F ROM D ALTON TO M ODERN 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)Atoms of different elements combine in simple whole-number ratios to form chemical compounds 4)In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element. 1)All elements are composed of tiny indivisible particles called atoms 2)Atoms of the same element are identical. Atoms of any one element are different from those of any other element. John Dalton (1766 – 1844)

 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 – can vary in mass. Atoms of the same element with different masses are called isotopes.

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

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

Mass of reactants = mass of products → 24.3 g Mg g Br 2 → g MgBr 2 ATOMIC EXPLANATION:

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

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

HOMEWORK ) O 2 + C → CO 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 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 =

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 =

Homework 1-5 2) 6 Mg + 2 N 2 → 2 Mg 3 N 2 ? g 56 g g A) g B) Conservation of Mass

Homework 1-5 2) 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)

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

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 = g24.0 g64.0 g64/24 = g27.3 g /27.3 = 2.67 ATOMIC LEVEL EXPLANATION MOLECULES OF CO 2 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 ? A) B) C) 1 LITER OXYGEN 1 LITER OF NITROGEN

1-5 P ROBLEM #3 waterHydrogen peroxide

H YDROGEN PEROXIDE REACTS WITH MnO 2 WHILE WATER DOES NOT !! Water + MnO 2 = NR Hydrogen peroxide + MnO 2 = MnO 2 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 = H 2 O Hydrogen Peroxide = H 2 O 2