Chapter 4 Atoms and their structure

History of the atom n Not the history of atom, but the idea of the atom. n Original idea Ancient Greece (400 B.C.) n Democritus and Leucippus- Greek philosophers.

History of Atom n Looked at beach n Made of sand n Cut sand - smaller sand n Smallest possible piece? n Atomos - not to be cut

Another Greek n Aristotle - Famous philosopher n All substances are made of 4 elements n Fire - Hot n Air - light n Earth - cool, heavy n Water - wet n Blend these in different proportions to get all substances

Who Was Right? n Did not experiment. n Greeks settled disagreements by argument. n Aristotle was a better debater - He won. n His ideas carried through middle ages. n Alchemists tried to change lead to gold.

Who’s Next? n Late 1700’s - John Dalton- England. n Teacher- summarized results of his experiments and those of others. n Elements substances that can’t be broken down n In Dalton’s Atomic Theory n Combined idea of elements with that of atoms.

Dalton’s Atomic Theory  All matter is made of tiny indivisible particles called atoms.  Atoms of the same element are identical, those of different atoms are different.  Atoms of different elements combine in whole number ratios to form compounds.  Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

Parts of Atoms n J. J. Thomson - English physicist n Made a piece of equipment called a cathode ray tube. n It is a vacuum tube - all the air has been pumped out. n A limited amount of other gases are put in

Thomson’s Experiment Voltage source +- Metal Disks

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

Thomson’s Experiment n By adding an electric field + -

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

Thomson’s Experiment n Used many different metals and gases n Beam was always the same n By the amount it bent he could find the ratio of charge to mass n Was the same with every material n Same type of piece in every kind of atom

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

Rutherford’s Experiment n Ernest Rutherford English physicist. (1910) n Believed the plum pudding model of the atom was correct. n Wanted to see how big they are. n Used radioactivity. n Alpha particles - positively charged pieces given off by uranium. n Shot them at gold foil which can be made a few atoms thick.

Rutherford’s experiment n When the alpha particles hit a florescent screen, it glows. n Here’s what it looked like (pg 72)

Lead block Uranium Gold Foil Flourescent Screen

He Expected n The alpha particles to pass through without changing direction very much. n Because… n The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles.

What he expected

Because

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

What he got

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

+

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

Density and the Atom n Since most of the particles went through, it was mostly empty. n Because the pieces turned so much, the positive pieces were heavy. n Small volume, big mass, big density. n This small dense positive area is the nucleus.

Other pieces n Proton - positively charged pieces 1840 times heavier than the electron. n Neutron - no charge but the same mass as a proton. n Where are the pieces?

Subatomic particles Electron Proton Neutron NameSymbolCharge Relative mass Actual mass (g) e-e- p+p+ n0n / x x

Structure of the Atom n There are two regions. n The nucleus. n With protons and neutrons. n Positive charge. n Almost all the mass. n Electron cloud- most of the volume of an atom. n The region where the electron can be found.

Size of an atom n Atoms are small. n Measured in picometers, meters. n Hydrogen atom, 32 pm radius. n Nucleus tiny compared to atom. n IF the atom was the size of a stadium, the nucleus would be the size of a marble. n Radius of the nucleus is near m. n Density near g/cm 3.

Counting the Pieces n Atomic Number = number of protons n # of protons determines kind of atom. n the same as the number of electrons in the neutral atom. n Mass Number = the number of protons + neutrons. n All the things with mass. n NOT on the periodic table

Symbols n Contain the symbol of the element, the mass number and the atomic number. X Mass number Atomic number

Symbols n Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number –Name Na 24 11

Symbols n Find the –number of protons –number of neutrons –number of electrons –Atomic number –Mass Number –Name Br 80 35

Symbols n if an element has an atomic number of 34 and a mass number of 78 what is the –number of protons –number of neutrons –number of electrons –Complete symbol –Name

Symbols n if an element has 91 protons and 140 neutrons what is the –Atomic number –Mass number –number of electrons –Complete symbol –Name

Symbols n if an element has 78 electrons and 117 neutrons what is the –Atomic number –Mass number –number of protons –Complete symbol –Name

Unit 3 notes n Ions – charged atoms n Most atoms in their natural state are not stable. In order to understand stability, we have to look at the last energy level the electrons fill. This shell is called the valence shell. If that shell is full the atom is happy, if not, then the atom will go out and react to make itself full. n If the atom needs to steal electrons to become stable it will, if it needs to give up electrons to become stable it will, and some have the ability to do both. n Rule is this, the atom will do whatever is easiest.

Unit 3 notes n Ex. Flourine :2 in the first level 7 in the second. That second level can hold 8, so it needs one more, so it will go out and steal one from something to make itself happy. n Now the PNE will change from 9, 10, 9 to 9, 10, 10, now the protons and electrons are not equal to each other and so the overall charge is not zero but now -1. So the flouride ion has an F-1 overall charge.

Unit 3 notes n Where does it get this extra electron from? n If you look at Lithium, the PNE for lithium is 3, 4, 3 2 electrons in the first level and 1 in the second. If lithium wants to fill its second level it would need to steal 7 electrons, very difficult. But if it could dump it to flourine, then the first level (which is now the only level) would be full and everything could be stable. n Lithium ion would have a PNE of 3, 4, 2 and because it gave up an negative particle would have a charge of Li +1 n These go hand in hand, you have to have one thing giving it up if you want to have something take it in. It is the backbone for every chemical reaction.

Unit 3 notes n If the atom gives up an electron we say that it has been oxidized (overall charge increases) n If the atoms takes in an electron we say that it has been reduced (overall charge decreases) n These come from Benjamin Franklin’s names of oxidation and reduction during a chemical reaction. n Cation- + ions n Anion - - ions n Something is unique about C though, it can do either gain 4 or lose 4, so it can have a charge of + or – 4 depending on whatever it needs to do.

Isotopes n Dalton was wrong. n Atoms of the same element can have different numbers of neutrons. n different mass numbers. n called isotopes.

Naming Isotopes n Put the mass number after the name of the element. n carbon- 12 n carbon -14 n uranium-235

Relative Abundance n The atomic mass given to you is the overall average of all of the isotopes found. n It is based on the relative abundance of each of the isotopes found in nature. n For example, Cu has two isotopes: Cu-63 and Cu-65 n Because the average is closer to 63, then Cu-63 is more abundant in nature.

Relative Abundance n To calculate the average of all the isotopes, use the atomic mass and the decimal form of the percent abundance to find the mass contributed by each isotope and then add them together.

Relative Abundance n What??????

Relative Abundance n Take the mass of one isotope and multiply it by its percentage that exists in nature (just use the decimal form) n Ex. You have element X-10 that is 19.91% abundant in nature and element X-11 that is 80.80% abundant in nature. 10 (.1991) = (.1991) = (.8080) = (.8080) = Now add them up to get the average: = amu

Relative Abundance n Now you try it! n Calculate the average atomic mass of strontium. Here are the relative abundances: n Sr % n Sr % n Sr % n Sr %

Relative Abundance n Bromine has two isotopes Br- 79 and Br- 81. If the average mass of bromine is 79.9, calculate the relative abundances?

Relative Abundance n Lead has 4 isotopes. Pb-204 Pb-206 Pb-207 and Pb-208. If the relative abundance of Pb-204 is 1.4% and the relative abundance of Pb-206 is 24.1% Calculate the relative abundance of the other two isotopes.

Relative Abundance n Element X has three isotopes X-100, X- 102 and X-104. If the average mass of element X is amu and the relative abundance of X-104 is 68.45%, what are the relative abundances of the other two isotopes?

Symbols n Contain the symbol of the element, the mass number and the atomic number.

A Z X  0 -1 e Bi Ba  La + A Z X Rn  4 2 He + A Z X3 A Z Po  A Z X Z Pb4 5. Potassium- 42 has a half-life of 12.4 hrs. How much of a 560 g sample remains after 74.4 hrs? 6. If the half-life of iodine-131 is 8.1 days, how long will it take a 50 g sample to decay to 12.5 g? 7.What is the half-life of a 100 g sample of nitrogen- 16 that decays to 6.25 g in 28.8 s? 8. C-14 has a half-life of 5730 yrs, if after 5730 yrs 35 g of C-14 remain, what was the original amount?

Bean Lab Solve for the average mass of the element “Bean” - In nature to find average mass, we take the % of each isotope x the mass of one isotope. - Directions: Split beans up into their 12 isotopes (varieties, call them whatever you want) - Count the number of each bean and record. (#1) for each one (12 lima, 10 split peas etc.) - Count total number of beans record that. (#2) - Record mass of the total of each isotope (ex. Put all lima beans on scale and get the total mass.) (#3) for each one - Find the % of each bean (#1) for each/ (#2) = (4) for each (take # of lima beans/total then repeat for each type) - Find average mass of each bean (#3)/(#1) for each = (#5) for each (take total mass of lima/ # of lima, then repeat for each one) - Find the average mass of all the isotopes:

Millikan’s Experiment Atomizer Microscope - + Oil Metal Plates

Millikan’s Experiment Oil Atomizer Microscope - + Oil droplets

Millikan’s Experiment X-rays X-rays give some drops a charge by knocking off electrons

- Millikan’s Experiment +

They put an electric charge on the plates

Millikan’s Experiment Some drops would hover

Millikan’s Experiment Some drops would hover

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

Atomic Mass n How heavy is an atom of oxygen? n There are different kinds of oxygen atoms. n More concerned with average atomic mass. n Based on abundance of each element in nature. n Don’t use grams because the numbers would be too small.

Measuring Atomic Mass n Unit is the Atomic Mass Unit (amu) n One twelfth the mass of a carbon-12 atom. n 6 p + and 6 n 0 n Each isotope has its own atomic mass n we get the average using percent abundance.

Calculating averages n You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? n Total mass = 4 x x 60 = 260 g n Average mass = 4 x x 60 = 260 g 5 5 n Average mass = 4 x x 60 = 260 g 55 5

Calculating averages n Average mass = 4 x x 60 = 260 g n Average mass =.8 x x 60 n 80% of the rocks were 50 grams n 20% of the rocks were 60 grams n Average = % as decimal x mass + % as decimal x mass + % as decimal x mass +

Atomic Mass n Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of amu and the rest has a mass of amu.

Atomic Mass n Magnesium has three isotopes % magnesium 24 with a mass of amu, 10.00% magnesium 25 with a mass of amu, and the rest magnesium 25 with a mass of amu. What is the atomic mass of magnesium? n If not told otherwise, the mass of the isotope is the mass number in amu

Atomic Mass n Is not a whole number because it is an average. n are the decimal numbers on the periodic table.

Law of Definite Proportions (#3) n Each compound has a specific ratio of elements. n It is a ratio by mass. n Water is always 8 grams of oxygen for each gram of hydrogen.

Law of Multiple Proportions n If two elements form more than one compound, the ratio of the second element that combines with 1 gram of the first element in each, is a simple whole number. n The ratio of the ratios is a whole number.

What? n Water is 8 grams of oxygen per gram of hydrogen. n Hydrogen peroxide is 16 grams of oxygen per gram of hydrogen. n 16 to 8 is a 2 to 1 ratio. n True because you have to add a whole atom, you can’t add a piece of an atom.