Intro to Biochemistry

4/9/25

living organisms are composed of about

25 chemical elements

what four elements make up the bulk of living matter?

carbon, hydrogen, oxygen, nitrogen

mass number

• at the top in atomic standard notation

• protons + neutrons

atomic number

• at the bottom in standard notarion

• # of protons

• # of electrons if neutrlal

isotopes

atoms of an element that have different number of neutrons

radioisotopes

• isotopes that emit radiation when decaying; radioactive

• used in carbon dating: measuring the decay of carbon-14 (radioisotope) to determine age of sample

what holds compounds together?

• intramolecular forces

• intra meaning within - between atoms in a compound

ionic bond

• nonmetal and metal

• form molecules

covalent bond

• two nonmetals

• sharing of electrons

non polar and polar

• nonpolar: equal electron sharing, Cl-Cl

• polar: unequal electron sharing, H-Cl

• polarity gives macromolecules its characteristics

EN differences and bond type

• 0≤ Δ EN≤ 0.4, non-polar or slightly polar

• 0.4 < Δ EN < 1.7, polar covalent bond

• Δ EN ≥ 1.7, ionic compound

intermolecular forces

• forces between molecules/ molecules and ions

• much weaker than intramolecular forces

types of intermolecular forces

• london dispersion: in all molecules and atoms

• dipole-dipole: in polar molecules

• hydrogen bonding: in molecules containing hydrogen bonded to F, N, or O

• ion-dipole: mixtures of ionic compounds and polar compounds

london dispersion forces

• also called van der waal’s forces

• weakest force

• all molecules contain them

• nonpolar molecules

• ex. H₂, O₂, Cl₂, CH₄

dipole-dipole forces

• exist between polar molecules only

• slightly negative and slightly positive sides of dipoles interact with each other

• ex. HCl

hydrogen bonds

• exist between polar molecules that contain H-F, H-O, or H-N nonds

• F, O, N are the most electronegative elements

• ex. H₂O, HF, NH₃

ion-dipole

• very strong due to fully charged ion

• between fully charged ion and polar moelcules

• ex. Na⁺ and H₂O

stronger forces =

• higher melting point

• higher boiling point

why does H₂O have a higher boiling point than CH₄?

• water has hydrogen bonding, which is strong intermolecular force

• methane is nonpolar and only has weak london dispersion forces

• stronger forces in water require more energy (higher temperature) to break

a single CO₂ molecule is held together by

intramolecular covalent bonds

dipole-dipole forces occur between

polar molecules

Ion induced dipole

Prescience if ion induces a temporary dipole in a non polar molecule