Lec 5: Chemical Atom Flashcards
Count Claude Louis Berthollet (1748-1822)
Joseph Louis Proust
Berthollet:
- > idea that elements entering combinations need not be the same
- > elements combine in indefinite proportions
- > water in the Nile != water in the Seine
Proust:
-> water has same composition regardless origin
Definite proportions:
-> chemicals combine in whole units, not real amounts
John Dalton (1766 - 1844)
- > settled Proust/Berthollet debate
- > not an experimentalist, worked in metereological phenomena (weather)Meteorological Observations and Essays (1793)
- > weather drove him to explore gas
Atmosphere known to have many gasses
- > why is it homogenous, not like oil&water?
- > oxygen heavier, why isn’t it at the bottom?
- > proportions same even at different altitudes
- > Creates Law of Partial Pressures.
Puts A New System of Chemical Philosophy (1808)
- > the UNSPLITTABLE ATOM, unique to each element
- > Represents atoms with symbols (new!)
- > though today, clear they were molecules
- > with atomic weights, could express reactions exactly with numbers
- > wanted to determine the ratio elements react in
- > assigned H arbitrarily as 1
- > then the rest as ratios of H
- > but it’s guesswork and he had nothing to go on
e. g. Water:
- >assume simplest compound is 1+1 atom
- >saw O and H combined 8:1 ratios
- >assumed HO, so weight of O = 8
- >however it’s really H2 + O not HO
- >so weight of 1 O is 8x 2H, or 16
- >did the same with Ammonia - NH instead of NH3, giving Nitrogen 1/3 its real value
Law of Partial Pressures
Dalton discovered that gases weigh same as combined weight of separate gases
Different gases do not attract; pressure of a mixture is sum of individual pressures.
Particles have different sizes (some larger & heavier)
- > exert different forces, no equilibrium
- > reactions are just recombining them - atoms do not change
- > ratios are nonunique, whole numbers
- > LAW OF MULTIPLE PROPORTIONS
- > thought if atoms are smallest unit, they must combine in wholes, as there are no fractions
- > if multiple combos possible, they are related with simple numerical ratio
A New System of Chemical Philosophy (1808)
Dalton postulates each element is made of particles, named atoms (from Democritus)
- > similar to our chemical atom
- > says all matter made of unsplittable building blocks
- > the atoms are identical size/shape/weight, and possess properties of the element
- > each element has own atom, with unique properties
- > reactions are change in atom combinations
“matter, though divisible, in an extreme degree, is nevertheless not infinitely divisible. That is, there must be some point beyond which we cannot go in the division of matter”
Law of Multiple Proportions
Dalton; a deductive hypothesis with little evidence.
A and B may combine in many possible ratios, but in fixed proportions. The ratio of A to B, compared between multiple possible molecules, can be expressed with simple numbers.
e.g. 1
CO: 100g of C with 133g of O
CO2: 100g of C with 266g of O
Ratio is 2:1
e.g. 2
C2H6: 1g of C with 0.252g H
C3H8: 1g of C with 0.224g H
The ratio is 9:8
William Hyde Wollaston (1766-1828)
Gives the term “equivalent” to chemistry
Gives as table of elements with Oxygen = 10 as the base unit
- > today is the chemical slide rule
- > preferred to Dalton’s, no spooky atoms
By 1814 he loses enthusiasim for Dalton’s theory
- > proposes equivalent weights instead of atomic weights
- > while atomic theory is best explanation for multiple proportions, calculating weights is hopeless since you have to guess the ratios
Jons Jakob Berzelius (1779 -1848)
Practical guy
- > introduces rubber tubing & filter paper
- > modern chemical symbols (not popular then)
Supported Dalton’s atomic theory
- > showed law of multiple proportions experimentally
- > gave Dalton some proof
New system: DUALISTIC THEORY
- > atoms are electrically charged; excess gives electropositive or electronegative
- > polarity gives chemical affinity - whether it will combine with other elements
- > chem reactions accompanied by electrical rxn
Modifies Dalton’s system for comparing atom weight - uses Oxygen as base
- > many more elements combine with O than H, so it’s easier to compare reference vs Oxygen
- > oxygen is 16x heavier, better for measurement
- > rapidly catches on, with O = 100 (eventually, 16)
Big Players in Notation?
Dalton:
- > symbols for atoms (actually molecules of compounds)
- > e.g. H is a dot, N is a line
- > In The New System, table of weights, H = 1
Wollaston:
- > “equivalent”
- > recomputes table with O = 10
- > today, slide rule
Berzelius:
- > gives modern symbols, with first letter in Latin, and second where needed
- > gives table with O as base
- > O = 100, eventually it became 16
- > superscripts to indicate number of times element is present
- > births the modern chemical notation
Stradonitz: Use Berzelius’ symbols in diagrams to represent molecules
e.g. H2O = H-O-H
Dualistic Theory
Berzellius - so called because all chemical compounds consist of an electropositive and electronegative part.
Atoms are charged, and net charge gives elements a polarity. This affects affinity.
Gay-Lussac (1778-1850)
Exploded VOLUMES (not weights) of elements
- > saw 2L of H + 1L H = water
- > suggests reactant gasses have simple ratios between their volumes, and volume of product (T, P constant)
Dalton disagreed though
- > carbonic oxide should be denser than O (as he thought it was CO, but it is LESS DENSE)
- > conflicts with indivisibility of atoms
Amedeo Avogadro (1776-1856)
Reconciles two ideas:
- > compounds have definite # of atoms (Dalton)
- > volumes of compound gas simply related to volume of component gas (Gay-Lussac)
Particles of gas may split in two in a rxn
- > equal volumes of different gas can contain same total number of particles
- > e.g. NO could be 2 half O’s + 2 half N’s to make 2 NO gas
So it is: EQUAL VOLUME OF GAS = EQUAL NUMBER OF MOLECULES (not atoms!)
- > e.g. 2H + O gives two water vapour
- > same volume as 2H
Distinguished between:
1) Atom (Elementary molecule )
- >smallest particle of element to have chemical properties
2) Molecule (integral/integrating molecule)
- >smallest particle to retain physical character of A SUBSTANCE
Molecules of elements = 2+ atoms
- > agrees with Dalton - atoms react in simple numerical ratios
- > particles making up gases are molecules, not atoms
However did not gain acceptance
- > tough words
- > hard to accept fundamental particles of gas have more than one atom
- > Berzelius stuck to Oxygen’s central role in acids
- > theory didn’t allow identical atoms to combine
