms have a dense positive nucleus that is surrounded by electrons, which form a spherically shaped cloud around the nucleus. Thus, spheres are used to represent atoms in three-dimensional molecular models. When the atoms interact to form a molecule, their electron clouds overlap (See Figure 1a). Space- filling models represent the space taken up by the electron clouds of atoms. Figure 1b shows a space-filling model of a molecule consisting of the same two atoms. The center-to-center distances between the atoms are proportional to the distances between actual atomic nuclei.Dverall inheractinrthalanoed Figure Bo: Electron-cloud overlap of two atoms interacting to form a molecule. Fry-ore Pb: Space-lling model of a molecule containing two atoms. You can also see that in a spacelling model, the spheres overlap where atoms are connected. Why do you think the spheres overlap? What do you think the overlapped part of each sphere represents? In class, you learned that when atoms get close to each other, line nucleus of one atom attracts the electrons of the other atom, and when the electrons are shared between atoms, a bond is formed. In a spacefilling model, the overlapping parts of the spheres show where chemical bonds are formed in a molecule when electrons are shared. Ball-and-stlck Model Scientists also use ballandstick models to show how the atoms in a molecule are connected to each other. As in line space-filling model, the hall-andstick model shows the H1ree-dimensional shape of molecules by using a sphere to represent an atom. However, unlike the spacefilling model, the ballandstick model uses a stick to show where two spheres are connected. The stick represents a chemical bond between atoms. A c '. I a '. e W I n d C- The representation below (Figure 2) is a ball-and-stick model of a molecule containing two atoms. The chemical bond is represented by the stick between the spheres. However, it is important to know that these sticks are only representations-they are not real. In class, you learned about the chemical bonds that hold atoms together. The electrostatic interactions between atoms due to charged subatomic particles pull them together, and they stay together at the distance where the forces between them are balanced. There are no physical objects, such as sticks, linking the atoms Figure 2: Ball-and-stick model of molecule together. containing two atoms. Although the space-filling models are more accurate, for more complex molecules, ball-and-stick models can make it easier to see all of the components of the molecule. Figures 2a and 2b compare the space-filling and ball-and-stick models of ethanol. While all of the atoms of the molecule are visible in the ball-and-stick model. H Activate Windows Go to Settings to activa Figure 30: Space-filling molecular model of Figure 3b: Ball-and-stick molecular model of ethanol (CzHO). ethanol (CHEO).Investigation 3 Activity 3 Auto saved POSSIBLE POINTS: 1 In previous investigations, you explored simulations of changes in the potential energy of a spring and a pendulum. Now you will explore a simulation to investigate how the potential energy between two atoms changes when they come together to form a bond. 1. People often think bonds store or hold energy. If that is true, should the energy be high or low when a bond forms? BIU 0 / 10000 Word Limit 1 2 3 4 5 6 7 8 9 10 Gh to SInvestigation 3 Activity 3 2 . 2 of 11 In this simulation, you will change the distance between two atoms, just as you did in the simulation in Activity 3.2. This time, notice how the atoms' potential energy changes as you move them. X Share About Left element Drag an atom's nucleus to explore bond formation. O Hydrogen Oxygen 21 Carbon Right element O Hydrogen O Oxygen Carbon Overall interaction: Attractive Visualization . Electron cloud Spacefill high Potential Energy Potential Energy Activate Windows Go to Settings to activate Window3. Investigation 3 Activity 3 3 of 11 POSSIBLE POINTS: 1 Using the simulation, find a pattern in how the potential energy of the system changes when the relative distance between two atoms changes. Describe the pattern. B I U 0 / 10000 Word Limit 1 2 3 4 5 6 7 8 9 10 Next Activate windowsInvestigation 3 Activity 3 4 of 11 POSSIBLE POINTS: 4 Draw a stacked bar graph of potential energy vs. the distance between two atoms that shows when the atoms are too close to form a bond form a bond . are too far apart to form a bond Make sure to label your graph. x Clear + Undo * Redo Activate Windows Go to Settings to activate WindInvestigation 3 Activity 3 5. 5 of 11 POSSIBLE POINTS: 1 Using the simulation, select three different pairs of atoms. For each pair, find the distance between the two atoms at which the potential energy is lowest. Is the distance the same for all three pairs? O yes O no 21 1 2 3 4 5 6 7 8 9 10 Next Activat Go to Settings to activate WiInvestigation 3 Activity 3 6'3 POSSIBLE POINTS: 2 1. What patterns 00 you notice when comparing the points where the potential energy is lowest for different pairs of atoms? 2. Use ideas of energy, stability and attractive and repulsive forces to explain why two separate atoms form a bond. a 1 52;: F 2: IC 0 J 10000 Word leit E] Investigation 3 Activity 3 7 of 11 Are all molecules more stable than separate atoms? Many substances, such as oxygen gas (02), water (H2O), and ammonia (NH3), are made up of molecules. However, some substances, such as helium gas (He), are made up of single atoms. Why is helium made up of single atoms rather than molecules? To answer this question, you need to compare the potential energy of the separate atoms with their potential energy when they are together in a molecule. In the figure, the difference in the potential energy between Point 1 (stable molecule) and Point 2 (individual atoms staying separate) is called the binding energy of the molecule. 21 The atoms repel each other when they are too close together At this distance, the attraction between the atoms is negligible. Potential Energy Binding energy: Difference in potential energy between the point when atoms interacting at the most stable point and when they are separate At the optimal distance (most stable Activate Windows point), attractive and repulsive forces Go to Settings to activate Wir are balanced Distance between AtomsCompare element pairs: Drag an atom's nucleus to explore bond formation. OH-H He-He Visualization Electron cloud Spacefill Overall interaction: Allracdive high Potential Energy Potential Energy low Distance (pm) The Concord Consortium Reset Activate Windows Se 2 3 4 5 6 7 8 9 10 11 Next ctivate Win80 Investigation 3 Activity 3 8 of 11 POSSIBLE POINTS: 4 1. Observe what happens when two hydrogen atoms (H-H) move close together. Describe the atoms when their potential energy is the lowest. 2. In the graphs is the energy at a high point or low point when the atoms form a bond? X 3. According to the graphs, do bonds have high energy or low energy? 4. Observe what happens when two helium atoms (He-He) move close together. Describe the atoms when their potential energy is lowest. 21 0 / 10000 Word Limit