Philosophy to Scientific Theory The Atom Philosophy to Scientific Theory

I. The Atom: Philosophy to Scientific Theory Ancient Greek Philosophers theorized on what the universe was made of Democritus (450-370 BC) Sand can be broken down into tiny indivisible components -- “atomos”

Aristotle (384-322 BC) Did not believe in the atomos theory Believed all matter was continuous No void exists 5 elements Aristotle’s views influenced Western thought for 2000 yrs.

Early Modern Times/Enlightenment Aristotle’s ideas questioned Sir Francis Bacon (1561-1626) developed the Scientific Method Early alchemist Works of Robert Boyle (1627-1691) led to the belief of more than four elements existing

Late 1700’s All chemists accepted the modern definition of an element 1790’s -Antoine Lavoisier-father of Modern Chemistry Stated the law of conservation of mass Recognized and named oxygen and hydrogen Late 1700’s

Late 1700’s Emphasis placed on quantitative analysis Led to discovery of the conservation of mass Law of multiple proportions Law of definite proportions

Dalton’s atomic theory 1808 English schoolteacher Applied the law of conservation of mass, the law of multiple and definite proportions

Dalton’s atomic theory 1. All matter is composed of extremely 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

Dalton’s atomic theory (cont’d) 3. Atoms cannot be subdivided, created, or destroyed. 4. Atoms of different elements combine in simple whole number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged.

Dalton’s atomic theory (cont’d) Some of Dalton’s theories were disproved. Which ones? How did Dalton “visualize” the atom at this time? The atom is impossible to see-instruments must be used to indirectly see.

Experiments to determine what an atom was J. J. Thomson- used Cathode ray tubes

Thomson’s Experiment Voltage source - +

Thomson’s Experiment Voltage source - +

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end.

Thomson’s Experiment Voltage source + - By adding an electric field, he found that the moving pieces were negative

Thomson’s Model Found the electron. Couldn’t find positive (for a while). Said the atom was like plum pudding. A bunch of positive stuff, with the electrons able to be removed.

Millikan’s Experiment Atomizer Oil droplets - + Oil Telescope

Millikan’s Experiment X-rays X-rays give some droplets a charge.

Millikan’s Experiment From the mass of the drop and the charge on the plates, he calculated the mass of an electron Some drops would hover

Radioactivity Discovered by accident Bequerel Three types alpha- helium nucleus (+2 charge, large mass) beta- high speed electron gamma- high energy light

Rutherford’s Experiment Used uranium to produce alpha particles. Aimed alpha particles at gold foil by drilling hole in lead block. Since the mass is evenly distributed in gold atoms alpha particles should go straight through. Used gold foil because it could be made atoms thin.

Florescent Screen Lead block Uranium Gold Foil

What he expected

Because

Because, he thought the mass was evenly distributed in the atom.

What he got

How he explained it + Atom is mostly empty Small dense, positive piece at center. Alpha particles are deflected by it if they get close enough.

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Modern View of the Atom The atom is mostly empty space. Two regions Nucleus- protons and neutrons. Electron cloud- region where you might find an electron.

Sub-atomic Particles Z - atomic number = number of protons determines type of atom. A - mass number = number of protons + neutrons. Number of protons = number of electrons if neutral.

Symbols A X Z 23 Na 11

Isotopes Atoms of the same element that have different masses because of different number of neutrons. Ex/Hydrogen Nuclide- a general term for a specific isotope of an element.

Isotope Compounds are made of two or more atoms or ions chemically combined. Can a compound be made of different isotopes?

Relative Atomic Masses Masses of atoms in grams are very small. Chemists use a more convenient unit The atomic mass unit or amu One amu is equal to exactly one twelfth the mass of a carbon-12 atom Hydrogen-1 is about 1/12 th e mass of carbon-12 1.007825 amu

Average Atomic Mass Most elements occur naturally as mixtures of isotopes. Isotopes occur in different percentages. Average atomic mass is the weighted average of the naturally occurring isotopes of an element.

Relating Mass to Numbers of Atoms The Mole Avogadro’s number Molar Mass Mass to Mole conversions Mole to number of particle conversions

The Mole Abbreviation for molecule A mole (mol) is the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of carbon-12. Counting unit-similar to a dozen A mole contains 6.022 X 1023 particles

Avogadro’s number Avogadro’s number is the number of particles in exactly one mole of a pure substance. 6.022 X 1023 Named after Italian scientist Amedeo Avogadro

Relevance of Avog. number Related the microscopic to the macroscopic Brought the amu (1/12 mass of Carbon-12) definition together with the gram How many particles do you need to equal 12 grams of Carbon-12? 1 gram = 1 atomic mass unit Needs more clarification

Relevance of Avagadro’s # 1 amu is equal to exactly one twelth the mass of a carbon-12 atom One mole is the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of carbon-12 So. . .

1 amu = 1/12 mass of Carbon-12 1 mole = # in 12 grams of Carbon-12 How much does 1/12 of a mole of Carbon-12 weigh? 1 gram If we are talking on the atomic level, we use amu If we are in the lab, we use grams and moles

Molar Mass The mass of one mole of a pure substance. Do not confuse with atomic mass Same number but . . . Atomic mass is measured in amu Molar mass is in grams

Conversion factors from these definitions 1 mole = 6.022 X 1023 1 mole = molar mass of the element Molar mass number is equal to the average atomic mass but in grams instead of amu Average atomic was calculated from isotopes and percent abundances

Gram to mole conversions How many moles of calcium are in 5.00 g of Calcium? How many moles of gold are in 3.6 mg of gold? How many moles of zinc are in .535 g of zinc?

Mole to mass conversions What is the mass in grams of 2.25 mol of the element Fe. What is the mass in grams of .375 mol of the element K? What is the mass in gram of 0.0135 mol of the element Na?

Mole to particle conversions Particles can be molecules, atoms, or formula units. How many atoms of Al are in 2.75 mol of aluminum? How many atoms of S are in 3.00 mol of Sulfur?

Particles to Mole conversions How many moles of Pb are in 1.50 X 1012 atoms? How many moles of Sn are in 2500 atoms of tin?

Mass to particle conversions Helps us count by weight Different Molar masses because elements weigh differently (# protons and # neutrons) How many atoms are in 3.2 grams of He? How many molecules are in 16.0 grams of Oxygen? How many atoms are in 16.0 grams of Oxygen (diatomic)?

Chemical Bonds The forces that hold atoms together. Covalent bonding - sharing electrons. Makes molecules. Chemical formula- the number and type of atoms in a molecule. C2H6 - 2 carbon atoms, 6 hydrogen atoms, Structural formula shows the connections, but not necessarily the shape.

H H H C C H H H There are also other model that attempt to show three dimensional shape. Ball and stick.

Ions Atoms or groups of atoms with a charge. Cations- positive ions - get by losing electrons(s). Anions- negative ions - get by gaining electron(s). Ionic bonding- held together by the opposite charges. Ionic solids are called salts.