1. Molecular Structures the electrons influence on molecular shape

2. The Basics Atoms bond with other atoms using pairs of electrons. The bonding electrons come from the outermost “s” and “p” orbitals (known as the valence electrons.) When an atom is bonding, it’s valence electrons will form four pairs of electrons that will repel each other.

3. x z y A Little Geometry In two dimensions, object position can be described with an “x-y” coordinate system. In three dimensions, objects need to be described with an x-y-z coordinate system. X Y Y Z X

4. Electron Pairs and Geometry: 2 Dimensions At first thought, 4 pairs of electrons could spread themselves out in the x-y plane But, with three dimensions, there is a better solution… y z x 90 o e-e

5. Electrons in 3 Dimensions If the electrons on the y axis move back a bit to the z axis, and the electrons on the x axis move forward, then the electron spread is maximized, from 90 o to 109.5 o Y z x e-e

6. Electrons in 3 Dimensions If the electrons on the y axis move back a bit to the z axis… And the electrons on the x axis move forward, towards the z axis Then the electron spread is maximized, from 90 o to 109.5 o Y Z X e-e

7. Bonding Pairs with 8 electrons The four electron pairs spread themselves out, at 109.5 o from each other, forming a tetrahedral structure. 109.5 o

8. Bonding Electrons Central Atom

9. Consider Methane, CH 4, which has 4 hydrogen atoms attached at equal distances from the carbon atom. Carbon Hydrogen

10. Ammonia, NH 3, is a trigonal pyramidal structure. Notice the lone pair of electrons at the top. Nitrogen Hydrogen Atoms Lone Pair of Electrons

11. Water, H 2 O is an angular structure. Notice the two pairs of lone electrons at the topand back. Oxygen Hydrogen Atoms Lone Pairs of Electrons

12. Hydrochloric Acid, HCl has a linear structure. Chlorine Hydrogen Atom Lone Pairs of Electrons

13. Some atoms can bond with 5 or 6 pairs of electrons. The elements include Phosphorus an below (As, etc.), that can form 5 bonds. The elements Sulfur and below, can form molecules with 6 pairs of electrons and 6 bonds.

14. For column 15 elements, except N Two electron pairs would be on the “y” axis Three more would be on the “x-z” plane, at 120 o from each other. x z y Y Z X

15. 5 Electron Structures The five electron pairs spread out at 120 o and 90 o from each other They form a trigonal bipyramidal structure, a double pyramid with a three sided base 90 o 120 o

16. 6 Electron Pair Structures The maximum spread for 6 electron pairs is 90 o It forms an octahedral structure, a double pyramid with a four-sided base y z x 90 o e-e