Pre-AP Chemistry Atoms: The Building Blocks of Matter

The Development of the Atomic Theory Democritus~ 400 BC Possibly first to propose the “atom”. Atom in Greek means indivisible Aristotle ~ 300 BC Thought matter was continuous and made of 4 basic elements: Fire Wind Earth Water

The Development of the Atomic Theory ALCHEMY – based on Aristotle’s idea of continuous matter Attempt to “transmutate” one type of matter to another EX: Lead  Gold Studied for ~ 2000 years! IT DIDN’T WORK!!!!! Helped lead to the development of the SCIENCE of Chemistry

Law of Conservation of Mass Mass is neither created nor destroyed during chemical or physical reactions. Total mass of reactants = Total mass of products Antoine Lavoisier

Laws of Proportions Law of Definite Proportions: A compound will always contain the same ratio (mass) of elements EX: H2O will always have 2 H and 1 O Law of Multiple Proportions: If the same elements combine in different ratios (mass) the result will form different compounds EX H2O and H2O2

Dalton’s Atomic Theory (1808) All matter is composed of extremely small particles called atoms Atoms of a given element are identical in size, mass, and other properties RIGHT! John Dalton WRONG! Atoms cannot be subdivided, created, or destroyed Atoms of different elements combine in simple whole-number ratios to form chemical compounds In chemical reactions, atoms are combined, separated, or rearranged WRONG! RIGHT! RIGHT! WAS HE CORRECT????

Modern Atomic Theory Several changes have been made to Dalton’s theory. Dalton said: Atoms of a given element are identical in size, mass, and other properties; Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element. = ISOTOPES

Modern Atomic Theory #2 Dalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to the presence of a negatively charged particle with a very LARGE charge and very SMALL mass. Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Discovery of the Electron In 1909, Robert Milikan confirmed Thomson findings of a negative particle in the cathode tube.. He was even able to determine the mass of the electron compared to a hydrogen atom.

Some Modern Cathode Ray Tubes

Conclusions from the Study of the Electron All elements must contain electrons because they all produced beams. Electrons are negative b/c they move from the negative to the positive area in the tube. Electrons have mass b/c they moved the wheel placed in the beams path. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons Electrons have so little mass that atoms must contain other particles that account for most of the mass

Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Rutherford’s Gold Foil Experiment Alpha particles are helium nuclei (+ charged) Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

The Answers Target #1 Target #2

Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected “Like howitzer shells bouncing off of tissue paper!”

Rutherford’s Findings Conclusions: Nucleus The nucleus is small Reason: Very few particles repelled. The nucleus is dense Some particles didn’t pass through The nucleus is positively charged Reason: Positive particles were repelled Conclusions: Electron Cloud The electron cloud is large Reason: Most particles passed through. The electron cloud is not dense Reason: Particles passed through. The electron cloud is negatively charged Reason: Atom is neutral, so e- cloud must balance out charge of the + nucleus.

Atomic Structure Atoms are made of 3 basic particles which have a characteristic charge, mass, and location inside the atom. Particle Charge Mass (g) Location Electron -1 9.109 x 10-28 Electron cloud Proton +1 1.673 x 10-24 Nucleus Neutron 1.675 x 10-24

Atomic Number Atomic number of an element is the number of protons in the nucleus of each atom of that element. Element # of protons Atomic # Carbon 6 Phosphorus 15 Gold 79

Mass Number Nuclide p+ n0 e- Mass # Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p+ + n0 Nuclide p+ n0 e- Mass # Oxygen - 10 - 33 42 - 31 15 18 8 8 18 Arsenic 75 33 75 Phosphorus 16 15 31

Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons

Composition of the nucleus Atomic Masses Atomic mass is the average of all the naturally isotopes of that element. Carbon = 12.011 Isotope Symbol Composition of the nucleus % in nature Carbon-12 12C 6 protons 6 neutrons 98.89% Carbon-13 13C 7 neutrons 1.11% Carbon-14 14C 8 neutrons <0.01%

Isotopes…Again (must be on the test) Isotopes are atoms of the same element having different masses due to varying numbers of neutrons. Isotope Protons Electrons Neutrons Nucleus Hydrogen–1 (protium) 1 Hydrogen-2 (deuterium) Hydrogen-3 (tritium) 2

The Mole 1 dozen = 12 1 gross = 144 1 ream = 500 1 mole = 6.022 x 1023 THIS IS A CONVERSION FACTOR!!!!!! There are exactly 12 grams of carbon-12 in one mole of carbon-12.

I didn’t discover it. Its just named after me! Avogadro’s Number 6.022 x 1023 is called “Avogadro’s Number” in honor of the Italian chemist Amadeo Avogadro (1776-1855). I didn’t discover it. Its just named after me! Amadeo Avogadro

# of atoms or # of molecules Atomic Conversions There are two basic steps that can be performed for atomic conversions. # of atoms or # of molecules # of moles Mass = # grams 6.022 x 1023___ = 1 mol 1 mol = atomic mass (from pt) Conversion Factors

Calculations with Moles: Converting moles to grams How many grams of lithium are in 3.50 moles of lithium? 1 mol = atomic mass (from pt) 3.50 mol Li 6.94 g Li = g Li 24.3 1 mol Li

Calculations with Moles: Converting grams to moles How many moles of lithium are in 18.2 grams of lithium? 1 mol = atomic mass (from pt) 18.2 g Li 1 mol Li = mol Li 2.62 6.94 g Li

Calculations with Moles: Using Avogadro’s Number How many atoms of lithium are in 3.50 moles of lithium? 6.022 x 1023___ = 1 mol 3.50 mol Li 6.022 x 1023 atoms Li = atoms Li 2.11 x 1024 1 mol Li

Calculations with Moles: Using Avogadro’s Number How many atoms of lithium are in 18.2 g of lithium? # of atoms or # of molecules # of moles # grams 1 mol = atomic mass (from pt) 1 mol Li 18.2 g Li = mol Li 2.62 6.94 g Li

Calculations with Moles: Using Avogadro’s Number- step 2 How many atoms of lithium are in 18.2 g of lithium? # of atoms or # of molecules # of moles # grams 6.022 x 1023___ = 1 mol 2.62 mol Li 6.022 x 1023 atoms Li 1.58 x 1024 = atoms Li 1 mol Li

Calculations with Moles: Converting moles to grams in Compounds How many grams of CO2 are in 3.50 moles of CO2? 1 mol = atomic mass (from pt) C = 12.01 g O = 16.00 g CO2 = 44.01 g Just add the masses of all the atoms together 3.50 mol CO2 44.01 g CO2 = g CO2 154 1 mol CO2

The Atomic Scale Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space “q” is a particle called a “quark”

About Quarks… Protons and neutrons are NOT fundamental particles. Protons are made of two “up” quarks and one “down” quark. Neutrons are made of one “up” quark and two “down” quarks. Quarks are held together by “gluons”