AP Chemistry Evolution of Atomic Theory Basic structure of the atom

Democritus – 400 B.C. Greek Philosopher Imagined particles that were indivisible Constituents of matter Atom comes from “atomos” Opposed Aristotle

Aristotle – 350 B.C. Widely accepted theory that all matter can be continually divided. Set science back for thousands of years.

John Dalton Theory Four postualtes All matter is made of atoms. Atoms are indivisible and indestructible. All atoms of a given element have identical masses and properties. Compounds are formed by a combination of two or more different kinds of atoms. A chemical reaction is a rearrangement of atoms.

Laws! Law of Constant Composition – In a given compound, the relative numbers and kinds of atoms are constant. (CO2 and H2O) Conservation of Mass – The total mass of products equals the mass of the reactants. Mass is not created or destroyed in a reaction. Law of Multiple Proportions – If 2 elements combine to form more than one compound, the masses exist in small whole number ratios.

DALTON’S MODEL OF THE ATOM

WHAT’S RIGHT / WRONG Right – Wrong – Postulates 3 and 4 Postulate 1 – smaller particles exist, and atoms can be collided Postulate 2 – isotopes!

J.J. THOMSON Professor of physics in Manchester Used the cathode ray tube! Determined the mass/charge ratio of the electron. 5.69 x 10-9

CATHODE RAY TUBE Different charges (+) and (-) attract Like charges (-) and (-) repel Bent beam and negative magnet Showed negative charge in atom called… ELECTRON! Proposed a model of the atom that was called the “plum pudding” model

WILLIAM THOMSON

WHAT’S RIGHT / WRONG RIGHT – WRONG – electrons exist the rest of the atom isn’t just positive space

ERNEST RUTHERFORD Performed the famous gold foil experiment Determined 3 things The atom is mostly empty space The nucleus is positively charged The nucleus is a small dense part of the atom

RUTHERFORD’S GOLD FOIL EXPERIMENT

GOLD FOIL EXPLAINED Why did most past through? Most of atom is empty space. Why were some deflected? They hit the nucleus. What does that say about the nucleus? It had a positive charge. It was very dense and massive.

RUTHERFORD’S MODEL OF THE ATOM

WHAT’S RIGHT / WRONG Right – Wrong – Dense, positive nucleus at the center of the atom Most of atom is empty space Wrong – Electrons can’t just float around in empty space

NIELS BOHR Observed spectral lines for hydrogen Proposed an orbit theory of the electron around the atom.

Bohr’s Model Electrons can only be in certain energy levels Electrons don’t just float around, they have a distinct orbit. There is a certain amount of energy in these levels.

Bohr’s Model This certain energy is the energy to pull the electron away. The closer you get to the nucleus….. a) the harder it is to pull electron away b) the greater the negative (-) c) the greater the attraction

What’ correct about Bohr’s Model? 1) Spectral lines are produced by electrons falling 2) Works for hydrogen

What’s wrong with Bohr’s Model? 1) Thought that electrons orbit around nucleus like planets. 2) Doesn’t work for any other element. 3) He treats electrons like particles.

Schrodinger’s Model The Quantum Mechanical Model Erwin Schrödinger - 1926 Developed a wave equation. Mathematical function that described the nature of the electron. The Quantum Mechanical Model

Schrodinger’s Model The Quantum Mechanical Model Treats electrons like waves All about the probability of finding the location of an electron We get atomic orbitals and what these orbitals look like. All mathematical

Heisenberg Uncertainty Principle Can’t know speed and location of electron at the same time. The more you know of one, the less you know of the other.