From Dalton to Modern Atomic Theory

Background- History of Atomic Theory 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

ATOMIC THEORY John Dalton (early 1800’s) – Statement of Atomic Theory based on EXPERIMENTAL EVIDENCE 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.

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)

Modifications of Dalton’s Atomic Theory Based Experimental Data discovered after Dalton’s Lifetime 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.

Atomic Theory 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)

Supporting evidence for atomic theory: what pattern do you notice in these data? Be + F2 → BeF2 9.01 g + 38.0 g → 47.01 g Ca + Cl2 → CaCl2 40.1 g 71.0 g → 111.1 g

Evidence 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 rxn Backup link AlBr3

Mass of reactants = mass of products → 24.3 g Mg + 159.8 g Br2 → 184.1 g MgBr2 ATOMIC EXPLANATION:

Mass of reactants = mass of products → 24.3 g Mg + 159.8 g Br2 → 184.1 g MgBr2 ATOMIC EXPLANATION: In a rxn atoms are rearranged but # and type of atoms stays the same (atoms conserved) Reactants: 1 Mg atom; 2 Br atoms Product: 1 Mg atom; 2 Br atoms 1 atom Mg + 1 molecule Br2 1 molecule MgBr2

HOMEWORK 1-5 1.) O2 + C → CO2 16.0 g ? 22.0 g A) 6.0 g (22.0 g – 16.0 g) B) Law of Conservation of Mass

HOMEWORK 1-5 2.) O2 + C → CO2 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

Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br2 = 2 Br2 = 3 Br2 =

Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br2 = 2 Br2 = 3 Br2 =

Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br2 = 2 Br2 = 3 Br2 =

Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br2 = 2 Br2 = 3 Br2 =

Notes on Coefficients and Subscripts Subscript: # below line; indicates # of atoms bonded together Br2 subscript Coefficient: # in front of formula; indicates number of units (atoms or molecules) present Coefficient 2 Na Br = 2 Br = 3 Br = Br2 = 2 Br2 = 3 Br2 =

Homework 1-5 3) 6 Mg + 2 N2 → 2 Mg3N2 ? g 56 g 201.8 g A) 145.8 g B) Conservation of Mass C) Atoms are conserved in a rxn but are rearranged

Homework 1-5 3) 6 Mg + 2 N2 → 2 Mg3N2 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.

Homework 1-5 3) 6 Mg + 2 N2 → 2 Mg3N2 C) Atoms are conserved in a rxn but are rearranged.

Homework 1-5 3) 6 Mg + 2 N2 → 2 Mg3N2 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)

Homework 1-5 3) 6 Mg + 2 N2 → 2 Mg3N2 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)

Law 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 CO2, the ratio of the masses of oxygen to carbon is always 2.67:1 Carbon dioxide carbon oxygen

Law of constant composition or definite proportions Total mass of sample Mass of carbon Mass of oxygen RATIO OXYGEN/ CARBON 44.0 g 12.0 g 32.0 g 32/12 = 2.67 88.0 g 24.0 g 64.0 g 64/24 = 2.67 100.0 g 27.3 g 72.7 72.7/27.3 = 2.67 ATOMIC LEVEL EXPLANATION MOLECULES OF CO2 ALWAYS CONTAIN 1 C ATOM AND 2 O ATOMS

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

1-5 Problem #3 water Hydrogen peroxide

Hydrogen peroxide reacts with MnO2 while water does not!! Video link Water + MnO2 = NR Hydrogen peroxide + MnO2 = MnO2 water

HW 1-5 ; 3C and 3D) 3C) NO ! Different chemical behavior means that they are not the same substance. 3D) 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 = H2O Hydrogen Peroxide = H2O2

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