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F ROM D ALTON TO M ODERN A TOMIC T HEORY. B ACKGROUND - H ISTORY OF A TOMIC T HEORY The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the.

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Presentation on theme: "F ROM D ALTON TO M ODERN A TOMIC T HEORY. B ACKGROUND - H ISTORY OF A TOMIC T HEORY The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the."— Presentation transcript:

1 F ROM D ALTON TO M ODERN A TOMIC T HEORY

2 B ACKGROUND - H ISTORY OF A TOMIC T HEORY The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”) He believed that atoms were indivisible and indestructible His ideas did agree with later scientific theory, but did not explain chemical behavior, and were not based on the scientific method – but just philosophy

3 ATOMIC THEORY KEY EXPERIMENTAL EVIDENCE 1) Law of Conservation of Mass : in a chemical reaction mass of reactants = mass of products 2) Law of Definite Proportions : The chemical identity and ratio of the masses of elements in a given compound is always the same. John Dalton (early 1800’s) – Statement of Atomic Theory based on EXPERIMENTAL EVIDENCE

4 ATOMIC THEORY 1) All matter is composed of tiny particles called Atoms. Atoms are the smallest unit of chemical combination. (Later: atoms are made of protons, neutrons, and electrons) 2) Atoms of the same element are identical. Atoms of any one element are different from those of any other element. (Later: Isotopes – same chemical properties but different mass)

5 M ODIFICATIONS OF D ALTON ’ S A TOMIC T HEORY B ASED E XPERIMENTAL D ATA DISCOVERED AFTER D ALTON ’ S L IFETIME Atoms are not the smallest particle of matter. Atoms are composed of protons, neutrons and electrons (and these particles are in turn composed of even smaller particles). Atoms of the same element are not necessarily completely identical – mass can vary. Atoms of the same element with different masses are called isotopes.

6 A TOMIC T HEORY 3) In chemical reactions, atoms are regrouped but the total number and type of atoms present does not change Supporting evidence: Law of Conservation of Mass 4) A compound is formed when atoms of two or more elements chemically combine. The compound has the same type and ratio of atoms. Supporting evidence: Law of Definite Proportions (Constant Composition)

7 S UPPORTING EVIDENCE FOR ATOMIC THEORY : WHAT PATTERN DO YOU NOTICE IN THESE DATA ? Be + F 2 → BeF 2 9.01 g + 38.0 g → 47.01 g Ca + Cl 2 → CaCl 2 40.1 g 71.0 g → 111.1 g

8 E VIDENCE FOR ATOMIC THEORY Law of Conservation of Mass – in a chemical reaction MASS IS CONSERVED. The sum of the masses of the reactants (LEFT side of the equation) equals the sum of the masses of the products ( RIGHT side of the equation.) Example: Link to MgBr2 rxnLink to MgBr2 rxn Backup link AlBr3

9 Mass of reactants = mass of products → 24.3 g Mg + 159.8 g Br 2 → 184.1 g MgBr 2 EXPLANATION USING ATOMIC THEORY: 1) Matter is composed of tiny particles called ATOMS 2) In a chemical reactions atoms are CONSERVED. TOTAL # AND TYPE OF ATOMS IS CONSTANT Reactants: 1 Mg atom; 2 Br atomsProduct: 1 Mg atom; 2 Br atoms 1 atom Mg + 1 molecule Br2 1 molecule MgBr2

10 Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br 2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br 2 = 2 Br 2 = 3 Br 2 =

11 Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br 2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br 2 = 2 Br 2 = 3 Br 2 =

12 Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br 2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br 2 = 2 Br 2 = 3 Br 2 =

13 Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br 2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br 2 = 2 Br 2 = 3 Br 2 =

14 Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br 2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br 2 = 2 Br 2 = 3 Br 2 =

15 HOMEWORK 1-4 CORRECT ERROR: iii ERROR: ii ERROR: i

16 HOMEWORK 1-4 1.) O 2 + C → CO 2 16.0 g ? 22.0 g A) 6.0 g (22.0 g – 16.0 g) B) Law of Conservation of Mass

17 HOMEWORK 1-4 2.) O 2 + C → CO 2 16.0 g ? 22.0 g A) 6.0 g (22.0 g – 16.0 g) B) Law of Conservation of Mass C) Atoms can be rearranged in a reaction but atoms are conserved (same total # of each type before and after rxn) + Reactants: 1 C atom + 2 O atoms Products : 1C + 2 O atoms

18 Homework 1-4 3) 6 Mg + 2 N 2 → 2 Mg 3 N 2 ? g 56 g 201.8 g A) 145.8 g B) Conservation of Mass C) Atoms are conserved in a rxn but are rearranged

19 Homework 1-5 3) 6 Mg + 2 N 2 → 2 Mg 3 N 2 C) Atoms are conserved in a rxn but are rearranged. In a chemical rxn the NUMBER and type of atoms present on the reactant side match the number and TYPE of atoms present on the product side.

20 Homework 1-4 3) 6 Mg + 2 N 2 → 2 Mg 3 N 2 C) Atoms are conserved in a rxn but are rearranged.

21 Homework 1-4 3) 6 Mg + 2 N 2 → 2 Mg 3 N 2 C) Atoms are conserved in a rxn but are rearranged. 6 Mg atoms + 4 N (2 x2) atoms = 6 Mg (2 x3) + 4 N (2 x 2)

22 Homework 1-4 3) 6 Mg + 2 N 2 → 2 Mg 3 N 2 C) Atoms are conserved in a rxn but are rearranged. 6 Mg atoms + 4 N (2 x2) atoms = 6 Mg (2 x3) + 4 N (2 x 2)

23 L AW OF CONSTANT COMPOSITION For a given compound, the elements that make up the compound and the ratio of their masses are always the same. Example: Carbon dioxide is always composed of the same two elements, carbon and oxygen. Regardless of the total mass of CO 2, the ratio of the masses of oxygen to carbon is always 2.67:1 carbon Carbon dioxide oxygen

24 L AW OF CONSTANT COMPOSITION OR DEFINITE PROPORTIONS Total mass of sample Mass of carbon Mass of oxygen RATIO OXYGEN/ CARBON 44.0 g12.0 g32.0 g32/12 = 2.67 88.0 g24.0 g64.0 g64/24 = 2.67 100.0 g27.3 g72.772.7/27.3 = 2.67 ATOMIC LEVEL EXPLANATION MOLECULES OF CO 2 ALWAYS CONTAIN 1 C ATOM AND 2 O ATOMS

25 WHICH CHOICE INDICATES THE CORRECT RELATIVE AMOUNTS OF N AND O IN A 1 LITER SAMPLE OF NO2 GAS ? A) B) C) 1 LITER OXYGEN 1 LITER OF NITROGEN

26 1-4 P ROBLEM #4 waterHydrogen peroxide

27 H YDROGEN PEROXIDE REACTS WITH MnO 2 WHILE WATER DOES NOT !! V IDEO LINKV IDEO LINK Water + MnO 2 = NR Hydrogen peroxide + MnO 2 = MnO 2 water

28 HW 1-4 ; 4C AND 4D) 4C) NO ! Different chemical behavior means that they are not the same substance. 4D) Since the elements are the same, the key difference must be the RATIO of atoms (Law of Constant Composition). These compounds have different formulas. Water = H 2 O Hydrogen Peroxide = H 2 O 2

29 3E) Data: Water: H: 2 g O: 16 g Hydrogen Peroxide: H: 2 g O: 32 g Law of Definite Proportions Different Mass Ratios: Water: 16/2 = 8 : 1 Hydrogen Peroxide: 32/2 = 16 : 1

30 HW 1-4, #5 Copper and chlorine form two different compounds. One is a light blue solid, while the other is a green solid. According atomic theory, what is the atomic level difference between the two compound? Ans: The compounds must have different RATIOS of ATOMS (Different chemical formulas).

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