Navigation
Determine if an entire molecule is polar based on its shape and bonds.
Have you ever wondered why oil and water refuse to mix, no matter how hard you shake the bottle? It isn't just a coincidence—it's a microscopic game of 'tug-of-war' where the shape of the molecule decides the winner.
To understand a molecule, we first look at its polar bonds. A polar bond occurs when one atom is more 'greedy' for electrons (higher electronegativity) than the other, creating a partial negative charge and a partial positive charge . However, having polar bonds doesn't automatically make the entire molecule polar. Think of it like a tug-of-war: if two equally strong people pull a rope in exactly opposite directions, the rope doesn't move. In chemistry, if the polar bonds are arranged symmetrically, their charges cancel out, resulting in a nonpolar molecule.
1. Carbon is in the center with two Oxygen atoms on either side. 2. The electronegativity difference makes each bond polar. 3. Because the molecule is linear (a straight line), the two Oxygens pull electrons in opposite directions (). 4. The pulls cancel out perfectly, making a nonpolar molecule.
Quick Check
If a molecule has polar bonds but is perfectly symmetrical, is the molecule polar or nonpolar?
Answer
Nonpolar, because the individual bond polarities cancel each other out.
When a molecule is asymmetrical (lopsided), the polar bonds do not cancel out. This creates a dipole, where one side of the entire molecule is slightly negative and the other is slightly positive. A major cause of asymmetry is the presence of lone pairs—unbonded electrons that take up space and push atoms into 'bent' or 'pyramidal' shapes. Because these shapes are not balanced, the molecule acts like a tiny magnet with a North and South pole.
1. Oxygen has six valence electrons; it bonds with two Hydrogens and keeps two lone pairs. 2. These lone pairs push the Hydrogen atoms down, creating a bent shape. 3. The Oxygen end becomes partially negative , and the Hydrogen end becomes partially positive . 4. Because the shape is not linear, the charges don't cancel. is a highly polar molecule.
Quick Check
What feature of the Oxygen atom in water forces the molecule into an asymmetrical 'bent' shape?
Answer
The lone pairs (unbonded electrons) on the Oxygen atom.
Polarity determines how substances interact. The rule is 'like dissolves like.' Polar molecules (like water) are attracted to other polar molecules and ionic compounds (like salt) because their charges can stick together. Nonpolar molecules (like oil or wax) prefer to hang out with other nonpolar molecules. This is why oil (nonpolar) and water (polar) separate; the water molecules are so attracted to each other that they squeeze the nonpolar oil molecules out of the way!
Consider Methane () vs. Chloromethane (): 1. Both have a tetrahedral geometry. 2. In , all four outer atoms are Hydrogen. The pulls are equal in all directions, so it is nonpolar. 3. In , one Hydrogen is replaced by a 'greedy' Chlorine. 4. Chlorine pulls electrons much harder than Hydrogen, creating an unbalanced pull toward the Chlorine atom. This makes a polar molecule.
Which of the following shapes is most likely to result in a nonpolar molecule if all surrounding atoms are the same?
Why does salt () dissolve in water but not in oil?
A molecule can contain polar bonds but still be a nonpolar molecule overall.
Review Tomorrow
In 24 hours, try to draw the Lewis structures for and and explain to someone why one is polar and the other is not.
Practice Activity
Look at the ingredients on a bottle of salad dressing. Identify which ingredients are likely polar (like vinegar/water) and which are nonpolar (like vegetable oil).