Agenda Development of Atomic Model History Atomic Structure Simulation

Watch Video & Answer Questions 1-6 http://www.schooltube.com/video/52bb23 430c9c4e0ebb74/Bill%20Nye%20- %20Atoms 5:13 8:37

Evolution of the Atomic Model Since atoms are too small to see even with a very powerful microscope, scientists rely upon indirect evidence and models to help them understand and predict the structure of an atom.

Democritus & Leucippus (~400 BC) Greek philosophers first to propose that matter made up of tiny, indivisible particles called atomos, the Greek word for atoms

Robert Boyle Proposed that an element is a substance that cannot be broken down into simpler substances

Antoine Lavosier: 1800s His experiments supported the law of conservation of mass: matter cannot be created or destroyed Supported Boyle’s claim that an element could not be broken down by chemical methods.

John Dalton (1803) Dalton theorized that the basic unit of matter is a tiny particle called an atom Dalton’s Theory: All elements are composed of indivisible (can’t be broken down) atoms All atoms of a given element are identical Atoms of different elements are different; Compounds are formed by the combination of atoms of different elements

John Dalton (1803) Billiard Ball Model: An atom is represented by a hard sphere (like the particle diagrams we have been drawing)

JJ Thomson (1897) Used a cathode ray tube to show one of the smaller units that make up an atom Because the cathode ray deflected towards the + charged plate when an electric field was applied, Thomson concluded that the ray was formed by particles and the particles were negatively charged

JJ Thomson (1897) Thomson discovered that the atom is made up of small, negatively charged particles which he called electrons Developed the Plum Pudding Model of the atom

Rutherford: Gold Foil Experiment

Ernest Rutherford (1909) Conducted the gold-foil experiment: Directed alpha particles (small, positively charged particles), at a thin piece of gold foil Most of the alpha particles passed straight through the foil and a few were slightly deflected Some of the alpha particles were greatly deflected and bounced back  

Ernest Rutherford (1909) Rutherford concluded that atoms have a dense, central core called the nucleus, while the remainder of the atom is essentially empty space Positively charged particles known as the protons are found in the nucleus

James Chadwick First to prove the existence of the neutron Provided explanation as to why the positively charged protons in the nucleus stayed intact and did not repel each other.

Neils Bohr (1913) Bohr Model a.k.a. “Planetary Model” or “Electron-Shell Model” The nucleus contained the protons and neutrons The electrons orbited around the nucleus (like planets orbiting the sun)  

Electron-Shell Model Electrons are shown in concentric circles or shells around the nucleus The 1st shell can hold a maximum of 2 electrons The 2nd shell can hold a maximum of 8 electrons The 3rd shell can hold a maximum of 18 electrons Electrons in the outermost shell are called the valence electrons

Wave-Mechanical/Cloud Model (modern, present-day model) Developed after the famous discovery that electrons can be viewed as both a wave and particle Like planetary model, atom is pictured as having dense, positively charged nucleus The difference in this model is how the electrons are pictured

Wave-Mechanical Model Electrons have distinct amounts of energy and move in areas called orbitals (clouds) An orbital is a region in which an electron is most likely (most probable) to be located Value of l Orbital (subshell) Orbital Shape Name* s sharp 1 p principal 2 d diffuse 3 f fine

Structure of an Atom Complete WS regarding atomic structure based on simulation: http://phet.colorado.edu/en/simulation/build- an-atom