The Atom Chapter 4

History of the Atomic Theory A. Democritus B. Aristotle C. Lavoisier D. Proust E. Dalton F. Modern Atomic Theory History of Atomic Structure A. Thomson B. Milikan C. Rutherford D. Bohr E. Chadwick F. Quantum Atom Subatomic Particles A. Atomic Number B. Mass Number and Isotopes C. Electrons and Ions D. Nuclear and Hyphenation Notation E. Average Atomic Mass Weighing and Counting Atoms A. Mole Atoms B. Mole Mass C. Mass Atoms

I. History of the Atomic Theory Remember: a scientific theory explains behaviors and the ‘nature’ of things Theories can be revised when new discoveries are made The theory describing the composition of matter has been revised many times

I. History of the Atomic Theory Democritus (460-370 BC) 1.Matter is made up of “atoms” that are solid, indivisible and indestructible 2.Atoms constantly move in space 3.Different atoms have different size and shape 4.Changes in matter result from changes in the grouping of atoms 5. Properties of matter result from size, shape and movement

I. History of the Atomic Theory B. Aristotle (384-322 BC ) & Others 1. Four kinds of matter a. Fire – Earth – Water – Air 2. One kind of matter can transform into another 3. Rejected idea of the “atom” (idea then ignored for almost 2000 years 4. This theory was more popular and it was easier to accept

Aristotle’s Theory of Matter

I. History of the Atomic Theory Antoine Lavoisier (1770s) 1. Experiment: 2 Sn + O2  2 SnO tin oxygen tin (II) oxide mass before reaction = mass after reaction 2. Law of Conservation of Mass a. Matter cannot be created or destroyed (in a chemical or physical change)

I. History of the Atomic Theory Joseph Proust (1779) 1. Develops Law of Definite Composition- all samples of a specific substance contain the same mass ratio of the same elements a. ex: all samples of CO2 contains 27.3% carbon and 72.7% oxygen b. therefore ‘elements’ are combining in a whole number ratio – WHY????

I. History of the Atomic Theory - Dalton John Dalton (1803) 1. Develops Law of Multiple Proportions a. describes the ratio of elements by mass in two different compounds composed of the same elements 2. Example: carbon monoxide carbon dioxide 1 part oxygen : 2 parts oxygen *when compared to the same amount of carbon in each compound

I. History of the Atomic Theory- Dalton 3. Dalton collects data and develops his atomic theory in 1803

4. Dalton’s Background Dalton became a school teacher at the age of 12 (he left school at age 11) loved meteorology - pioneer in this field studied works of Democritus, Boyle and Proust Wrote New System of Chemical Philosophy in 1808

5. Dalton’s Atomic Theory 1. Matter is made of small particles-atoms 2. Atoms of a given element are identical in size, mass, but differ from those of other elements*. 3. Atoms cannot be subdivided or destroyed*. ( supports law of conservation of mass) 4.Atoms combine in small whole number ratios to form compounds. (def comp,Mult prop) 5. Atoms combine, separate, or rearrange in chemical reactions. * Modified in Modern Atomic Theory

JUST A THEORY……. But it lead to the Modern Atomic theory

F. Modern Atomic Theory All matter is made up of small particles called atoms. Atoms of the same element have the same chemical properties while atoms of different elements have different properties Not all atoms of an element have the same mass, but they all have a definite average mass which is characteristic. (isotopes)

F. Modern Atomic Theory Atoms of different elements combine to form compounds and each element in the compound loses its characteristic properties. Atoms cannot be subdivided by chemical or physical changes – only by nuclear changes

I. History of the Atomic Theory 1803 1897 1909 1913 1935 Today solid particle electron proton e- orbit nucleus neutron Quantum Atom theory Dalton Thomson Rutherford Bohr Chadwick Schrodinger and others

History of the Atomic Structure A. J.J. Thomson (1856-1940) Experiments with cathode ray tubes a. atoms have (-) charged particles which are smaller than atoms b. determined charge/mass ratio of the “electron”

Voltage source - + Vacuum tube Metal Disks

Voltage source - +

Voltage source - +

Voltage source - +

Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

Voltage source - + Passing an electric current makes a beam appear to move from the negative to the positive end

Voltage source By adding an electric field

Voltage source + - By adding an electric field

Voltage source + - By adding an electric field

Voltage source + - By adding an electric field

Voltage source + - By adding an electric field

Voltage source + - By adding an electric field

Voltage source + - By adding an electric field he found that the moving pieces were negative

Demonstration of the cathode ray experiment.

Thomson’s Model The Pudding Model a. electrons present b. atom is like plum pudding - bunch of positive stuff (pudding), with the electrons suspended (plums)

History of the Atomic Structure B. Robert Milikan (1868-1953) 1. Oil Drop Experiment (1909) a. Discovered mass and actual charge of electron (-1) b. Mass is 1/1840 the mass of a hydrogen atom 1) e – has a mass of 9.11 x 10-28 g

Oil Drop

History of the Atomic Structure – Summary thus far So, at this point we know: - Atoms are divisible into smaller particles Electrons are negatively charged The mass of an electron is very small HOWEVER Atoms should have a (+) portion to balance the negative part - Electrons are so small that some other particles must account for mass

II. History of the Atomic Structure Ernest Rutherford (1871-1937) C. Ernest Rutherford (1909) 1. Discovered the proton p+ 2. Received Nobel Prize in Chemistry 3. Gold Foil Experiment (Expectations) a. Shot alpha particles at atoms of gold b. expected them to pass straight through

Florescent Screen Lead block Uranium Gold Foil

He thought this would happen:

According to Thomson Model

He thought the mass of the positive charge was evenly distributed in the atom

Here is what he observed:

4. Gold Foil Experiment Results a. Most positive alpha particles pass right through b. However, a few were deflected c. Rutherford reasoned that the positive alpha particle was deflected or repelled by a concentration of positive charge

The positive region accounts for deflection

5. Gold Foil Experiment Conclusions a. the atom is mostly empty space b. the atom has a small, dense positive center surrounded by electrons Rutherford Model of the Atom

II. History of the Atomic Structure At this point in 1909, we know: p+ = 1.67 x 10-24 g e- = 9.11 x 10-28 g The charges are balance! But, How are the electrons arranged? There is still mass that is unaccounted for

II. History of the Atomic Structure Niels Bohr (1913) 1. Electrons orbit nucleus in predictable paths

History of the Atomic Structure E. Chadwick (1891 – 1974) 1. Discovers neutron in nucleus 2. Neutron is neutral - does not have a charge n0 3. Mass is 1.67 x 10-24 g a. slightly greater than the mass of a proton

II. History of the Atomic Structure F. The Quantum Atom Theory 1. The atom is mostly empty space 2. Two regions: a. Nucleus- protons and neutrons b. Electron cloud- region where you have a 90% chance of finding an electron

II. History of the Atomic Structure Charges balanced Mass accounted for However – what about the behavior of the electrons?