Atoms, Molecules & Ions

AP Chemistry

Notable Scientists & LawsRobert Boyle, English, 1627-1691 -first “chemist” to perform quantitative experiments of pressure versus volume. -developed a working definition for elements”.Antione Lavoisier, French, 1743-1794, -published the first modern chemistry textbook -was the first to insist on quantitative experimentation. -verified the Law of Conservation of Mass:

Mass is neither created nor destroyed.

Joseph Proust, French, 1754-1826 -stated the Law of Definite Proportions (once called Proust’s law):

A given compound always contains exactly the same proportion of elements by massJohn Dalton, English, 1766-1844 -stated the Law of Multiple Proportions:

When two elements combine to form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers.

Mass of Oxygen that combines with 1 gram of CarbonCompound I 1.33 gCompound II 2.66 g

Therefore Compound I may be CO while Compound II may be CO2.

-compound II has 2x the number of O’s as compound 1I = 1.33g = 1

II 2.66g 2

John Dalton, English, 1766-1844 -developed his Atomic Theory in 1808 (p46)

Joseph Gay-Lussac, French, 1778-1850 -in 1809, performed experiments [at constant T & P] to measure volumes of gases that react with each other.

Amadeo Avogardro, Italian, 1776-1856 -in 1811, proposed his hypothesis regarding Gay- Lussac’s work (Avogadro’s Hypothesis) At the same temperature and pressure, equal volumes of different gases contain the same number of particles.

-was basically ignored, so 50 years of confusion followed

Gay-Lussac and Avagadro’s experimental work held the keys to determining absolute formulas for compounds, which Dalton was unable to do. -it was Dalton’s incorrect assumptions about formulas of certain compounds that had him incorrectly calculating atomic masses -more research was done on the concept of atoms eventually leading to the:

**MODIFICATIONS TO DALTON' S THEORY**1. Subatomic particles were discovered.2. Isotopes were discovered.

Characterizing an AtomThe ElectronJ.J. Thomson, English (during 1898-1903) -discovered the cathode ray, a negative particle we now call an electron

Characterizing an AtomThomson, cont. -measured the charge-to-mass ratio

e = -1.76x108 C

m g

where e is charge on electron in Coulombs, (C) and m is its mass in grams. -repeated experiments found all metals, and atoms, contained electrons

Thomson, cont. -all atoms were neutral there must be some (+) ∴ charge within the atom and the “plum pudding” model was born.

Robert Millikan, American, during 1909 -oil drop experiment -calculated the mass of an electron by determining the size of the electron charge and using the mass-to-charge ratio discovered by Thomson

mass of e- = 9.11 × 10-31 kg,

Ernest Rutherford, England, in 1911 -pioneer in radioactive studies -tested Thomson’s plum pudding model -if Thomson was correct, the massive α particles would pass through the gold foil with only occasional deflections.

Ernest Rutherford, cont. -was astounded when many particles bounced back or were deflected -determined the plumb pudding model was incorrect -for the (+) a particles to be deflected or reflected, there must be a (+) core he called the nucleus - (+) core contains most of the atom’s mass -nuclear atom was born

Modern View of Atomic StructureElement: matter composed of only one type of atom Atom: smallest particle that retains the chemical properties of that element • nucleus-- protons and neutrons; very dense - proton--positive charge, defines atomic number - neutron--no charge, same size & mass as a proton, alters mass number • electron--negative charge, responsible for bonding, located outside the nucleus.

mass number A

atomic number Z

isotope: atom with the same number of protons but different number of neutrons -12C, 13C, 14C -most elements contains mixtures of isotopes

X element