What is bond enthalpy? Bond enthalpies of polyatomic molecules are average values, whereas those of diatomic molecules can be accurately determined. Why?
Chapter 2: Chemical context of life 1/7/16 Why Chemistry ● Bombardier beetle defense against ants ● Redox reaction ● Multidisciplinary science Matter Sodium + chlorine → sodium chloride (solid + liquid → salt (solid compound)) ● Diverse form ● Elements (92 in nature), compounds ● 6 Essential elements (CHONPS) (carbon, hydrogen, oxygen, nitrogen, prosperous, sulfur) ● Trace elements Atom ● Smallest unit with the properties of an element ● Nucleus ● Protons (+charges) ● Neutrons (no charge) ● Atomic number: protons, and determines what the element is, it doesn’t changes ● Mass number: protons + neutrons, can change, like isotopes Isotopes ● Same element with different # of neutrons ● Same atomic # ● Different mass # ● Can be used for o Carbon dating o Atomic bomb ▪ Iran and Uranium enrichment Energy levels of electrons ● First shell (2e-) ● Second shell (8 e-) ● Third shell (8e-) ● Moving outward from atomic nucleus: energy absorbed ● Moving inward from atomic nucleus : energy lost ● Electron configurations ( SPDF) ● Chemical behavior of atom depends on o Valence shell o Valence electrons Periodic Table ● Atomic number, element symbol, atomic mass ● Ordered by shells Chemical bonds ● When atoms combine to form molecules and ionic compounds o Incomplete→ complete valance shells o Will only combine with another element that completes each other shells o Full 8e- ● Atoms share or transfer valence electrons o Hydrogen bonds Covalent bond ● Two atoms sharing a pair of valence electrons ● 2H can share bonds to become H2 ● Pure element ● Compounds vs. molecule ● Single bond ● Double bond ● Note valence electrons ● Examples o Hydrogen H2 o Oxygen O2 (double bond) o Water H2O o Methane CH4 Electronegativity ● Attraction of a particular atom for the electrons of a covalent bond ● Nonpolar covalent bond (equal pulling) ● Polar covalent bond (unequal pulling) o Ex. Water ▪ 1H and 1 O are negative ▪ 1H is positive o Partial charges; negative and positive o Not pulling hard: positive o Pulling hard: negative ● Ionic bond o So unequal in valence shell attraction that electron stripped away o Bond is formed due to opposite charges ● NaCl→ Na+Cl- o Sodium atom + chlorine atom→ sodium chloride o Na=cation o Cl=anion ● Strong bonds, when you break them apart, they release more energy ● Weak chemical bonds o What’s the advantage ▪ Easier to break, o Hydrogen bonds ▪ Weak bond individually, but when there are a lot of them they can become strong ▪ Ex. Water (H2O) and Ammonia (NH3) ● H positive, N negative