Atomic Theory The thesis that all elements are composed of tiny, indestructible particles called atoms All atoms of the same element are alike and have the same mass

How Small? How Big? If an atom occupied a football field, the nucleus would be like a marble in the center.

Democritus (460 – 370 B.C.) All matter is made of small, indivisible particles called “atomos” also know as atoms. His model was a tiny, hard, uncuttable sphere.

John Dalton (1766-1844) The ratios of the masses of elements in a compound can always be reduced to small whole numbers Law of Multiple Proportions

Dalton’s Atomic Theory 1) all matter is composed of tiny particles called atoms 2) the atoms of an element are always identical while the atoms of different elements are different 3) compounds form when atoms combine; atoms combine in small whole number ratios 4) reactions involve reorganization of atoms; the atoms themselves do not change

Dalton Proposed the “Billiard-ball model” of the atom His model was a solid, indivisible sphere

J.J. Thomson 1897 Produced a “cathode ray” which was deflected by a negative electric field Thus the ray must be made of negative particles (electrons) Thompson was awarded the 1906 Nobel Prize for his discovery

J.J. Thomson and Crooke Since atoms are neutral, they must also have a positive area Plum pudding model

Ernest Rutherford (1911) Tests Thomson’s Plum Pudding Model by shooting alpha particles through a sheet of gold foil

Ernest Rutherford Nuclear Model of the Atom

Protons were found to be 1836 X the mass of an electron Charge of proton is +1

James Chadwick 1932 Discovered high energy particles with no charge and the same mass as the proton – the neutron

Niels Bohr (1912) Electrons “orbit” the nucleus somewhat like planets orbit the sun Planetary Model

Arnold Sommerfeld Expanded the Bohr model Electrons travel in orbitals, but the orbitals are not the same shape -- this leads to the electron cloud model of the atom

Electron Cloud Model

ERWIN SCHRODINGER Was awarded the 1933 Nobel Prize for his Electron Cloud Model His model was based on mathmatical probabilities, not a physical model.

Modern View of the Atom Tiny nucleus surrounded by electron “cloud” Nucleus accounts for all of the mass Arrangement of electrons causes different chemical properties

Electron Cloud Model Note: Just as no map can equal a territory, no concept of an atom can possibly equal its nature. These models of the atom simply served as a way of thinking about them, though they contained limitations (all models do).

How do we know the identity of an element? The number of protons (atomic #) distinguishes one element from another

Mass Number The total number of protons and neutrons in the nucleus (protons + neutrons=mass number of an element)

Isotopes Isotopes are atoms that have the same atomic number but a different mass number Same element Different numbers of neutrons in the nucleus Ordinary Hydrogen Deuterium Tritium proton neutron

Atomic Mass The average mass of an element Most elements occur as two or more isotopes in nature, and therefore have differing atomic masses—this is why the atomic mass is an average, each isotope is figured in to the average

What Can We See? This is the surface of a platinum-nickel alloy as imaged by a Scanning Tunneling Microscope. Vienna University of Technology Light colors are closer, black deeper in the surface. The brighter blobs are the Ni atoms. The very large white blobs are surface impurities. Ni has 28 protons and 30 neutrons, which are not distinguishable.

An Alloy In The Process … These are lead atoms working their way into copper to form an alloy.

Metals Hard Shiny – have luster Ductile Malleable Good conductors of heat and electricity

Nonmetals Dull Brittle solids or gases at room temp. Insulators

Metalloids They have properties of both metals and nonmetals. Found on each side of the zig zag line between metals and nonmetals

Alkali Metal Group Group 1 or IA (except for H) Most active metal group Soft, shiny, very reactive with H2O Wants to lose 1 electron

Alkaline Earth Metal Group Group 2 or IIA Harder than the alkali metals Wants to lose 2 electrons

Transition Metals Group 3-12 or IB-VIIIB

Halogens Group 17 or VIIA Most active group of nonmetals Wants to gain 1 electron

Noble Gas Family Group 18 or VIIIA Inert So stable they do not want to react with other elements

Periodic Law The properties of the elements are periodic functions of their atomic numbers Henry Moseley-arranged the periodic table by increasing atomic number (# of protons) instead of increasing atomic mass.

Period or series Horizontal rows on the Periodic Table

Groups or Families Vertical columns on the Periodic Table

The End