ECA
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Study Notes

Unit 1: Water & Chemical Bonds

Water Is Polar

In a water molecule (H₂O), oxygen pulls shared electrons closer to itself than hydrogen does. This gives oxygen a partial negative charge (δ−) and each hydrogen a partial positive charge (δ+). A molecule with uneven charge distribution like this is polar.

  • Oxygen = partial negative (δ−); hydrogens = partial positive (δ+)
  • Polarity is what makes water such a good solvent for ions and other polar molecules
  • Polar and charged substances are hydrophilic ("water-loving"); non-polar substances are hydrophobic ("water-fearing")

Hydrogen Bonds

A hydrogen bond is a weak attraction between a partially positive hydrogen (one that is covalently bonded to O or N) and a partially negative atom (such as oxygen) on a neighboring molecule. Hydrogen bonds are NOT covalent bonds — no electrons are shared.

  • Each oxygen atom can make 2 hydrogen bonds (using its two lone pairs)
  • Each hydrogen atom can make 1 hydrogen bond
  • The hydrogen must be attached to a very electronegative atom (O or N). Hydrogens bonded to carbon do not form hydrogen bonds
  • When drawing hydrogen bonds between alcohol or water molecules, the dotted line goes from the δ+ H (on an O–H) to the O of another molecule

Water as a Solvent

When an ionic solid like table salt (NaCl) dissolves, water molecules surround each ion. The oxygen end of water (δ−) is attracted to the positive sodium cation (Na⁺), and the hydrogen ends (δ+) are attracted to the negative chloride anion (Cl⁻). This pulls the crystal apart.

  • Na⁺ attaches to the OXYGEN of water because oxygen has a partial negative charge
  • Cl⁻ attaches to the HYDROGENS of water because hydrogens have a partial positive charge

Covalent vs. Ionic Bonds

Covalent bonds form when atoms share electrons. Ionic bonds form when electrons are transferred from one atom to another, creating oppositely charged ions that attract each other.

  • CO₂ — covalent: carbon and oxygen atoms share electrons
  • NaCl — ionic: an electron moves from sodium to chlorine, making Na⁺ and Cl⁻ ions
  • In its most stable configuration, oxygen forms 2 bonds and nitrogen forms 3 bonds
  • Carbon forms 4 bonds; hydrogen forms 1
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