UNIT 2 Atomic Structure Section 1: History & Development of Atomic Theory (Chapter 3) History of the Atom Video
The Greeks Democritus World made of empty space and tiny particles ( atoms ) Thought there were different types of atoms for every material Problem: NO experimental evidence Credited for 1 st atomic theory Nothing exists except atoms and empty space; everything else is opinion.
The Greeks Aristotle Proposed that the world was continuous and that there were no tiny particles Very influential so his ideas were widely accepted until the 18 th century
Aristotle s 4 element theory
Early Atomic Theory JOHN DALTON Examined the work of others Lavoisier (early 1700s): Law of Conservation of Mass: Mass is neither created nor destroyed during a chemical reaction or physical change Antoine Lavoisier
The total mass of reactants = total mass of products Lavoisier: The Law of Conservation of Mass Law of Conservation of Mass During a chemical change, matter is neither created nor destroyed.
Example Problems If 5.68 g of X combine with 3.66 g of Y, how many grams of the compound XY are produced? X + Y XY 5.68 + 3.66 = XY XY = 9.34 g If 3.7 g of element A combines with element B to make 8.32 g of compound AB, how many grams of B were needed? A + B AB 3.7 + B = 8.32 B = 4.62 g
John Dalton continued Dalton also examined the work of: Proust: Law of Definite Proportions Specific substances always contain elements in the same ratio by mass (i.e. Salt (NaCl) always contains 39.34% Na by mass & 60.66% Cl by mass) Joseph Proust
Example Problem Take the reaction A + B AB If 2 g of A combine with 5 g of B to form AB, how many grams of B are needed to combine with 9 g of A to form AB? 2 + 5 AB 9 + x AB 9/2 = 4.5; 4.5 * 5 = 22.5 g of B needed In the second reaction there are 4.5 times more A than in the first reaction, so multiple 4.5 * 5 (Grams of B in 1 st reaction) to determine how many grams of B are needed
Early Atomic Theory Based on others work, Dalton developed: Law of Multiple Proportions: If two or more different compounds are composed of the same two elements, the mass of the second element combined with a specific mass of the first element can be expressed as a ratio of small whole numbers.
In a nutshell.. when elements combine, they do so in a ratio of small whole numbers carbon and oxygen react to form CO or CO 2, but not CO 1.6
Law of Multiple Proportions Carbon monoxide (CO): 12 parts by mass of carbon combines with 16 parts by mass of oxygen. Carbon dioxide (CO2): 12 parts by mass of carbon combines with 32 parts by mass of oxygen. Ratio of the masses of oxygen that combines with a fixed mass of carbon (12 parts) 16: 32 or 1: 2
Continued Take an example of two mineral samples iron pyrite (FeS 2 ) and iron troilite (FeS). Both contain iron and sulfur atoms. However, for a given fixed amount of iron it requires exactly twice the mass of sulfur needed to make pyrite than that of troilite with the same amount of iron.
John Dalton Cont Dalton also developed his own Atomic Theory See information on Dalton vs. Modern Atomic Theory you are responsible for knowing all information on this chart!!!
Dalton s Atomic Theory 1. All matter is composed of small particles called atoms. 2. Atoms of a given element are identical in size, mass and other properties; atoms of different elements differ in size, mass and other properties. 3. Atoms cannot be subdivided, created or destroyed. 4. Atoms of different elements can combine in simple, whole number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated or rearranged. Modern Atomic Theory 1. All matter is composed of small particles called atoms. 2. Atoms of the same element have the same chemical properties; while atoms of different elements have different chemical properties. 3. While individual atoms of a given element may not all have the same mass, any natural sample of an element will have a definite average mass that is characteristic of that element. 4. Compounds form when atoms of two or more elements unite and each lose their characteristic properties as a result. 5. Atoms are not subdivided in physical or chemical reactions.
Biggest Differences between Theories Dalton stated all atoms of the same element are identical; they are not (isotopes and ions are the same element with different numbers of neutrons or electrons) BUT the same atoms have the same chemical properties Atoms cannot be subdivided/destroyed by CHEMICAL means (Dalton said they cannot be at all)