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Molecular Shapes

Molecular Shapes. Covalent Molecules. Determining Molecular Shapes. Lewis Structures only show us the 2-Dimensional shapes. Not the 3-D molecules we see in the real world This 3-D shape is important in determining the chemical and physical properties of a molecule.

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Molecular Shapes

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  1. Molecular Shapes Covalent Molecules

  2. Determining Molecular Shapes • Lewis Structures only show us the 2-Dimensional shapes. Not the 3-D molecules we see in the real world • This 3-D shape is important in determining the chemical and physical properties of a molecule. • Sugar, as an example, fits certain receptors on the tongue that sends signals to the brain. The brain interprets these signals as “sweet” tasting (a physical property). • It is this particular shape that fits nicely into the proper receptors.

  3. Lewis Structures Help Us With Shapes • A Lewis Structure can help predict molecular shapes • Using the Valence Shell Electron Pair Repulsion (VSEPR) Theory we can determine the shapes. • The unshared pairs of electrons repel one another and migrate to as fair away as they can. • Consider and draw the Lewis Structures for CO2 and SnCl2

  4. CO2 Vs. SnCl2 • Carbon Dioxide is a “linear” molecule • The double bond takes up two bonding sites. The other two migrate to opposite ends where the two oxygens bond to the central carbon. • Tin (II) Chloride is a “bent” molecule • This occurs because the two loan pairs of electrons on the Tin push the two oxygens away to form the four bonding sites we are use to.

  5. VSEPR Theory • According to the VSEPR Theory, the shape of the molecule is determined by the valence electrons surrounding the central atoms. • Think about what would happen when the central atom is surrounding by three atoms. • How will they arrange themselves around the central atom? • Now, what if there were three atoms with the central atoms having an addition unbonded pair? • HINT: Keep in mind we are always considering three-dimensions!

  6. What if there are unshared electrons?? • Draw out NH3 • What is happening here? • What shape would you call this?

  7. Practice With VSEPR Theory • Draw out the Lewis structure for BF3

  8. Practice With VSEPR Theory • Draw out the Lewis structure for BF3 • This formation is known as Trigonal Planar. • So not we have linear, bent, and trigonal planar. Great test questions!! HINT, HINT!!

  9. Surrounded Central Atoms • Think about what happens when the central atom is surrounded by four pairs of electrons. • Draw out the Lewis Structure for methane, CH4

  10. Surrounded Central Atoms • Think about what happens when the central atom is surrounded by four pairs of electrons. • Draw out the Lewis Structure for methane, CH4 • Notice that on a FLAT PLANE these atoms are not as fair apart as they could possible be in a 3-D world. • This is known as a Tetrahedron Shape

  11. Unshared Pairs of Electrons • Unshared electrons influence the shape of a molecule but are not visible in the space-filling molcule. • For example, the shared and unshared pairs of electrons in SnCl2 form a trigonal planar geometry, but the molecule has a bent shape. • SO2 has a bent shape also, but has two unshared pairs of electrons. This is due to the “not visible in the space-filling molecule”

  12. Predict The Following Shapes • Page 211: Predict the shapes of the following molecules and polyatomic ions. NH2Cl H2O NOCl NH4+

  13. Molecular Shapes and Properties • A molecule’s shape affects both the physical and chemical properties of the substance. • One specific property that shape determines is the polarity of the molecule. • Recall: What is polarity??

  14. In Other Words… • Think about the molecule’s overall polarity like this: • You and a friend pull on a cart in equal and opposite directions. The cart does not move anywhere. The pull forces cancel one another out in the same way as the polarities do in carbon dioxide. • Now, think about what happens when you guys pull will equal forces in nonopposite directions. If you pull it north and your friend pulls it west. • The cart has a net force now and it will move. • The water molecule has a net partial positive charge on the H side and a net negative charge on the O side. As a result, the molecule has an overall charge and is therefore polar.

  15. Polarity Affects Many Things • Polarity can affect the strength of the molecule. • It can affect how it interacts with other ions • Charged particles will be attracted to the poles in a polar molecule. • This is why water dissolves many substances. • The Universal Solvent

  16. Test Tomorrow!!

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