Simple to Complex – Life ’ s Levels of Organization Our journey begins here.

Chemistry and Life What ’ s the difference between an atom and a molecule? What makes atoms hold together to form molecules? Does shape matter?

To Understand the Big, You ’ d Better Know the Small

Atoms and Atomic Structure Atoms are the smallest units of a chemical element. Elements are substances that cannot be broken down to simpler substances by chemical reactions.

In a simplified yet useful view of an atom, electrons orbit a nucleus composed of protons and neutrons. Atoms and Atomic Structure

Atoms Come in Different Forms Called Isotopes Isotopes of a given element have the same number of protons but different numbers of neutrons. Many isotopes are unstable, making them radioactive. Radioactive isotopes (radioisotopes) play an important role in health, medicine and biological research. Three isotopes of hydrogen.

Some Atoms are Sociable, Others Aren ’ t Atoms “ want ” (are most stable) to have a filled outer electron shell. Atoms without a filled outer shell will share electrons with other atoms to accomplish this “ goal. ” Filling outer electron shells controls which atom will pair with which others and in what combinations.

Filling Electron Shells An important rule: the innermost shell holds two electrons; subsequent shells hold 8 electrons.

Pairing for the Greater Good When atoms come together by sharing electrons the bond is a covalent bond. H2H2 A molecule is formed when two or more atoms are bound together covalently.

Drawing It Out The sharing of a pair of electrons between atoms (a covalent bond) is shown as: H-H (for H 2 ) or H-O-H (for H 2 O) or (for CH 4 )

Water - A Most Important Molecule Note how bonding fills all outer electron shells.

Polar and Non-Polar Covalent Bonding Some atoms have an equal affinity for electrons. The covalent bonds of H 2 and CH 4 are non-polar and so are the molecules. If so, the shared electrons spend equal amounts of time around each atom and the covalent bond is non-polar.

Polar and Non-Polar Covalent Bonding Some atoms have an unequal affinity for electrons. Therefore, the covalent bonds of H 2 O are highly polar and so is the molecule. If so, the shared electrons spend more time around one atom relative to another and the covalent bond is polar. Oxygen draws electrons to itself much more strongly than hydrogen.

Polar and Non-Polar Covalent Bonding

The polar versus non-polar distinction determines which molecules will dissolve in a particular solute. For example, sugar dissolves in water, but fat doesn ’ t. The general rule is like dissolves like. The familiar case of oil and water.

Some Atoms Do Almost Anything to Fill Electron Shells Sodium donates a lonely electron to chlorine to complete its outer electron shell. Chlorine is only too happy to accept. The result is ion formation. An ion is an atom or molecule with one or more full positive or negative charges.

Ions and Ion Formation

Ionic Bonds Two oppositely charged ions bind together. This type of chemical bond is an ionic bond. Salts are solids held together by ionic bonds. Ionic bonds are common and important in biology.

Hydrogen Bonding Oxygen and nitrogen are much more “ hungry ” for electrons than hydrogen. The result is a hydrogen bond. Bonds between nitrogen or oxygen and hydrogen are highly polar. This allows bonds to form between partially positive and partially negative atoms in different or (in large molecules) the same molecule.

Hydrogen Bonding Gives Water Unique Properties

Relative Bond Strengths Covalent Ionic Hydrogen >>

Molecular Shape Molecules have distinct shapes – and shape matters.

Molecular Shape A regulatory protein molecule (yellow) binding to DNA. Without complementary shapes, binding would not occur.

Molecular Shape Matters We perceive and distinguish odors because of the particular shape of the odorant (the molecule we smell) and receptor molecules on nose cells.

Biological Chemistry Takes Place in Solutions Molecules are often described as hydrophilic (water- loving) or hydrophobic (water-fearing) on the basis of their solubility in water.