Describe the beliefs of early Greek philosophers.
- believed in 4 elements: fire, earth, water, and air
- Democritus developed the early atomic theory on the divisibility of matter (“atomos”; ultimate particles)
Early alchemists spent a lot of their time trying to accomplish what? In this pursuit, what did they accomplish?
- turn metals into gold
- prepared mineral acids and isolated elements such as mercury and sulfur
Interpret this quote from a book Francis Bacon wrote: “take lead and melt it, second in the middlteh of it, when it begins to congeal, make a little hole and put quicksilver wrapped in a piece of linen into the hole. the quicksilver will fix and run no more and endure the hammer”
- recipe for an alloy
- quicksilver: mercury
- molten lead and molten mercury combined will blend and solidify into a material much harder than either
Early alchemy books sometimes wrote in code that only fellow alchemists could interpret. Explain the code in this passage about how to make fake emeralds
“Take white lead, one part, and of any glass you choose, two parts, fuse together in a crucible and then pour the mixture. To this crystal, add the urine of an ass and after forty days you will find emeralds.”
- urine of an ass = copper containing salt
What is the importance of sulfur in early alchemy?
sulfur: earliest element to be understood to be an element (along with gold)
If you look at the North shore you will see large mountains of yellow powder. What is this made of?
powdered sulfur!
Describe the achievements of Robert Boyle
- first “chemist”
- quantitative behaviour of gases
- element cannot be broken down into 2 or more simpler substances
- grew the list of known elements
Describe the achievements of Lavoisier
- “mass is neither created nor destroyed”
- Law of conservation of mass
- became a tax collector because he was friends with the king.
- also famous for discovering oxygen
- executed by revolutionists because he was a rich supporter of the king
- Lavoisier begged for a few extra weeks of life to finish what he believed to be one of his most important experiments but: “The revolution has no need of scholars or chemists”
- Lavoisier engraving: poster that says “we were wrong, how could we have killed a brilliant person
- made gas collection balloons out of leather pouches to tell if they were filling with gas
Describe the achievements of Proust
- Law of Definite Proportions
- a given compound always contains exactly the same proportion of elements by mass
- CO2 always contain 2.66g of O for every g of C
Water always contains 8 g of O for every 1 g of H
Why did past chemists have to be very rich?
had to finance their own experiments because chemists didn’t make money
Describe the achievements of Dalton
- Law of multiple proportions: when two elements combine with each other to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.
- Dalton’s atomic theory
- prepared first table of atomic masses based on relative masses of elements in known compounds (made lots of incorrect assumptions)
Explain Dalton’s atomic theory; Explain the revision it underwent
- Each element is made up of tiny particles called atoms.
- All atoms of the same element are identical
- All atoms of the same element contain the same number of protons and electrons.
- Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative number and types of atoms.
- Chemical reactions involve the reorganization of atoms; changes in the way they are bound together.
- Atoms themselves are not changed in a chemical reaction
Revision:
- Atoms of a given element may have different numbers of neutrons.
- All atoms of a given element are similar to one another
Describe the achievements of Gay-Lussac
- measured the volumes of gases that reacted with each other (at the same temperature and pressure):
Describe the achievements of Avogadro
- hypothesis: at the same temperature and pressure, equal volumes of different gasses contain the same number of particles
Describe the achievements of Stanislao Cannizzaro
• assigned the H2 molecule a relative mass of 2 (and oxygen must be O2 and water is H2O)
• measured the relative molecular masses of a large # of compounds
• led to approximate values of the relative atomic
masses
Describe the achievements of J. J. Thomson
• experimented with Cathode-ray tubes (Michael Faraday)
• produced a beam of electrons in an evacuated tube
• deflected beam with an applied electrical field
• measured charge-to-mass ratio of an electron (negatively charged)
- charge to mass ratio: -1.76x10^8 C/g
• proposed the first atomic model: plum pudding model (electrons distributed randomly in a diffuse positive cloud)
Describe the achievements of Millikan
• determined the charge of an electron in an oil drop experiment
- Charge = -1.60 x 10-19 C
• using e/m, the mass of an electron was determined:
mass = 9.11 x 10-31 kg
Describe the achievement of Becquerel
- first person to discover evidence of radioactivity
- a mineral of U can produce an image on a photographic film in the absence of light
What are the 3 types of radiation? Describe their defining characteristic
- gamma (γ): high energy light
- beta (β): high speed e-’s
- alpha (α): He2+ ions (mass 7300 x larger than the e-
Describe the achievements/experimentation Rutherford accomplished
-experiments to deduce something about the distribution of
electrons in the atom
- predicted existence of neutral particles to account for total mass of the atom
-bombarded gold foil with high energy α particles (+ve)
Expectations :
-α particles would pass straight through with little or no deflection
Results:
• most particles passed straight through
• some with slight deflection
• some with large angle deflection
• some bounced straight back
- “its almost as if you fired a 15 inch shell into a piece of tissue paper and it came back and hit you”
Conclusion:
- atom is mostly empty space
- massive positive centre (nucleus)
- electrons moved around the nucleus at a relatively large distance
- atom is neutral (#e = #p)
If atoms are made of the same components, why do different atoms have different chemical properties?
- chemical properties are due to the number and arrangement of electrons
- different elements have different number of protons, neutrons, and electrons
Describe the diameter of a nucleus and electron cloud
- small nucleus ~ 10-13 cm in diameter
- electron cloud 10-8 cm across
Mass of: - electrons - protons - neutrons Charge of: - electron and proton
electrons: 9.11 x 10^-31 kg
protons/neutrons: 1.67 x 10^-27 kg
charge: 1.60x10^-19 C
Describe the achievement of Chadwick
o Paraffin exposed to radiation from beryllium produces particles which could penetrate 20 cm of lead
o Charged particles are blocked by 0.25 mm of lead, thus particles are neutral
o These neutral particles have mass similar to protons
o “Neutrons”
