The early days and beyond The Atom The early days and beyond

Greek Life Philosophers form Grease had many ideas about the world that increased our understanding today They thought that matter came in four forms Earth Fire Water Air And combinations of these were why things were Dry (fire+earth) Wet (air+water) Cold (water+earth) Hot (fire+air)

New Ideas about things Philosophers (not scientists) came up with new ideas about things Democritus First to come up with ideas about atoms Believed all matter was mad of atoms and atoms Atoms move through empty space Atoms are solid, homogeneous, indestructible and indivisible Different kinds of atoms have different sizes and shapes Size, shape and movement of atoms determine the properties of matter This dude lived almost 2500 years ago and he came up with this without microscopes, computers, or any of that! Just WOW!

Everyone's a critic Everyone had questions for Democritus Questions like What holds the atoms together? Without experiments Democritus couldn’t answer One of the most influential critics was Aristotle He didn’t agree with Democritus He believed in the old way of earth, air, fire and water He also didn’t believe there was empty space There was no way to prove Democritus and because he was so influential nobody even looked again for atoms for almost 2000 years

Geek life In the 18th century scientists started to look at the atom again But it wasn’t for another 100 years before the idea gained ground John Dalton an English schoolteacher lead the charge He developed Dalton’s atomic theory It said Matter is composed of atoms Atoms are indivisible and indestructible Atoms of a given element are identical in size, mass, and chemical properties Atoms of a specific element are different from those of another elements Different atoms combine I simple whole-number ratios to form compounds Ina a chemical reaction, atoms are separated, combined or rearranged

So what did Dalton get right/wrong? Not all was right Isotopes have different masses but the same element Atoms can be destroyed/made Atoms are made of smaller things What was right Daltons theory easily explains the conservation of mass The rearrangement of elements in chemical reactions Combining in small whole number ratios END 4-1

The atom What makes it up So how small is an atom? Nucleus in the center Electrons on the outside in a cloud So how small is an atom? Lets take the population of the earth lets say 6.5 billion or 6.5* 10 9 Lets compare that to the number of copper atoms in a penny. So 2.9* 10 22 So one penny contains about 5 trillion more times atoms than people of earth.

Electrons During a series of vacuum experiments using a glass tube that almost had a vacuum in it and running an electric charge threw the vacuum some interesting things happened. Sir William Crookes noticed that a grew glow appeared in the tube Further experiments showed that these particles traveled from the cathode (-) to the anode (+) end of the tube Dubbed a cathode ray Particles have mass J. J. Thomson did experiments using a cathode ray and magnetic as well as electrical fields The ray curved toward the positive side of the field By saying opposites attract we now know the charge of the electron

Charge and mass kind of We knew electrons had a negative charge and we know they have mass but how much mass? Robert Millikan used oil to figure this out What he did was spay oil into the ray and the elctrons stuck to the oil droplets and then to the positive plate of a magnet. So knowing the charge he used a complicated charge to mass ratio to figure out the mass of an electron 9.1* 10 −28

Now with the knowledge of the electron Thompson came up with a model of an atom based on his knowledge of the electron He came up with…. The plum pudding model Or the chocolate chip cookie model So what he said was the electrons were randomly distributed throughout a spherical mass of positively charge material

Ernest Rutherford hates plum pudding (model) Rutherford did an experiment were he shoot a ray of alpha particals at a thin piece of gold foil There was a detection screen around the foil It the pudding model was right the particles should just right through But that’s not what happened While most of the particles went straight though some were deflected Some almost strait back This meant something with large mass was in the atom The nucleus was discovered

Rutherford makes his own model In this new model a positively charged nucleus in the center He later refined this model with more experiments to prove the existence of the proton Protons have a positive charge Later his coworker James Chadwick proved the existence of neutrons in the nucleus and a new model emerged.

What is it now Now we know that atoms contain a nucleus Contains protons and neutrons With negatively charged electrons moving on the outside Now in an electron cloud 99.97% of the mass of the atom in the nucleus End 4-2

How atoms differ The periodic table shows us some of these differences The atomic number This is the number of protons and the number of electrons in a neutral atom

Isotopes Atoms that have different neutron amounts are called isotopes Mass number is atomic number + number of neutrons Natural abundance of isotopes In nature elements have isotopes Using the natural abundance of isotope the average atomic mass is figured out That’s what is reported on the periodic table This is why carbon has a mass of 12.04 and not 12 Elemental carbon has 6 protons and 6 neutrons We can write isotopes by atomic mass Carbon -14 Uranium-235

Deciphering isotopes Mass #-Atomic# =neutons

How we report and calculate atomic mass First the masses are so small that scientist had to come up with a better way Created a new unit equal to the mass of 1 proton Called the atomic mass unit (AMU) To calculate the average atomic mass Take the percent of an isotope times its mass + the percent of isotope 2 times its mass and so on. end4-3 Isotope Molar Mass, g Percent Abundance Strontium-84 83.913 0.56 Strontium-86 85.909 9.86 Strontium-87 86.909 7.00 Strontium-88 87.906 82.58

Radioactivity If an atom is unstable it goes through a process called radioactive decay In this decay radiation is given off to lower the energy of the system The less energy the more stable There are three types of radiation emited in this process Alpha radiation is made up of two protons and two neutrons Has positive charge Moves towards negative plate in experiments Beta radiation Fast moving electron with a negative charge Move towards positive plate Gamma radiation Pure energy so no charge Not attracted to either plate