Unit 3 Atomic Theory Chapter 3

Early Beliefs Ancient Greek, Hindu, and Japanese systems contained 5 elements 1) Earth 2) Air or Wind 3) Water 4) Fire & 5) Void, Ether, or Heaven

Early Beliefs (cont’d) Substances (like gold) were combinations of the elements. You can continue to split a substance in half until you broke it down into the elements. Elements could be divided in half infinitely.

The Ancient Greeks Democritus (and his teacher Leucippus) had an idea that was different: Matter is… Composed of indivisible parts. The parts are called atomos. Properties of matter are due to the size, shape, and weight of the atomos. His idea was not popular and was abandoned for over 2,000 years.

Goto: http://bit.ly/1belMUl Democritus (460-370 BCE) By convention there is sweet, by convention there is bitterness, by convention hot and cold, by convention color; but in reality there are only atoms and the void. Current Research Indicates that your color perception is dependent upon the order in which you learned your colors! Goto: http://bit.ly/1belMUl

Crazy Ancients Since they had no microscopes or precise measuring devices, All experimental work was done crudely. Conclusions were mostly derived from pure thought. This led to some crazy ideas… Like Alchemy.

Alchemy Since substances were made up of the same 4 or 5 elements, they could be transmuted into other substances by changing the ratios of elements in them. The race began to change lead into gold.

Ffffffailure For eons, alchemists tried and failed, they could never transmute lead into gold. Did have some successes (making colors, fire, etc) It was more of an art. And they kept trade secrets.

The Death of Alchemy Many alchemists died… because they swindled a lot of powerful (and rich) people. For years, alchemy was a taboo. Eventually, a different class of people emerged. They built on the lessons of alchemy.

Fast Forward Scientists and the scientific method emerged. In the late 1700’s and early 1800’s, Scientists began to notice that certain substances combined With other substances in specific amounts.

Law Abiding Elements The Law of Definite Proportions was declared: A chemical compound always contains the same elements in exactly the same proportions by weight Two samples of same the compound will always have the exact same percent composition of elements It is intensive (doesn’t matter how much you have, the percentage will always be the same!)

Proportional Legality Another law was needed to describe different things made of the same elements… The Law of Multiple Proportions states: When two elements combine to form two or more compounds, the mass of one element that combines with a given mass of the other is in the ratio of small whole numbers.

Huh?!? In English, the Law of Multiple Proportions: When two or more compounds are made up of the same elements, their percent compositions will always be different.

Example Water is made up of 26.5 g water = 23.5 g O & 3.0 g H 88.81% oxygen and 11.19% hydrogen. 26.5 g water = 23.5 g O & 3.0 g H Hydrogen peroxide is 94.07% O and 5.93% H. 26.5 g H2O2 = 24.9 g O & 1.6 g H

Percent Composition The make-up of a compound can be expressed by a percentage of each element. To calculate percent composition, you need to know how much of each element it takes to make the compound. Total Weight of Element Percent = Total Weight of Compound 100

Total Percent MUST always equal 100% Example If you combine 15.7 g of sodium metal (Na) with 24.2 g chlorine gas (Cl), you will produce 39.9 g sodium chloride (salt). What is the % comp for each element? %Na = 15.7 / 39.9 * 100 = 39.3% %Cl = 24.2 / 39.9 * 100 = 60.7% Total = 39.9 / 39.9 * 100 = 100% Total Percent MUST always equal 100%

What goes in…must come out Early scientists began to notice that the combined weight of the reactants Is always the same as the combined weight of the products. We cannot destroy (or create) matter.

Conservative Matter The Law of Conservation of Mass: Matter can neither be created nor destroyed (in a chemical reaction), but only changed from one form to another. When we react substances, they must always be accounted for in the products.

Conserve This!!! 30.0 g iron reacts with 12.9 g oxygen to form 42.9 g rust – at 100% yield. 30.0 g + 12.9 g = 42.9 g Rust is 69.9% Fe and 30.1% O In order to obey the law, 30.0 g iron and 12.9 g oxygen must form 42.9 g of something – regardless of yield. 42.9 g go in, 42.9 g must come out.

John Dalton (1766-1844) English chemist / physicist / meteorologist Many areas of influence Colorblindness (he was colorblind) The behavior of gases, Interactions of light & heat, and The behavior of chemicals.

Colorblindness Before Dalton, no official description of it

More Blindness…

Dalton’s Work He was very persistent and stubborn. Accepted his own very crude results over better data of others because He’d been fooled before by erroneous data from others.

Returning to Democritus In 1803, Dalton published his “Atomic Theory”. 1) All matter is composed of extremely small particles called atoms, which cannot be subdivided, created, or destroyed. 2) Atoms of a given element are identical in their physical and chemical properties.

Atomic Theory (cont’d) 3) Atoms of different elements differ in their physical and chemical properties. 4) Atoms of different elements combine in simple, whole number ratios to form compounds. 5) In chemical reactions, atoms are combined, separated, or rearranged but never created, destroyed, or changed.

Atoms Shmatoms He was unable to prove the existence of atoms. My paper on Brownian Motion proved the existence of atoms…it was confusing, but not nearly as confusing as my paper on Special Relativity that came out in the same year… He was unable to prove the existence of atoms. Dalton’s work came out in 1803. It wasn’t until 1905 that the existence of atoms was proven by Albert Einstein.

Summary Law of Definite Proportions Law of Multiple Proportions Compounds will always have the same percent composition. Law of Multiple Proportions Different Compounds made up of the same elements will always have different percent compositions.

Summary (cont’d) Law of Conservation of Mass Dalton’s Atomic Theory Total mass of Reactants must equal total mass of products. Dalton’s Atomic Theory 1. Matter is composed of atoms. 2. Atoms of element are identical. 3. Properties of different elements are different. 4. Atoms combine in whole numbers. 5. Cannot create or destroy atoms.