10/7/20131Wed, Oct 9• Lecture 6 (Representation)– The Elements (4.1‐2)– Dalton’s Atomic Theory (4.3)– Formulas of Compounds (4.4)• Questions we’ll answer:– How do we know that there are atoms?Democritus –Atomism (5th Century BCE)• Democritus proposed that if you repeatedly divide an object, eventually you will get to a unit of matter that can no longer be divided…an atom (from the greekatomos, meaning undivided).• The properties of the macroscopic object are determined by the nature of its atoms.– Iron atoms must be solid and strong– Water atoms must be slippery and smooth– Air atoms must be light and whirling10/7/20132Law of Conservation of Mass (c. 1780)When pure substances undergo a chemical change to form new substances, the mass of the product(s) is identical to the mass of the reactant(s)In other words: Mass is neither created nor destroyed.Antoine Lavosier(1743‐1794) and Marie‐Anne Pierrette Paulze(1758‐1836)Law of Constant Composition (1800)A given compound always has the same proportion of elements by mass, regardless of where it comes from or how it was made.Imagine a combustion reaction involving exactly one gram of C and measuring how much O combines with C:First experiment: 1.33 grams of O.Second experiment: 2.66 grams of O.We can conclude that twice as much O must be present in the second compound relative to the first. Joseph Proust (1754‐1826)10/7/20133Dalton’s Atomic Theory (1803)J. Dalton (1766‐1844)• The law of constant composition serves as the gateway to thinking about the fundamental building block of nature.• John Dalton suggests that if the elements making up compounds are themselves composed of individual particles (“atoms”), a given compound would always contain the same number of atoms.• Note: Dalton’s theory provides an atomic description of matter, but there is still uncertainty about the ratio of atoms in compounds (Dalton assumes water is OH).10/7/20134Dalton’s Atomic Theory (1803)• Dalton proposed the following theory of matter:o Elements are made up of atoms.o Atoms of a given element are identical, and different for different elements.o Compounds formed from combinations of atoms. A given compound always has the same atomic composition.o Chemical reactions involve atomic reorganization, but atoms don’t change.• Dalton knows that compounds contain relative amounts of atoms, but he doesn’t know the absolute amounts! • i.e., 1 g C + 1.33 g O  CO, or C2O2, or C3O3, or…• He can only describe compounds in terms of ratios of macroscopic masses, which imply relative ratios of atoms. Law of Combined Volumes (1808)2 L hydrogen + 1 L oxygen = 2 L gaseous water (steam)Joseph Gay‐Lussac performs an experiment where he keeps temperature and pressure constant, and measures the volume of gasses that react with each other.Huge experimental findings: Joseph Gay‐Lussac (1778‐1850)This well‐controlled experiment opens the door to understanding absolute proportions of compounds!1 L hydrogen + 1 L chlorine = 2 L hydrogen chloride gasHydrogen gasChlorine gasHydrogen chloride gas+10/7/20135Avogadro’s Law (1811)2 L hydrogen + 1 L oxygen = 2 L waterGay‐Lussac’s results lead to a hypothesis by Amedeo Avogadro: At the same pressure and temperature, equal volumes of different gasses have the same number of particles.He also concluded from the data that hydrogen, oxygen, chlorine, and hydrogen chloride must be diatomics, and water must be triatomic. Amadeo Avogadro (1776‐1856)But he was ignored.10/7/2013611Clarity from Cannizzaro (1860)• Stanislao Cannizzaro begins to clarify things by using elements from two existing hypotheses:– Compounds contain whole numbers of atoms (Dalton)– Equal volumes of gases under the same condition have the same number of molecules (Avogadro)S. Cannizarro (1826‐1910)• Applying this logic to Gay‐Lussac’s experimental results, Cannizzaro is convinced that hydrogen exists as the diatomic species H2.• He assigns a mass of 2 to H2and uses that assignment to define the masses of other gaseous elements and compounds.Relative Atomic Masses (1860)• Stanislao Cannizzaro suggested a relative atomic mass scale based on hydrogen.• Since Cannizzaro had concluded from experimental data that elemental hydrogen exists as a diatomic, he assigned H2a relative mass of 2 and used this mass to define the masses of other elements and compounds.• For example:– 1.0 L of H2gas at 1 atm and 298 K weighs 0.082 g– 1.0 L of O2at 1 atm and 298 K weighs 1.31 g1.31 g160.082