History of Atomic Theory

Essential Question How did the atomic theory evolve from Dalton's Postulates to the current modern atomic theory?

Ancient Philosophy Who: Aristotle, Democritus When: More than 2000 years ago Where: Greece What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”. Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

Democritus Matter is composed of empty space through which atoms move. Atoms are solid, homogeneous, indestructible, and indivisible. Different kinds of atoms have different sizes and shapes. The different properties of matter are due to the size, shape, and movement of atoms. Apparent changes in matter result from changes in the groupings of atoms and not from changes in the atoms themselves.

Democritus Aristotle

Alchemists Who: European Scientists When: 800 – 900 years ago Where: Europe What: Their work developed into what is now modern chemistry. Why: Trying to change ordinary materials into gold.

Alchemic Symbols

Particle Theory Who: John Dalton When: 1808 Where: England What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom. Why: Building on the ideas of Democritus in ancient Greece.

John Dalton Matter composed of small particles (atoms) Atoms of an element have the same size, mass, and chemical properties. Atoms cannot be created, divided into smaller particles, or destroyed. Atoms of different elements combine in whole number ratios to form compounds (H2O, CO2, MgO, etc.). In a chemical reaction, atoms are separated, combined or rearranged.

John Dalton

Discovery of Electrons Who: J. J. Thomson When: 1897 Where: England What: Thomson discovered that electrons were smaller particles of an atom and were negatively charged. Why: Thomson knew atoms were neutrally charged, but couldn’t find the positive particle.

Cathode Ray Tube Cathode rays are a stream of charged particles Particles carry a negative charge (electrons) Thomson concluded that electrons were smaller than a hydrogen atom (lightest known atom) Conclusion: Electrons are smaller than atoms Dalton was wrong: Atoms were divisible into smaller subatomic particles Cathode rays are a stream of charged particles Particles carry a negative charge (electrons) Thomson concluded that electrons were smaller than a hydrogen atom (lightest known atom) Conclusion: Electrons are smaller than atoms Dalton was wrong: Atoms were divisible into smaller subatomic particles

The Atom, circa 1900: “Plum pudding” model, put forward by Thomson. Positive sphere of matter with negative electrons imbedded in it. In 1911 Ernest Rutherford became interested in studying how positively charged alpha particles interacted with solid matter. He designed a famous experiment sometimes referred to as “gold foil experiment”

Plum Pudding

J. J. Thomson

Atomic Structure I Who: Ernest Rutherford When: 1911 Where: England What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core. Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

Discovery of the Nucleus: Gold Foil Experiment Ernest Rutherford shot  particles at a thin sheet of gold foil and observed the pattern of scatter of the particles. (Zinc sulfide)

The Nuclear Atom Since some particles were deflected at large angles, Thomson’s model could not be correct. This experiment suggested that the atom had a core of some sort – what we now call the nucleus

THE NUCLEAR MODEL According to Rutherford, most of the atom consists of electrons moving rapidly through empty space and a very dense, tiny, positively charged nucleus. Put the size of the nucleus in perspective; green pea in the center of a football field would be the nucleus and the rest of the field would be the “electron” space.

Ernest Rutherford

Other Subatomic Particles By 1920 , Rutherford had refined his concept of the nucleus: He concluded that the very dense nucleus contained positively particles called protons. James Chadwick (1932), a coworker, showed that the nucleus also contained a neutral particle. This was the neutron – a particle with nearly equal mass as a proton.

Atomic Structure II Who: Niels Bohr When: 1913 Where: England What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus. Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

Bohr Model

THE BOHR MODEL

Niels Bohr

Electron Cloud Model Electrons travel around the nucleus in random orbits. Scientists cannot predict where they will be at any given moment. Electrons travel so fast, they appear to form a “cloud” around the nucleus.

                                  Electron Cloud Model

Properties of Subatomic Particles Symbol Location Charge Relative mass Actual mass (g) Electron e- Around nucleus 1– 1 1840 9.11×10-28 Proton p+ Nucleus 1+ 1.673×10-24 Neutron n0 nucleus 1.675×10-24 Summary table – copy into notes

Atomic Structure Timeline Requirements: Must include a title. Must include a spot for each of the 8 items. Must include at least the Who, When, and What. Must have a picture (include color) for each item. Options: Comic book format Standard timeline

Atomic Structure and the Periodic Table Atomic number = # protons 14 Si 28.086 Silicon Symbol Name Atomic Mass: Round to a whole #: 28 = Relative mass of nucleus 14 = # protons 14 = # neutrons

Practice From the Periodic Table Complete the following chart: Name Symbol Relative mass # protons # neutrons # electrons Lithium Mg 26 16