13D Shapes of Covalent Compounds

Electron pairs around an atom Electron pairs around an atom repel each other.repel each other.

Bonding and unshared pairs of eBonding and unshared pairs of e-- spread out in space to move as spread out in space to move as far apart as possible.far apart as possible.

Any bonded atoms move with the Any bonded atoms move with the electron pairs. electron pairs.

2So, to ask “What is the shape of a compound?” means we first must ask (and answer):

“What is the arrangement of the What is the arrangement of the electron pairs around an atom?electron pairs around an atom?”

two electron pairstwo electron pairs •• Z ••

maximum separation = 180° 180° = linear arrangementlinear arrangement

3three ethree e-- pairs pairs

Z

maximum separation = 120° 120° = trigonal planar trigonal planar arrangement

••

••••

4four efour e-- pairs pairs

•• Z

maximum separation in 3D =

109° 109° = tetrahedraltetrahedral arrangement.

•

••

•••

5Result & Conclusion... ifif the number of groups number of groups connected to an atom = number of electron = number of electron pairs pairs on the atom,

thenthen the geometry of the geometry of the molecule molecule around that atom = = geometry of the electron pairs geometry of the electron pairs around the atom.

6Examples

H-Be-HH-Be-H 2e- pairs = linear; 2 bonded atoms =

linear molecule.

BClBCl33 3e 3e-- pairs = trigonal pairs = trigonal planar; planar; 3 bonded atoms = 3 bonded atoms =

trig planar trig planar moleculemolecule

7

CHH

HH

HHHH

4e- pairs around C = tetrahedral , 109°;

4 bonded atoms around C = tetrahedral molecule, 109° bond angles

8Perspective Rendering CH4:

C

HH

HH

HHHH

= bond in plane of page= bond back plane of page

= bond projects out at viewer

9What if #eWhat if #e-- pairs doesn’t = # pairs doesn’t = # bonded atoms??bonded atoms??

N

HH

H•• 4 e4 e-- pairs around N; pairs around N;

max separation at 109° max separation at 109° = tetrahedral= tetrahedral

There are only 3 H atoms connected to the e- pairs: shape of molecule describes mass points in space. So...

10What we see in space are the heavy mass points-- the nuclei of the atoms arranged around the N. The three H atoms lie in one three H atoms lie in one plane plane (making a triangle), with with the N atom abovethe N atom above, like the apex of a pyramide. The molecular shape is said to be trigonal trigonal pyramidalpyramidal.

11Consider H2O

H •• O •• H••

•• Around Oxygen: 4e- pairs, arranged tetrahedrally.

# of Atoms connected to O = 2. Molecular shape appears bentbent or V-shapedV-shaped: O

about 109° H H

12Shapes of Compounds with multiple bonds

Formaldehyde, HCHO isFormaldehyde, HCHO is

H - C = OH - C = O 4e- pairs on C =>

HH BUT BUT 2 pairs are in a double bond ===> they must they must point in the same general point in the same general direction in spacedirection in space...

13

Thus: the C=O ethe C=O e-- pairs are pairs are considered as 1 pair for considered as 1 pair for geometry purposes.geometry purposes.

So: it seems there are only 3 eit seems there are only 3 e-- pairs around C, and no unshared pairs around C, and no unshared pairs.pairs.

Result: shape of molecule around shape of molecule around O is O is trigonal planar..

14Make predictions for...Make predictions for...

* Ethylene, C2H4 , or

H - C = C - H

H H

* Acetylene, C2H2 , or

H - C C - H

15

New subject... Covalent Bonds - all sharing isn’t equal

Some nuclei have a stronger pull Some nuclei have a stronger pull for electrons than others.for electrons than others.

ElectronegativityElectronegativity describes the relative strength of attraction an atom has for e-.

16MetalsMetals have very low low electronegativity electronegativity values -- they give up e- easily.

Non-metalsNon-metals have larger larger electronegativity electronegativity values -- they attract e-.

Electronegativity increases: across a periodacross a period, & going up a going up a Group or column.Group or column.

17Most electronegative element = Fluorine (F).

Least Electronegative = Cesium (Cs)/Francium (Fr).

Pauling electronegativity scale uses 0 - 4 rating. Most electronegative = 4 Not electronegative = 0

18

When two non-metals form a covalent bond, the e- pair may not be evenly shared between the two nuclei.

If one element is more electronegative than the other, the e- pair is pulled closer to that nucleus.

19Example

H - Cl Cl is more EN than H:Cl is more EN than H:

the electron pair in the bond is the electron pair in the bond is pulled closer to Cl.pulled closer to Cl.

The H-Cl bond is a polar covalent The H-Cl bond is a polar covalent bond, shown as: bond, shown as: + H-Cl The symbol + or - means “partial - charge.”

20A polar covalent bond has a “dipole.” (+ and - ends)

Molecules may have more than Molecules may have more than one polar covalent bond.one polar covalent bond.

Water, H2O has two O-H bonds each having a dipole as

+ H - O - . The entire molecule may be polar...

21if the molecular geometry creates an over-all net dipole moment:

O - net + H H +

Water can solvate ions because it has polarity. (More on this later.)

22A molecule with polar bonds may be non-polar overall if the individual dipoles cancel:

Cl

CCl4 is Cl - C - Cl

Cl

Each C-Cl bond is polar as +C-Cl -.

23

But each C-Cl bond points away in space by 109° in opposite directions:

there is no net dipole moment for CCl4.

We say it is a non-polar molecule.