Department of Chemistry CHEM1010 General Chemistry *********************************************** Instructor: Dr. Hong Zhang Foster Hall, Room 221 Tel:

CHEM1010/General Chemistry _________________________________________ Chapter 2. (L7)-Introduction to Atoms Today’s Outline..Dalton: The law of multiple proportions..Dalton’s atomic theory of matter and its changes..Comparison of Dalton’s atomic theory with the ancient Greek’s atomic idea about matter..Relative mass of various pure elements (atoms)..How Dalton’s atomic theory explains the three fundamental facts (laws) about matter and its changes: -how to explain the law of conservation of mass -how to explain the law of definite proportions in mass -how to explain the law of multiple proportions in mass

Chapter 2. (L7)-Introduction to Atoms The law of multiple proportions John Dalton ( ), an English schoolteacher Proust’s law of definite proportion states existence of only one proportion for a chemical of different components. Dalton discovered that more than one ratio or proportion can exist for combination of the same set of two or more components.

Chapter 2. (L7)-Introduction to Atoms The law of multiple proportions Let’s see an example to illustrate the law of multiple proportions g carbon g oxygen = g carbon monooxide 1.00 g carbon g oxygen = g carbon dioxide Clearly, here we see two mass ratios of carbon over oxygen: 1 st ratioCarbon : oxygen = 1 : = 3 : 4 2 nd ratioCarbon : oxygen = 1 : = 3 : 8 We notice that the second ratio doubles the first one in terms of oxygen.

Chapter 2. (L7)-Introduction to Atoms The law of multiple proportions So, Dalton’s law of multiple proportions is actually an extension of Proust’s law of definite proportions. In other words, Proust’s law of definite proportions is only a special case of Dalton’s multiple proportions.

Chapter 2. (L7)-Introduction to Atoms The law of multiple proportions In the history of science, oftentimes, a law about a special case was replaced, later on, by a law about more general phenomena. Example: Attraction exerted by the Earth on everything on the Earth was noticed first, but later on, it was discovered that attraction occurs between any two things each having a certain mass.

Chapter 2. (L7)-Introduction to Atoms Dalton’s atomic theory about matter and its changes Dalton’s atomic theory can be summarized by the following basic points: 1. All matter is composed of extremely small particles called atoms. 2. There are different kinds of atoms. 3. Each kind of atoms has many many identical individual atoms. Since they are all the same kind, they represent one particular chemical element. This element has a symbol to represent all the same atoms and each of them. Example: Durango is a name for all individual Durango cars. Durango represents all the same cars and each of them. Of course, atoms of one element differ from the atoms of any other element.

Chapter 2. (L7)-Introduction to Atoms Dalton’s atomic theory about matter and its changes Dalton’s atomic theory can be summarized by the following basic points (Continued): 4. Compounds are formed when atoms of different elements (different kinds of atoms) combine in fixed or definite proportions or ratios. The substances composed of different components are now called compounds. The substances composed of only one single component (one element or one kind of atoms) may be called elemental substances.

Chapter 2. (L7)-Introduction to Atoms Dalton’s atomic theory about matter and its changes Dalton’s atomic theory can be summarized by the following basic points (Continued): 5. A chemical reaction is simply a rearrangement of atoms, changing the combination of the different kinds of atoms. Example: Suppose we have two kinds of atoms, or two elements: A, and B; these two kinds of atoms can form two compounds: AB and A 2 B 2, and one elemental substance, A 2, and another elemental substance, B 2. Now, the chemical reactions between these chemical substances can be considered as rearrangement of atom A and atom B or element A and element B: A 2 + B 2 = A 2 B 2 ; A 2 + B 2 = AB + AB AB + AB = A 2 B 2

Chapter 2. (L7)-Introduction to Atoms Comparison of Dalton’s atomic theory with the ancient Greek’s atomic idea about matter..If you go through all the major points of Dalton’s atomic theory, you will find out the ancient Greek’s atomic idea has only the first point: that is, “a ll matter is composed of extremely small particles called atoms”...The ancient Greek’s idea does not have a lot of details and is far less sophisticated than Dalton’s atomic theory...The ancient Greek’s idea cannot satisfactorily explain the three basic chemical laws.

Chapter 2. (L7)-Introduction to Atoms Comparison of Dalton’s atomic theory with the “quala, bala, wala, zala theory”..Obviously, the advancements of the “quala, bala, wala, zala” theory are: -It includes four different atoms, called quala, bala, wala, zala -The chemical reactions can be explained as the rearrangement of quala, bala, wala, zala.

Chapter 2. (L7)-Introduction to Atoms Comparison of Dalton’s atomic theory with the “quala, bala, wala, zala theory”..The beauty of Dalton’s atomic theory lies that it does not specify how many different kinds atoms are present. It’s up to the experimental facts that will make the call...One limitation of the “quala, bala, wala, zala” theory is that it puts a limit for the atom kinds...Also, it does not have the sophistication that Dalton’s theory achieves.

Chapter 2. (L7)-Introduction to Atoms Relative atomic masses..Mass of atoms The mass values of atoms are extremely small, for example: electron mass = 9.11× kg proton mass = 1.67× kg It is very difficult to work with these small numbers.

Chapter 2. (L7)-Introduction to Atoms Relative atomic masses..Relative mass of atoms To overcome the difficulty working with extremely small values of atoms, instead, relative mass was used. Dalton assembled relative masses for a number of atoms based on hydrogen. That is: Relative mass = Mass of an atom/Mass of hydrogen This way, the small values are canceled and you get a ratio of the masses, which give you a number that is much easier to work with. For example: relative mass of fluorine is 19

Chapter 2. (L7)-Introduction to Atoms Relative atomic masses..Relative mass of atoms Example: Relative mass of fluorine is 19 This means: Mass of F/Mass of H = 19 Relative mass of carbon is 12 This means: Mass of C/Mass of H = 12

Chapter 2. (L7)-Introduction to Atoms How Dalton’s atomic theory explains the three fundamental laws about matter and its changes..As we mentioned before, science is working at two ends, scientific observations and facts vs. scientific theory...We have learned the three fundamental laws of chemistry (facts and observations)...We have also learned Dalton’s atomic theory about matter and its changes...Now, time to connect the two together, meaning how the theory explains the facts.

Chapter 2. (L7)-Introduction to Atoms How Dalton’s atomic theory explains the three fundamental laws about matter and its changes How to explain the law of conservation of mass: Following the fifth point: “A chemical reaction is simply a rearrangement of atoms, changing the combination of the different kinds of atoms.” -then, since any rearrangement of the combination of the atoms neither increases nor decreases the amount of atoms, the total mass of all the atoms remains the same before and after the reaction. -As a matter of fact, the total number of the atoms involved remains unchanged in the reaction.

Chapter 2. (L7)-Introduction to Atoms How Dalton’s atomic theory explains the three fundamental laws about matter and its changes How to explain the law of definite proportions: Fact: 1 g hydrogen + 19 g fluorine = 20 hydrogen fluoride the mass ratio of Hydrogen:Fluorine = 1:19 Known: Mass of Fluorine is 19 times the mass of hydrogen Theory : 1 atom of hydrogen combines with 1 atom of fluorine to form 1 compound of hydrogen fluoride (HF) Reasoning : Suppose mass of 1 hydrogen atom is 0.1 g, to form 1 HF, you need 1 atom of fluorine, and its mass should be 0.1×19 = 1.9 g. If you have 10 atoms of hydrogen, then you need 10 atoms of fluorine to form 10 HF, then the total mass of 10 H is 0.1 × 10 = 1 g, and the total mass of 10 F is 1.9 × 10 = 19 g. See, this theory predicts that the mass ratio should be 1 g of H over 19 g of F if 1 atom of Hg combines with 1 atom of F.

Chapter 2. (L7)-Introduction to Atoms How Dalton’s atomic theory explains the three fundamental laws about matter and its changes How to explain the law of multiple proportions: Since we have understood that we just need to adjust the ratio of the component atoms for a particular compound, then we can predict different mass ratios for different compounds having the same component atoms. Example: CO and CO 2, The trick here is the ratio of component atoms, then from this ratio, we can find out the mass ratio.

Chapter 2. (L7)-Introduction to Atoms Significance of Dalton’s atomic theory..The ancient Greek philosophical idea about matter is now developed into a scientific theory about chemistry...From now on, chemistry is all based on atoms. Everything and every chemical change are now explained in terms of atoms...This theory now brings order to the chemical world, and many many observations are now connected together. Now, we have a unified chemical view on our world around us.

Chapter 2. (L7)-Introduction to Atoms Significance of Dalton’s atomic theory, Continued..This theory can predict many new chemical changes...This theory is testable. We can check to see if the prediction can be verified and if it can explain new observations...This theory also, understandably, raises more questions: How small is an atom? How many atoms are present? How atoms combine together to form a compound? So, this theory will promote further development of the chemical science.

Chapter 2. (L7)-Introduction to Atoms Limitation of Dalton’s atomic theory This theory still cannot explain a bunch of other observations and facts and chemical reactions. Example: It cannot explain how a battery works. It cannot tell the structure of compounds. It does not tell how atoms get together to form the compounds or elemental substances. It does not tell why some reactions are easy to happen but some are not. It does not tell under what conditions a reaction can happen, and if it needs energy or releases energy? And more…..

Chapter 2. (L7)-Introduction to Atoms Chemical symbols.. Dalton also invented a set of symbols to represent atoms or elements, in stead of a word from everyday language...Nowadays, there are a pool of chemical symbols to represent or denote each of the different atoms or elements in modern chemistry...We will learn more about the symbols for the various atoms or elements. Example: H: for hydrogen; F: for fluorine; O: for oxygen; C: for carbon; and more.

Chapter 2. (L7)-Introduction to Atoms Classification of matter based on Dalton’s atomic theory Now we can classify matter more strictly or accurately on the basis of Dalton’s theory: -Atoms -Molecules that are composed of identical atoms combined together as a group of a certain number (e.g., n = 1, or 2, or 3, or more) or composed of different kinds of atoms combined together in various atomic ratios. -Elemental chemicals that are composed of identical atoms combined together as a group of a certain number (example: hydrogen gas, H 2, where n = 2) -Compounds that are composed of different kinds of atoms combined together in various atomic ratios (example: hydrogen fluoride, HF, H atom : F atom = 1:1) Hence, both elemental chemicals and chemical compounds are molecules, which are one level above the atoms in terms of structure of matter. So, elemental chemicals are special cases of compounds.

Chapter 2. (L7)-Introduction to Atoms Classification of matter based on Dalton’s atomic theory Now we can classify matter more strictly or accurately on the basis of Dalton’s theory (continued): -Pure chemicals: Both elemental chemicals of a certain kind and compounds of a certain kind are pure chemicals. In other words, a pure chemical is a pure molecule of a certain kind. -Mixtures are different kinds of chemicals physically mixed together; the mixed chemicals can be different kinds of elemental chemicals (H 2 and Cl 2 ) or different kinds of compounds (HF and HCl), or mixtures of both (H 2 and HF). In other words, mixtures are different kinds of molecules mixed together.

Chapter 2. (L7)-Introduction to Atoms Quiz Time The various mass ratios for reacting components in a reaction to form a new chemical, or in other words, the law of definite proportions, is explained in Dalton’s theory by (a) the number of hydrogen required; (b) mass of hydrogen; (c) size of hydrogen atom; (d) various ratios of the numbers of different kinds of atoms forming a compound

Chapter 2. (L7)-Introduction to Atoms Quiz Time Dalton’s atomic theory about matter is a reflection of which philosophical approach profoundly adopted in science, is that (a) the holistic approach, meaning things are connected all together as a whole, and cannot be reduced to sub- level components; (b) the romantic approach; (c) the reductionism, meaning reduction of different things at a higher level to certain components at a lower level, and by studying the components separately, these components can be used to explain the various things at the higher level; (d) the fuzzy approach.

Chapter 2. (L7)-Introduction to Atoms Quiz Time Dalton’s atomic theory in chemistry may be considered to be analogous to (a) car racing; (b) poll; (c) statistics; (d) the cell theory in biology.

Chapter 2. (L7)-Introduction to Atoms Quiz Time A chemical element represents (a) many different kinds of atoms; (b) one single atom of a certain kind; (c) numerous identical atoms of a specific kind with certain physical and chemical properties ; (d) an elemental substance containing identical atoms combined together in a certain ratio.