Models of the Atom

Ancient Greece 300 BC in Greece School of thought that matter is made up of tiny indivisible, invisible, indestructible, fundamental units of matter called atoms (means “that which cannot be divided”) Democritus of Abdera most well known atomist. Did no experiments No proof Philosopher

1770s Antoine Lavosier made observations and did experiments resulting in the Law of Conservation of Mass Law of Conservation of Mass Matter cannot be created or destroyed or destroyed Mass of materials before the chemical reaction is the same chemical reaction is the same mass after the reaction mass after the reaction

1799- Joseph Proust Law of Definite Proportions: that the proportion by mass of the elements in a compound is always the same (CO vs. CO 2 ) Part of our definition of a chemical - any substance with a definite composition

1808- John Dalton English School Teacher Theory based on the Greek idea of the atom Atoms were tiny indestructible particles

Dalton’s Atomic Theory 1.All matter is made of invisible and indestructible particles (atoms) 2.Atoms of same element are identical 3.Atoms of different elements differ in physical and chemical properties 4.Atoms of different elements combine in simple whole number ratios to form compounds 5.Chemical Reactions occur when atoms separate, join or rearrange. Atoms of one element NEVER change into atoms of another element.

Problems with Dalton’s Model Atoms are destructible (protons, electrons, neutrons) Atoms of the same element are not completely identical, isotopes exist Atoms of the same element combine with each other (O 2 ) Atoms can “turn into” other elements through nuclear decay

1897 Sir J.J.Thomson British physicist Theory: cathode ray is a stream of negatively charged particles Experiment: cathode ray experiment

Experiment Cathode ray: a tube filled with gas that would glow when an electric current was passed through tube (like a neon sign) Thomson held the “+” pole of magnet next to tube, and the light bent towards magnet Therefore he concluded that there must be some negative charge in atom!! (discovered electron) Also hypothesized that there must be some positive charge in atom (as atoms are neutral)

Plum Pudding Model Electrons are embedded in atom Positive charge floats around rest of atom

Thomson Misconceptions? Placement of electron Location of positive charge

1909 -Ernst Rutherford Physicist and Chemist from New Zealand Proposed: that the theory of Thomson was actually correct Any charge that occurs in atom must occur in whole number ratios Ex: +1 or -1, not + 1.5

Rutherford's Gold Foil Experiment Fired small radioactive particles (basically a helium nucleus) at a piece of gold foil Expected that most of these particles would pass right through the gold foil, deflecting only a little bit * what actually happened was that while that majority did pass right through, those that deflected deflected at many different angles Think of throwing a grain of salt at a chain link fence!

Rutherford’s Experiment

Rutherford’s Conclusion Key Idea -NUCLEAR ATOM All of the mass of the positive particles (protons) is at the center region of the atom Center region called the NUCLEUS Electrons surround the nucleus in a “cloud” Atom is mostly empty space

Rutherford’s Misconceptions Still could not place electrons correctly Was not aware of the neutron

Niels Bohr Danish Physicist Proposed his model in 1915 (this is the one most of you know) He coined the term, “Planetary Model” of atom Electrons orbit the nucleus of atom like planets around the sun Different orbits exist, each having a specified level of energy Suggested that outermost energy levels can hold more electrons than inner energy levels

Bohr’s Experiment Studied line emission spectrum of hydrogen Noticed that when electrons were excited (basically electrified) they would “jump”. Further, he noticed that these jumps were similar in energy This helped to solidify his theory of electrons orbiting the nucleus in specific energy levels

Bohr’s Atom Conclusions: There are certain circular ORBITS in which an electron can travel around the nucleus The farther away from the nucleus, the higher the energy level

Problems with Bohr Electrons do not exist in orbits!

Quantum Model/Schrodinger Theory: states that electron can act as a particle and a wave electrons can exist in any of an unlimited number of energy levels Experiment: Schrodinger wave equation

Quantum Model/Schrodinger Conclusions: Electrons do not orbit nucleus, they exist in orbitals Orbitals are probable locations of the electron There is an unlimited number of energy levels in which electrons can exist Energy increases as you get farther from nucleus Misconceptions?

Quantum Model/Schrodinger

What do we know now?? All models are not exactly correct, but they lead to further understanding and discovery All have important key ideas Atoms are made up of subparticles Atoms are divisible, but not by ordinary chemical means Atoms of elements can vary (isotopes) Electrons reside in ORBITALS, not orbits

Parts of Atom ParticleLocationRelative Mass & SizeCharge ProtonsNucleusHeavy mass E - 24 g + NeutronNucleusHeavy mass E - 24 g No charge ElectronOrbitals outside of nucleus 1.1 E -29 g mass is 1/1836 that of proton _

What happens when you change… Protons: Since the number of protons is the same as the atomic number, protons are what define an element. If you change the protons you change the element Neutrons: The neutrons add to the mass of the element. If you change the neutrons you change the mass of the element (make an isotope) Electrons: The electrons balance the charge of the protons. If you change the electrons you change the charge of the element (make an ion)

Quotes about atoms From A Short History of Nearly Everything by Bill Bryson “protons give an atom its identity, electrons its personality” p. 140 “if an atom were expanded to the size of a cathedral, the nucleus would be only about the size of a fly- but a fly many thousands of times heavier than the cathedral” p. 141