Molecular Structures

the electrons influence on molecular shape

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.

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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.

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Y

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Z

X

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…

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90o

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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 90o to 109.5o

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z

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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 90o to 109.5o

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Z

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Bonding Pairs with 8 electrons

• The four electron pairs spread themselves out, at 109.5o from each other, forming a tetrahedral structure.

109.5o

Bonding Electrons

Central Atom

• Consider Methane, CH4, which has 4 hydrogen atoms attached at equal distances from the carbon atom.

Carbon

Hydrogen

• Ammonia, NH3, is a trigonal pyramidal structure.

• Notice the lone pair of electrons at the top.

Nitrogen

Hydrogen Atoms

Lone Pair of Electrons

• Water, H2O is an angular structure.

• Notice the two pairs of lone electrons at the topand back.

Oxygen

Hydrogen Atoms

Lone Pairs of Electrons

• Hydrochloric Acid, HCl has a linear structure.

Chlorine

Hydrogen Atom

Lone Pairs of Electrons

• 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.

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 120o from each other.

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YZ

X

5 Electron Structures

• The five electron pairs spread out at 120o and 90o from each other

• They form a trigonal bipyramidal structure, a double pyramid with a three sided base

90o

120o

6 Electron Pair Structures

• The maximum spread for 6 electron pairs is 90o

• It forms an octahedral structure, a double pyramid with a four-sided base

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90o

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