Biology 1 Lecture 1.1

Chemical bond

An attractive force linking 2 atoms together in a molecule

Covalent bond

Forms when 2 atoms share electron pair(s) in their valence shells. Are very stable and break only with a large input of energy

Electronegativity

• A measure of how strongly an atom attracts shared electrons in a chemical bond

• Depends on # of protons and the distance between protons and valence electrons

Polarity of covalent bond

• Depends on differences in electronegativities of the 2 atoms making the bond

• Nonpolar covalent: x1-x2 < 0.4

• Polar covalent: 0.4< x1-x2 <1.8

• Ionic bond: x1-x2 > 1.8

Properties of carbon

• The most versatile element on Earth in bonding due to its atomic structure

• Has 4 unpaired valence electrons and can form 4 covalent bonds

• Can form molecules of widely different sizes, shapes, and composition

  • Length, double bond position, branching, presence of rings

Ionic bonds

• Formed by electrical attraction between positively and negatively charged electrons

• Are strong in solids (salts) but weaken in aqueous solutions

Hydrogen bond

• Is a weak electrical attraction that forms between a partial positive charge on H and a partial negative charge on O or N

• Forms between a H atom involved in a polar covalent bond and an electronegative atom

Water properties

• Has a unique structure and special chemical properties

  • Small size, polar covalent bonds, bent shape, polar molecule

• Has the ability to form hydrogen bonds

• Capacity to form H-bonds gives it unique properties important to life

  • Excellent solvent for polar compounds

  • High surface tension by cohesion

  • High heat capacity

  • High heat of vaporization

  • Liquid has higher density then solid

• Ice is less dense than liquid water b/c of H-bonding differences in states of water

  • Read slide

• One water molecule can form max of 4 H-bonds at a time

Hydrophobic effect

• Drives nonpolar molecules together in aq solution to avoid contact with water

• Hydrophilic (polar or charged) likes water

• Hydrophobic (nonpolar) avoids water

Van der Waals forces

• Are weak, transient attractions that occur when nonpolar molecules are close together

• Electrons are constantly moving/fluctuating which causes temp dipole

• One temp dipole can induce another

• Many vdw forces are significant