Lecture 2

BIOLOGY 172, Lecture 2, Unit 1

Orbitals

• Region that electrons move around nuclei in

• Each orbital can hold 2 electrons

• Are grouped into levels (electron shells)

Electron shells

• Orbitals grouped into levels

• Shells are numbered

  • Smaller numbers closer to nucleus

  • Electrons in outermost shell (valence electrons)

Valence ELECTRONS

• Electrons in the outermost shell

Valence

• Number of unpaired electrons in an atom

• Elements commonly have one unpaired valence electron

Electron Shell Organization (How Many Electrons Each Shell Can Hold)

• First Electron Shell: 2 electrons

• Each Subsequent Shell: 8 electrons

Octet Rule

• Atom is MOST STABLE if it has 8 electrons in its valence shell

  • Elements without 8 electrons in valence shell will share/steal electron from another atom

Chemical Bonds in Cells

• Atoms are most table when outermost orbital has either 8 electrons

• LEAST PAIRS of electrons

Molecules

• +2 atoms held together by chemical bonds

Covalent Bonds

• Bonds that hold molecules together

• Atoms share pairs of valence electrons

• Atoms can pair valence electrons in outermost shell to form covalent bond

• Either polar or nonpolar

Electronegativity

• Measure of elements ability to attract electrons

• O>N,S,P>C≈H

Nonpolar Bond

• Covalent bond

• Equal sharing of electrons

• Same electronegativity

Polar

• Covalent bond

• Atoms share electrons unequally

• Partial charges exist

  • Difference in electronegativity

Hydrogen Bonds

• Unequal electron sharing

  • Electrons pulled toward oxygen

  • Difference in electronegativity

• Hydrogen with F,O, N

Ionic Bond

• Molecules gain/ or lose electrons

• Electrons transfer

• Cations and anion interactions

• Are salts

Van der Waals

• Hydrophobic bond

• Results from non-polar covalent bond

• Brief “spots’ of positive and negative charge in MOLECULES with non-polar covalent bond

• Weak interactions when molecules very close

Solvent

• Solution that substances easily dissolve in

H2O and Hydrogen Bonds

• O-H bonds are polar covalent

• Partial positive H attracted to partial negative O

• H2O interacts with other H2O via hydrogen bonds

H2O Interactions with H2O

• H2O interacts with other H2O via hydrogen bonds

• Cohesion and adhesion

  • Adhesion: H2O near surface adhere to glass

  • Cohesion: Net DOWNWARD pull due to surface hydrogen bond interacting with water molecules below

H20 Surface Tension

• H2O has high surface tension

• Ice: Crystal lattice formed

• Liquid water: No crystal lattice formed

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