DEVELOPMENT OF THE ATOMIC THEORY Atom- smallest particle into which an element can be divided and still be the same substance Called the building blocks of matter! Atoms make up elements, elements combine to make compounds!

DEVELOPMENT OF THE ATOMIC THEORY Theory = unifying explanation for a broad range of hypotheses and observations that have been supported by testing and experimentation

DEMOCRITUS PROPOSES THE ATOM 440 BC- Greek philosopher Democritus Proposed that you would eventually end up with the “uncuttable” particle called the atom All atoms are small, hard particles made of a single material formed into different shapes and sizes Atoms are always moving and form different materials by joining together

A. Ancient Greece (2000+ years ago) Democritus Believed that matter could not be continuously divided Matter consists of small indivisible particles “Atom” = indestructible Particles are in continuous motion Four elements make up all matter and energy Earth,Wind,Water, & Fire No scientific evidence to show this.

ARISTOTLE DISAGREES! Greek philosopher Aristotle disagreed with Democritus He believed that you would never end up with an indivisible particle TURNS OUT: Aristotle was wrong!

EARLY 1800S Scientists learned that elements combine in specific proportions based on mass to form compounds

DALTON CREATES ATOMIC THEORY BASED ON EXPERIMENTS! John Dalton, British chemist and school teacher, 1803: 1.All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed 2.Atoms of the same element are exactly alike, and atoms of different elements are not alike 3.Atoms join with other atoms to make new substances So at this point we believe that an atom is like a small solid ball of matter that cannot be split up

19 TH CENTURY: NOT QUITE CORRECT YET! 19th century: scientists agreed that Dalton’s theory explained many of their observations but not all of them Dalton’s atomic theory was revised to more correctly describe the atom

THOMSON FINDS ELECTRONS IN THE ATOM! British scientist JJ Thomson discovered an error in Dalton's theory with a cathode ray tube! Thomson discovered: There are small particles inside the atom Atoms cannot be divided into smaller parts Positively charged plate attracts the beam, so the beam was negatively charged particles called corpuscles, now called electrons

THOMSON REVISES ATOMIC THEORY! Why? To account for electrons Proposed a plum pudding model! Also called the chocolate chip cookie model! model= representation of an object or system, a picture

In 1897 J.J. Thomson discovered the electron The distribution of charge and mass in the atom was unknown. Thomson proposed a 'plum pudding' model, with tiny, negatively charged electrons embedded in a positively charged sphere. This plum pudding model was generally accepted. Even Thomson's student Rutherford, who would later prove the model incorrect, believed in it at the time.

REVIEW QUESTIONS! 1.What discovery demonstrated that atoms are not the smallest particles? 2.What did Dalton do in developing his theory that Democritus did not do? 3.Why was it important for Thomson to repeat his experiments?

RUTHERFORD OPENS AN ATOMIC “SHOOTING GALLERY” Thomson's student Ernest Rutherford tested his teachers theory Aimed a small beam of small positively charged particles at a thin sheet of gold foil surround by zinc sulfide which glowed when particles struck it Thomson thought all particles would pass through the gold in a straight line But some of the particles were deflected and bounced back!

RUTHERFORD’S EXPERIMENT SET UP

RUTHERFORD REVISES THE ATOM! 1911: Plum pudding model proved to be wrong with his experiment! New model: Has a nucleus= center of the atom that is tiny, dense and positively charged Most of the atom was empty space

Ernest Rutherford wanted to determine what an atom looked like. Fired + charged alpha particles at a very thin piece of gold (Gold foil) Put a screen behind the foil to determine what happens Radioactive source Gold foil Screen Most of the rays went straight through the foil Some rays deflected A very few rays came straight back

- - - Rutherford explained this by stating that a. Atoms have mostly empty space This is why most rays went straight through b. There must be a dense positive center to an atom This is why the positive rays deflected. (Positive rays are repelled by positive charged objects) Later experiments showed that electrons exist in the space between nuclei + Rutherford model

BOHR STATES THAT ELECTRONS CAN JUMP BETWEEN LEVELS! 1913: Danish scientist, Niels Bohr suggested that electrons travel around the nucleus in a definite paths. The paths are located on different levels which were different distances from the nucleus. Electrons can jump to different paths. Electrons (negative particles) Bohr suggested electrons travel around the nucleus like planets around the sun

e- E. Bohr Neils Bohr looked at the arrangement of electrons Electrons exist in definite areas around the nucleus Further from the nucleus, an electron had more energy Electrons can gain energy and “jump” to higher levels They can then give off the energy as they jump back down nucleus e- Energy levels e- M L K Planetary model

THE MODERN THEORY: ELECTRON CLOUD Current theory: revised by Erwin Schrödinger and Werner Heisenberg by using quantum mechanics! Electrons are found in the electron cloud because we are uncertain of exactly where they are at a given time

F. Electron Cloud (Wave Mechanical Model) Modified Bohr’s model Electrons are not in circular orbits, but exist in specific spaces around the nucleus (orbitals) Pattern is random, unpredictable level nucleus

As a planet moves around its sun, a definite path can be plotted for it called an orbit. The familiar 20th century atomic model originally postulated by Neils Bohr in 1913 looks similar and features the electron orbiting around the nucleus as in the hydrogen image below. The actuality is somewhat different, as electrons inhabit regions of space known as orbitals. An atomic orbital is essentially an electron "cloud" describing the probability of the position of the electrons. Therefore, an electron can be found within the range of an atomic orbital's shape at any point in time. This however, does not determine the exact position of the electron because it is constantly moving. However, if an electron is in a particular orbital it will have a particular definable energy.

REVIEW QUESTIONS! 1.In what part of an atom is most of it’s mass located? 2.What are the 2 differences between the atomic theory described by Thomson and that described by Rutherford? 3.Identify the difference in how Bohr’s theory and the modern theory describe the location of elements.