ATOMS The word atom comes from the Greek atomos, meaning “indivisible.” An atom is the smallest identifiable unit of an element. There are about 91 different naturally occurring elements, and over 20 synthetic elements (elements not found in nature). 1

Early Ideas about the Building Blocks of Matter Leucippus (fifth century B.C.) and his student Democritus (460–370 B.C.) were first to propose that matter was composed of small, indestructible particles. Democritus wrote, “Nothing exists except atoms and empty space; everything else is opinion.” They proposed that many different kinds of atoms existed, each different in shape and size, and that they moved randomly through empty space. 2

Early Building Blocks of Matter Ideas Plato and Aristotle did not embrace the atomic ideas of Leucippus and Democritus. They held that matter had no smallest parts (infinitely divisible) different substances were composed of various proportions of fire, air, earth, and water. 3

Early History of Chemistry Greeks were the first to attempt to explain why chemical changes occur. Alchemy dominated for 2000 years. Several elements discovered. Mineral acids prepared. 4 4

Early History of Chemistry Robert Boyle was the first “chemist”. Performed quantitative experiments. Separated chemistry from alchemy Developed first experimental definition of an element. 5 5

Early History of Chemistry Robert Boyle was the first “chemist”. Published landmark publication The Sceptical Chymist in 1661 - Boyle presents his hypothesis that every phenomenon was the result of collisions of particles in motion - emphasized role of experimentation - experimentally rejected idea of elements as earth, wind, fire, and water 6 6

Three Important Laws Law of conservation of mass (Lavoisier): Mass is neither created nor destroyed in a chemical reaction. Law of definite proportion (Proust): A given compound always contains exactly the same proportion of elements by mass. 7 7

Law of Conservation of Mass “We may lay it down as an incontestable axiom that, in all the operations of art and nature, nothing is created; an equal amount of matter exists both before and after the experiment. Upon this principle, the whole art of performing chemical experiments depends.” --Antoine Lavoisier, 1789 © 2012 Pearson Education, Inc.

Definite Proportion

Three Important Laws (continued) Law of multiple proportions (Dalton): When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers. Used idea of relative masses to justify that elements were made of tiny particles (atoms)…. Masses were always proportional because number of atoms was proportional 10 10

Dalton’s Atomic Theory (1808) Each element is made up of tiny particles called atoms. The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.

Dalton’s Atomic Theory (1808) Chemical compounds are formed when atoms of different elements combine with each other. A given compound always has the same relative numbers and types of atoms. Chemical reactions involve reorganization of the atoms—changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.

Avogadro (1811) Unified theories proposed by Gay-Lussac and Dalton Measured (under same conditions of T and P) the volumes of gases that reacted with each other. 13 13

Representing Gay—Lussac’s Results Copyright © Cengage Learning. All rights reserved 14 14

Avogadro (1811) Unified theories proposed by Gay-Lussac and Dalton Ratios of masses in compounds represents ratio of number of atoms 15 15

Avogadro (1811) Avogadro’s Hypothesis At the same T and P, equal volumes of different gases contain the same number of particles. Volume of a gas is determined by the number, not the size, of molecules. 16 16

Avogadro’s Hypothesis

Avogadro’s Hypothesis

Atomic Mass Dalton first proposed idea of comparing RELATIVE weight of elements Constructed a table relative weights of a handful of elements (errors!) While he could measure mass, Dalton had no way of determining number of atoms involved in a compound Assumed that ALL compounds are binary

Atomic Mass Avogadro’s hypothesis allowed chemists to use volumes to determine relative number of atoms and compare relative masses Hydrogen (lightest element) arbitruarily given a mass of 1

Atomic Mass Unit (amu) Defined as 1/12 the mass of a carbon-12 isotope Relative comparative scale for atomic masses Now understood to be equivalent to 1 gram per mole

Modern Understanding of Atom The atom contains: Electrons – found outside the nucleus; negatively charged. Protons – found in the nucleus; positive charge equal in magnitude to the electron’s negative charge. Neutrons – found in the nucleus; no charge; virtually same mass as a proton.

The nucleus is: Small compared with the overall size of the atom. Extremely dense; accounts for almost all of the atom’s mass.

Nuclear Atom Viewed in Cross Section

Isotopes Atoms with the same number of protons but different numbers of neutrons. Show almost identical chemical properties; chemistry of atom is due to its electrons. In nature most elements contain mixtures of isotopes.

Two Isotopes of Sodium