Covalent bonds and shapes of molecules

Covalent bonds and shapes of molecules

Determining number of bonds We use Lewis dot structures The number of unpaired electrons will be the number of bonds How many bonds would each typically form? H Be C O Ne P Li B N F S As

Step 1: come up with a formula Example: determine the structure of a molecule containing Hydrogen and Oxygen From the periodic table and your knowledge of Lewis structures and valence electrons, you know O has a - 2 charge and H has a +1 charge Do the crisscross method, and your formula is: H 2 O

Step 2: make bonds Consider the table you made to be a set of rules H can have one bond, and only one bond O can have two bonds, and only two bonds Represent a covalent bond with a between the atoms sharing the bond So your molecule (for now) is H O H

Step 3: add in remaining valence You need to represent ALL valence electrons The electron pairs not in bond are called lone pairs electrons

PracVce H 2 BeF 2 BF 3 NH 3 CH 4 H 2 S Cl 2 C 2 H 6 CH 3 OH NaI MgF 2 HF HI CCl 4 Don t forget lone pairs!

12.22 Molecular models Today we will Learn names of geometric shapes in molecules and make lots of models

Shapes of molecules Number of bonds Number of lone pairs shape example 2 0 linear BeCl 2 3 0 Trigonal planar BF 3 2 2 Bent H 2 O 4 0 Tetrahedral CH 4 3 1 Trigonal pyramidal NH 3 5 0 Trigonal bipyramidal PF 5

Assemble write down geometry of central atom(s) H 2 O (water) CH 4 (methane) NH 3 (ammonia) CH 2 OH (methanol; methyl alcohol) C 2 H 6 (ethane) C 2 H 5 OH (ethanol; ethyl alcohol) C 3 H 8 (propane) CH 3 CHOHCH 3 (isopropyl alcohol) CO 2 (carbon dioxide) CH 2 O (formaldehyde) C 2 H 4 (ethene) CH 3 COCH 3 (acetone) CHCl 3 (chloroform) H 2 O 2 (Hydrogen peroxide)

1.03 Go to hdp://mw.concord.org/modeler/ Click on the more link about 2/3 of the way down the page Wait for the new page to open Look for the secvon labeled AcVviVes for the Science of Atoms and Molecules Project, click on the link envtled molecular geometry Complete all quesvons do not print; copy the quesvons and answer them

VSEPR Valence Shell Electron Pair Repulsion Orbitals repel other orbitals Because? What is the relavonship between the spaval arrangement of orbitals and the number of orbitals?

Shapes of molecules Number of bonds Number of lone pairs shape Bond angle example 2 0 linear 180 BeCl 2 3 0 Trigonal planar 120 BF 3 2 2 Bent ~104.45 H 2 O 4 0 Tetrahedral 109.5 CH 4 3 1 Trigonal pyramidal 5 0 Trigonal bipyramidal 107.5 NH 3 90/120 PF 5

Assemble write down geometry of central atom(s) H 2 O (water) CH 4 (methane) NH 3 (ammonia) CH 3 OH (methanol; methyl alcohol) C 2 H 6 (ethane) C 2 H 5 OH (ethanol; ethyl alcohol) C 3 H 8 (propane) CH 3 CHOHCH 3 (isopropyl alcohol) CO 2 (carbon dioxide) CH 2 O (formaldehyde) C 2 H 4 (ethene) CH 3 COCH 3 (acetone) CHCl 3 (chloroform) H 2 O 2 (Hydrogen peroxide)

1.05.12 Warmup: imagine molecules as polygons. What is the relavonship between the number of electron domains around the central atom to the number of sides on a polygon? Today s objecvve: Construct 2- D and 3- D models of select atoms, determine geometry Determine polarity of molecules based on structure

Polarity If a molecule has an asymmetric structure, it is considered a polar molecule This means it has an unequal distribuvon of electrons throughout, which means one end of the molecule is slightly negavve, the other end is slightly posivve Examples H 2 O, CHCl 3, CH 2 O, acetone, all alcohols Counter- examples CH 4, CO 2, BF 3, BeCl 2

See handout Molecular shapes lab

1.9.12 What does organic mean? How can you tell if a molecule is organic or not? Today finish acvvity on assembling molecules. Be able to determine geometry and polarity from formulas Also 2 worksheets (8-1 and 8-2) must be done by end of class

Organic chemistry Any compound containing Carbon AND Hydrogen is considered an organic compound CH 4, alcohols, acetone, propane, butane, octanes, steroids, lipids, proteins, DNA/RNA, etc. are all organic CO 2, diamonds, cyanide, graphite are not organic. Why?

Finish labs When finished, pick up worksheet 8-1. When finished with 8-1, turn that in and pick up 8-2. Both must be done by end of class.

More complicated bonding, S and P Sulfur and Phosphorous (as well as others) can form as many covalent bonds as they have valence electrons Contrast this to Nitrogen and Oxygen This allows molecules such as PCl 5, SF 4, SF 6 note you may not have unpaired electrons on an atom, electrons must be grouped in twos (orbitals)