Each box below illustrates an atomic model proposed during the course of history.

1. Each box below illustrates an atomic model proposed during the course of history. Predict the order in which each model was introduced into the science community. Justify why you think your arrangement is correct. Bohr Model

2. DEMOCRITUS'S ATOMIC THEORY • This is the Greek philosopher Democritus who began the search for a description of matter in 400 B.C. (over 2400 years ago). To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together. He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? He Claimed: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”

3. Dalton’s Atomic Theory“Billiard Balls”1803 DALTON'S ATOMIC THEORY John Dalton (1766-1844) was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into color blindness . He proposed the Atomic Theory of Matter based on his experimental observations. With few exceptions, his theory is accepted today. Thus, Dalton is known as the ”Father of Modern Chemistry”. Dalton claimed: • All matter is made of atoms. Atoms are indivisible and indestructible. • All atoms of a given element are identical in mass and properties. • Compounds are formed by a combination of two or more different kinds of atoms. • A chemical reaction is a rearrangement of atoms. • Atoms can be neither created nor destroyed. BilliardBall Atomic Model

4. JJ THOMSON'S ATOMIC THEORY“Plum Pudding”1897 Untilthe end of the nineteenth century the concept of atom was similar to a small solid billiard ball.In the year 1897, Joseph John Thomson • totally changed the view of an atom by discovering electron. • Thomson’s atomic theory suggested that the atom containssmaller pieces calledelectrons and protons. J.J. Thomson suggested the "plum pudding" atomic model. In this model the electrons and protons are uniformly mixed throughout the atom:   • JJ Thompson claimed: • An atom consists of a sphere of positively charged protons with negativelycharged electronsembedded in it. • The positive and the negative charges inside an atom are equal to one another. Thus, the overall charge of the whole atom is neutral.

5. HantaroNagaoka’sSaturnian Model of an Atom1904 HantaroNagaoka developed the earliest published quasi-planetary model of the atom. This graduate of the University of Tokyo from 1887 spent his postdoctoral period in Vienna, Berlin and Munich before obtaining a professorship in Tokyo to become Japan's foremost modern physicist. Nagaoka claimed: • An atom is a large, massive, positively charged sphere, encircled by very many light-weight, negatively charged electrons. • An atom’s electrons are bound by electrostatic forces that are analogous to Saturn's ring, which is stabilized and attracted to the heavy planet by gravitation and consists of a myriad of small fragments.

6. Rutherford Atomic Theory“Planetary Model”1911 Rutherford tested Thomson's hypothesis by devising his "gold foil" experiment. Rutherford reasoned that if Thomson's model was correct then the mass of the atom was spread out throughout the atom. Then, if he shot high velocity alpha particles (helium nuclei) at an atom then there would be very little to deflect the alpha particles. He decided to test this with a thin film of gold atoms. As expected, most alpha particles went right through the gold foil but to his amazement a few alpha particles rebounded almost directly backwards.     These deflections were not consistent with Thomson's model. Rutherford was forced to discard the Plum Pudding model and reasoned that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in a dense center. Rutherford claimed: 1. Most of an atom’s mass is located at the center of an atom 2. Outside an atom’s nucleus is mostly empty space. Rutherford developed the planetary model of the atom which put all the protons in the nucleus and the electrons orbited around the nucleus like planets around the sun.  

7. Bohr’s Atomic Theory1913 Neils Bohr refined Rutherford's model in 1913. Bohr's model was so successful that he immediately received world-wide fame. Unfortunately, Bohr's model worked only for hydrogen. Thus, the final atomic model was yet to be developed. Bohr claimed: 1. Electrons orbit the nucleus without losing energy. 2. Electrons could move only in fixed orbits of specific energies. 3. Electrons with low energy would orbit closer to the nucleus while electrons with high energy orbit further from the nucleus. Bohr’s Atomic Model

8. Erwin Schrödinger’s Atomic Theory1926 Erwin Schrödinger built upon the thoughts of Bohr yet took them in a new direction.  Schrödinger claimed: • One can not say with any certainty, where the electron actually is at any point in time, yet can describe where it ought to be.   • One can predict where an electron has probably been and where it is likely going to be. • As an electron moves, it leaves a trace of where it was and the collection begins to resemble a cloud.

9. James Chadwick’s Atomic Theory1932 Until 1932, the atom was believed to be composed of a positively charged nucleus surrounded by negatively charged electrons. In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. Chadwick claimed: • The radiation he produced was composed of particles with a neutral electrical charge and the approximate mass of a proton. • The neutral particles became known as neutrons. With the discovery of the neutron, an adequate model of the atom became available to chemists. • Isotopes of the same element have the same number of electrons and protons but differ in the number of neutrons in their nucleus. This explains why Carbon (C), for example, has a radioactive isotope known as Carbon-14. Modern Atomic Model