Chem. 1-2 Chapter 8 Molecular Shape

Formulas Molecular Formula: –Specifies how many atoms are in a single molecule of a compound: –Ex. Glucose is C 6 H 12 O 6 Empirical Formula: –Specifies the ratios in which molecules are present. –Ex. Glucose empirical formula is C 3 H 6 O 3 –You simply divide until a subscript cannot be divided by a whole number

Formulas….. Structural Formula: –Shows the arrangement of atoms in a molecule… The Lewis structures you have been doing. However, structural formulas don’t give information on the shape of the molecule. This information is often shown with ball and stick models

Shapes Shapes are most often symmetrical. Why? Valence electrons will arrange themselves as far away from each other as possible. This is called the Valence-Shell Electron Pair Repulsion Theory (VSEPR) –Exceptions to this are molecules involving transition metals. There are five main shapes of molecules

Linear A straight line. All molecules with only 2 atoms. But CO 2 is too….. –This is because the valence electrons try to get as far away from each other as possible. –No unshared central electrons. Bond angle is 180°

Trigonal Planar “Triangular laid out flat” Example BCl 3 Generally when a central atom has 3 surrounding atoms AND the central atom has no unshared pairs of electrons. Bond angle is 120°

Tetrahedral “4 surfaced shape” Ex. CH 4 It is like a pyramid with a flagpole on the top. It is not a flat shape because the electrons can be further apart (at an angle of 109.5°) using a 3D spread instead of 90° as a flat shape.

Pyramidal Specifically a 3-sided base (not like the Egyptian pyramids) When we have a central atom with unshared electron surrounded by 3 other atoms. Ex. NH 3 Bond angle is 107° Unshared pairs repel stronger than bond pairs

Bent Example : H 2 O Again, the unshared pairs exert a greater repulsion force. So the 2 H atoms are separated by a slightly smaller angle than one would expect. Bond angle is 105°

Home work Write the Structural Formula (Lewis Structure) and predict/draw the shape of the following molecules: CCl 4, HCN, BF 3, BeCl 2, H 2 S, NF 3, PCl 3, OF 2, SiO 2, CF 4 Due Tomorrow

Bond Length The distance between nucleii of atoms in a molecule. Two trends…. –Triple bonds are shorter than double bonds which are shorter than single bonds. –As you move down a group, atoms are _________, therefore bond length is _______

Polarity Polar molecules remember from ch 7? Polar molecules form Dipoles, meaning two poles – the molecule has a negative and a positive end. If a molecule has only non-polar (less than 0.4 E.N. difference) bonds it is a non-polar molecule. Strangely, if a molecule contains polar bonds, it is not necessarily polar. If symmetry cancels out the polarity, the molecule is non-polar.

Polarity…. The shape of a molecule AND the polarity of its bonds determine molecule polarity. We must look again at electronegativity… the ability of an atom to attract electrons in bonds. Remember, electronegativity increases going left to right in a period. Electronegativity increases going up a group. See table on page 241

Why is polarity important? Polar molecules only dissolve polar molecules Non-polar molecules only dissolve non- polar molecules.

How do we figure out if a molecule is polar? Draw ball and stick model. Look at the electronegativity difference between each atom involved in a bond. Draw arrows in the direction of highest E.N. on each bond. The length of each arrow is proportional to the E.N. difference across each bond. Imagine the arrows show an electron tug of war….

If the tug-of-war is a draw, the molecule is non-polar. There is symmetry of electronegativity. If there is not good symmetry, the molecule is polar.

Polarity …. Lets take the examples of H 2 CO, CO 2 and H 2 O.